# 13" F/3 telescope build



## Mitch Alsup (Jan 20, 2018)

18 19 years ago I built myself a 20" F/4 telescope:




This telescope has served me well, but as I was approaching retirement the year previous to last
I wanted to build another set of telescopes to last me the rest of my life. I had several criterion
a) I wanted a telescope with a shorter focal length--in particular 1/2 the focal length of the 20".
b) I wanted as large an aperture as could be made to work well with the optics available today.
c) I wanted to do more and better machining

As to (c) above, the above telescope was built with nothing more than a hack saw, files, drill
press, sandpaper, and I farmed out the woodwork to a friend with a cabinet making shop.

Being an engineer by profession, an amateur physicist, amateur mechanical engineer, and
a all round driven person, I invented a new mirror cell architecture which one can puruse at

https://www.cloudynights.com /topic/547689-mitchs-mirror-cell-architecture/

The optical train in a Newtonian telescope is well established. With both (a) and (b) requirements
that largest primary mirror and pin-point star images with coma corrector is in the F/2.75 range
enabling a 14.5" mirror. F/2.75 is the recommended "fastest" Newtonian one should make with
todays optics (in particular a Paracorr 2 coma corrector). At this "speed" not very many kinds
of eyepieces will put up the the <now>F/2.75*1.15 = F/3.15 light cones. Luckily (or on purpose)
I had only been collecting EPs that would put up with such fastoptics.

But I backed down to a primary of F/3 which left me with a 13" aperture. To build the scope
around. At F/3 this telescope has 2× the Field of View as the current 20" F/4 enabling wide 
field viewing of <say> the andromeda Galaxy and with 13" of aperture a bright image of same.

The EPs mentioned above all weight in in the 1-3 pound range, the Paracorr 2 is just over 1
pound, and if a Barlow is used for further magnification, we have 6+ pounds of weeight out
cantilevered 6+ inches from the typical focuser mounting position. So, instead of following
conventional reasoning (some might say any reasoning whatsoever) I designed a mounting 
system that minimizes the cantilevering of forces, greatly stiffening the ability to hold an EP
at just the correct spot. A summary of this can be found at:

https://www.cloudynights.com/topic/545307-mips

In Jan of 2015 the mirrors were ordered, with a proposed ship data 9-10 months later and
ended up arriving in May '16 for the 13".


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## Mitch Alsup (Jan 20, 2018)

So, with my destiny set, I proceeded to fret about machinery--I should have also been fretting 
about running electricity to the machines, but we will get to that in time.

In April of last year, I ordered a 12-36 lathe, an 8*32 knee mill, a 14" bandsaw and a 10" table
saw. About a week later they arrived. On the day they arrives the friends who said they would
be there and help me move the machines into shop would become the shop wigged out. The 
delivery guy was kind enough to allow me to place 2 sheets of  4*8*3/4 plywood alternatingly
so we could maneuver the 1000+ pound machines towards the shop.

The next day I had some real friends show up, one with an engine hoist. If I had filmed this
It could have been in contention for the funniest home video ever--but alas. If we knew
at the beginning what we knew at the end of the move it would have only taken about 2 hours
to get everything inside. as it was it took 5+hours and all we did was get them inside. Success.

A few weeks earlier I had made arrangements with an electrician to drop 6 220V services to 
the shop. He kept putting me off so long I finally had to fire him and hire someone who could
get to the job at hand.

The shop is located in the back of a Duplex we have owned for 20-odd years. It was originally
built out as the 3rd apartment in a triplex, but the cityu said NO, and the previousowner sold
the property to my wife and the back area had been used as simple storage for 18 years.

The new electrician was faced with the daunting task of bringing the electrical service drop at 
the duplex "up to code". So we started with a service drop for the 2 apartments and the dead
back room as:




None of this meeting current code. After a week of negotiaion with the city we had an
acceptable plan and began the electrical work. And 5 weeks later the outside looked like:




And I had electricity to run the machine, and LED lights in the shop and all was well with the 
world. I got six (6) 30A 200V circuits into the shop, 5 of which were consumed immediately.


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## Mitch Alsup (Jan 20, 2018)

With the shop now operational, a quick view is in order:




A long time ago my wife had a tenant who did stained concrete as a living, adn these
floors came in a trade fro rent money. All of the machines are on adjustable machine
feet, and over the next 6 months I ahve had to adjust them a couple of times as the floor
"takes a set".

But is is time to actually use the machine and start building stuff.............

As mentioned in the mirror cell architecture thread, the goal of the mirror supports are:
            a) low friction
            b) robust support
            c) dynamic balance.

Most cells support their mirror on plastic pads of some low friction stiff plastic material. The mirror 
slide around on the pads. But when sticktion arises, the mirror can stick and get its surface distorted. 
Pads also have the property that the point so accurately located by PLOP is distributed over a fairly 
large area. I wanted lower friction, lower sticktion, and stiffer connection to the backside of the mirror 
while maintaining accurate support locations.

The 13" mirror is supported by 6 ball transfers on 3 beams at points (3.902" apart) determined by 
PLOP. The mirror is free to slide over the ball transfers with about 10% of the friction of the lowest 
friction pads one can find. This ensures that the mirror is not held artificially in any orientation. 
Similarly, the edge supports will utilize the same ball transfers, this time set at the center of gravity 
of the mirror.

Each beam rotates on an axle supported by a pair of 1/4-1/8-3/32 ball bearings. Thus, the beam rotation 
is light and delicate and has low friction. ultimately the beams will be dynamically balanced so that as the 
mirror cell moves in altitude, the beams cannot impart any forces onto the back of the mirror, other than 
the support the mirror requires.

Three <ahem> perfectly machined beams for the 6-point cell in my 13" DOB project:




The ball transfers are 0.470 in diameter and I have them set 0.300 deep. The ball seats are 0.468 
in diameter created with a boring head on the mill. There is a 0.440 hole in the back for push out 
access. The ball transfers are pressed in with a thumb, then can be pressed out with a screw driver.

As seen above, I still have to machine a tool to cut a bearing-fit on the 1/4-1/8-3/32 ball bearings 
the axle pivots upon. The bearings measure 0.2498 in diameter. A good seat was created  with a 
D-sized drill bit.

Once the bearings are fit, I will machine (skim) the bottoms to achieve balance. Right now it should 
be bottom heavy if all measurements have been done correctly. Balancing the beams means that the 
beams will not push on the back side of the mirror as the scope moves in altitude (or azimuth.)


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## Mitch Alsup (Jan 20, 2018)

The holder for the secondary mirror requires a 45º angle on the nose of the support plate and 
means to attach the spider vanes. It is desired that this component be light but also stiff.

The secondary mirror is attached to the secondary holder in such a way as to minimize most 
cantilevering forces . In the following figure, the spider vanes are attached to the left side of the 
spider holder with 4-40 socket head screws, and the plane where the vanes are attached runs 
directly through the CoG of the secondary mirror.




Likewise, the secondary is attached to the spider assembly with three socket head collimation 
screws and one tension adjustment rod with a thumb screw bolt. The bolt applies a pulling force 
directed at the CoG of the secondary mirror, while the three socket head collimation screws apply 
pushing forces defining the plane in which the secondary mirror is pointed.




Only the offset weight of the secondary itself applies cantilevering forces to the spider assembly. 
The spider vanes attach to the spider holder close to the backside of the secondary mirror reducing 
cantilevering forces.

Notice that the optical axis (black cross) is closer to the focuser than the CoG (green cross) of the 
secondary, thus the secondary holder is offset away from the focuser. In order that the four spider 
vanes create 4 diffraction spikes the opposing vanes must remain parallel, which causes the vane 
load to be articulated into the upper assembly offset away from the focuser.

An F/3 Newtonian is essentially unusable without a Paracorr 2 (or similar coma corrector) in the 
light path coming to focus.  If one inserted a Paracorr 2 with tunable top into the focuser, and then 
a heavy 2" eyepiece into the tunable top of the Paracorr 2, large cantilever forces are applied to the 
tunable top, to the focuser, and to the base plate. To top it all off, the moment of inertial of the upper 
assembly is lower due to the small radius of the upper assembly round plywood tube. as illustrated 
in the following figure:




Instead, knowing that the Paracorr 2 is an integral part of the upper assembly, it is designed into 
the optical path in order to achieve the stiffest possible design at the light weight an upper assembly 
desires. The tunable top of the Paracorr is removed, and the Paracorr 2 itself is inserted into a tube
 held concentric with the focuser. The Paracorr does not move during focusing.

Thus the weight of the Paracorr is on one side of the focuser and the weight of the eyepiece on the 
other side canceling out much of any cantilevering force. Furthermore, the moment of inertia of the 
upper assembly is dramatically improved because the round tube on which it is built has a 20" radius 
instead of a 15" radius. Finally, the truss poles articulate directly into this tube, further stiffening the 
upper assembly.




Seen in plan view, we have:


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## Mitch Alsup (Jan 20, 2018)

Now that the upper assembly architecture is understood, let is proceed to the fabrication of 
the secondary holder and spider system.

The secondary holder would have been easy to construct if someone made 135º angle aluminum 
like they make 90º angle aluminum. 6061 T6 is just about the ideal substance to construct ATM 
telescope parts, strong, light, easy to cut, file, machine, and corrosion resistant.

Creating the 45º nose on the 1/4" 6061T6 aluminum was a bit difficult as I had to make a bracket 
to hold the spider-plate to the machining vise while clamping the nose piece in the vise and have 
the ability to mill a flat at 45º on the plate before drilling a 0.086" hole through the plate at 45º and 
directly into the nose piece. Before moving the mill head, the hole in the plate was enlarged to 0.111. 
The holes drilled into the nose piece are threaded 4-40. This took 3 tried to get it right, the second 
one would have been alright, except the spacing on the holes caused one hole to intersect a 4-40 
tapped hole in the nose piece, which snapped the drill bit--Grrrrr.

This support plate was also the first time I had used the ability to set the vise at and angle other than 
perpendicular to the table movements.  I lightened the center of the support plate with a 5/8" end mill 
ide cutting the 4 triangular holes. Various holes are 0.086 drilled and 4-40 tapped.

The nose piece has a 0.257 hole (F-drill bit) on the line which will pas through the center of gravity of 
the secondary mirror to provide clearance to the 1/4-20 attachment bolt. There are three 4-40 tapped 
holes on the 40% elliptical radius. Into these holes will be 4-40 socket head screws with their ends cut
into 60º points. These points will fit into points on the plate silicone glued to the back of the secondary 
mirror. These 4-40 bolts are manipulated by hand during collimation. I have used a similar arrangement 
for 18 years in my 20" DOB and it works well.

Every surface is cut by machine.




This first view shows how I used 4-40 screws to join the spider plate and the 45º secondary nose.

I just wish somebody extruded 135º 6061T6 angle aluminum 1/4 thick 4-5" on each side. Would 
have saved me days of setup for two simple 2 minute cuts.




This second image shows the miter angle between the nose piece and the body. Forces exerted 
by hand on this nose piece give the impression that the nose piece attachment is at least as 
strong as the plate to which it is attached.

{For those more easily amused, this took 3 tries, the first two got ruined when the machining 
forces were greater than the clamping forces leading to bad cuts and mispositioned holes.}

The secondary mirror is silicone glued to a collimation plate. In order to avoid thermal problems, 
the plate is machined in a Y form and each leg of the Y is cut twice 75% of the way through creating 
a long spring between the place where a pulling force is exerted on the plate and the place where the 
pushing forces are imposed onto the plate. This long force path will reduce the  forces due thermal 
difference in expansion with temperature. The silicone pads are directly over the coned 4-40 socket 
head cap screws. The seat is drilled with a #1 center drill so that each is fully supported but allowed 
to tip/tilt in collimation.




I did decide to leave the flanking sides where the vanes attach on this iteration, unlike the drawing.

Here is a picture of the collimation plate sitting on top of its collimation screws and tensioned with 
a 4-40 nut. The 1/4-20 threaded rod is secured into the collimation plate with a set screw.


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## Mitch Alsup (Jan 20, 2018)

Now, with the router radius attachment, I routed three rings, 2 for the upper assembly, one for 
the lower assembly:




The lower ring and the upper top ring are 20.5" with 14.5" hole in the middle.

The lower top ring is 20.5" in diameter with 16.5" hole in the middle.

This allow the missing disk from the lower top ring to be used as the dust cover in transit and 
storage.

I thought I would take some time and explain a few architectural things about truss poles that 
make for good design and good engineering. These concepts are from Albert Highe's book 
"Portable Newtonian Telescopes".

The first item on the list is eccentric loadings--you don't want any. When the load carried by 
the truss pole enters the pole on the axis of the pole, there are no bending moments imposed 
by the application of the force, otherwise there are. We don't want bending forces imposed on 
the poles, the poles bend under compression anyway. We do this by applying the load forces 
onto the poles through a <spherical> ball end.

The second item on the list is bending loads on pairs of truss poles. A pair of truss poles comes 
together at a bracket. When one extends the axis of the poles to where they both meet, and this 
virtual meeting point is at the center of gravity of the assembly being supported, then there are 
no bending moments imposed by the bracket onto the pair of poles.

The third item is that we want the system of poles to be stress free prior to the clamps being clamped, 
the unclamped truss allows the assemblies to find the point of least stress. If the clamping action can 
be applied without moving from this balanced position, the whole truss will be stress free.

Without knowing up front, one can guess that the center of gravity for the lower assembly will be 
close to the back face of the primary mirror, and similarly, one can guess that the CoG of the upper 
assembly will be at the CoG of the secondary mirror. The following figure illustrates these CoGs and 
the archetypal pole spans:




One wants the convergence point of the poles to coincide with the plane of the CoG at the upper 
assembly and the lower assembly. Slight inaccuracies (as much as a whole inch) do not effect the 
efficiency of the truss poles much, so a guess on the CoG is accurate enough for ATM builds.

One needs the poles to avoid vignetting the light column that reflects off the primary and reaches 
the focal plane. In addition, as the radius of the truss increases, its stiffness increases cubically. I 
followed Albert Highe's recommendation to use Drum Shells as round plywood for the upper assembly. 
Secondarily, I wanted the cantilever moment of the focuser on the upper assembly to be small, so by 
choosing a 20" drum shell for the 13" and having the Paracorr apply weight inside the shell and the 
focuser and EP applying similar forces on the outside of the shell, the majority of the cantilevering 
force cancels. Thus, the poles are aimed at the 20" shell at the upper assembly. This prevents any 
reasonable sized pole from vignetting the light path.

One needs the poles separated by enough distance at the pole clamping bracket such that a single 
simple clamping can be applied which is equal to both poles. A simple single bracket guarantees that 
both balls are clamped with the same force. The following figure illustrates where one would desire the 
pole <ball> ends to be located:




As the pole balls increase in distance from the primary mirror the mirror box grows in height. As the 
pole balls decrease in distance from the primary the separation between the two balls decreases, until 
at some point a single simple lamp will not fit between the pole balls.

The single clamping mechanism has been decided to be a 1/4-20 thumb screw and thus one needs 
enough distance the human hand can get between the poles and tighten this thumb screw.

Now the bracket can be attached to the mirror box from below or from above, as illustrated in the 
following figure:




This ATM decided to place the bracket below the <plate> ring on the mirror box, and to use the upper 
clamping bracket to capture the poles in transit. Furthermore, the clamping thumb screw is semi-captured 
on the upper bracket <plate>.

The same logic holds at the upper assembly, as illustrated in the following figure:


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## Mitch Alsup (Jan 20, 2018)

The second thing I machined were the seats for the truss poles. The next picture shows the components:




The requirements here are a firm clamping action on the balls on the ends of the poles, a strong 
attachment to the upper or lower assembly, fast, easy, and simple clamping mechanism. In addition, 
for geometric reasons, the upper bracket and the lower bracket must have the ball seats accurately 
positioned to properly apply tension forces to the balls, converting the pinned joint (pre clamp) to a 
rigid joint (post clamp) which doubles the stiffness of the truss.

Towards the right are the pole-captive attachment brackets with semi-attached thumb nut. A nice ball 
seat was milled into the captive bracket to hold the balls on center as a clamp.

Towards the left are the frame brackets. These screw onto the upper and lower assemblies of the scope 
giving a firm location for the truss pole ends. Ball seats were also milled to properly locate the truss ball 
ends. The frame brackets have countersunk wood screw holes located so that the bracket can be held 
firmly to the wooden rings on the upper and lower scope assemblies.

At the top is the thumb screw with 1/4" of threads removed. This tightening device threads through the 
captive bracket and then turns freely.

Now, to be fair, it took me several tries to get these things made precisely enough--all part of learning 
how to use the machines I put in my shop, how to measure accurately, and in what order does one 
machine things to avoid tolerance stackup problems.


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## Mitch Alsup (Jan 20, 2018)

Today, I got a mirror cell support plate <jig> made.




This support plate has 6 accurately positioned accurate diameter and height stand-offs to hold the 
mirror beams in <near> perfect position so that the beam pivot supports can be attached to the 
mirror cell frame. As machined, the tolerance on the positioning of any given point at the end of the 
beam is on the order of 0.002".

The Blue tape is just to hold the beam pivot supports in line so the mirror frame (Y) can be positioned. 
I was intending to use the brackets shown above and using a Muggy-Weld product to solder aluminum 
to steel. Ultimately I could not figure out how to make sure that the two axles from each side were 
sufficiently co-axial that I made different brackets (see 2 paragraphs later.)

This was the third try at making such a jig. The previous ones failed in the accuracy department and in 
the usability department.

Basically over the last 3 weeks I have tried a number of ways to mount my 6-point cell beams to the Y-frame 
such that the points at the ends of the beams are at exactly the right positions (within 0.003"). In addition to 
this, the beams themselves are balanced so that they can be oriented in any orientation and remain balanced. 
This requires the CoG of the beam is on the axis of rotation.

A few weeks ago, I had beams that were perfectly balanced, but it turns out I read the wrong dimension and 
while the beams balanced perfect, the points were no where close (about 0.2" off) to where PLOP wanted them 
(3.902"). So, drat, I whacked out another set. At each end of each beam a 0.420 hole was drilled, then each hole 
was bored to 0.468 so that the ball transfer bearings could be pushed in with thumb pressure and not fall out. 
This set of 3 came out as; 1 was 0.000,5 too long, one was 0.000,5 too short, and one was 0.003 too short. I 
decided this was good enough.

The axle was precisely located and cross drilled, each side was then drilled 0.110 deep with a 0.246 (D) drill so 
the 1/4-1/8-3/32 ball bearings could be pushed in under thumb pressure.

Woe:: It turns out that drills wander as they drill holes, and the axle was not exactly perpendicular to the beams 
even though the head on the mill is accurately trammed to the table of the mill. I found this out on the first set 
of beams, and although I tried drilling slower, at lower speeds, at higher speeds, at lower pressures, and as much 
as I could bear to put on the 0.89 drill bit, all of the holes wandered enough to be seen with dial calipers. I did 
some research and found out that it is not easy or inexpensive to bore holes this small, either.

The beams were balanced by first shaving material from the back side of the beam until the CoG was at the height 
of the axle (defined by the ball bearings sitting in their races on an axle.) Then the heavy end was hand filed until 
balance was achieved horizontally left. Then the beam was balanced again vertically, and then the beam was balanced 
in the horizontally right.

If the beams are not balanced, then as the mirror moves in altitude, the imbalance on the beams can apply forces 
to the back of the mirror and distort its surface. I definitely did not want this.

The trick is to make something that connects the axle pick up points to the Y-frame. I won't bother you with the 
number of things I tried (6) before finally getting one that ALMOST worked. I machines some u-brackets out of 
aluminum and drilled a 0.089 (minor diameter of 4-40 threads), drilling through both ends of the bracket in a single 
drilling operation. This is how one gets two holes drilled so that both are absolutely coincident--but this is only 
straight, not completely true (perpendicular.)

The holes were then threaded with a 4-40 tap 0.400" deep. The width of the threads allow bending forces to be 
picked up by the bracket efficiently. in the final assembly a 4-40 nut will be used to lock the axle in place.

I bought some precision ground rod for the axles. As it arrived, I miced it and it read 0.1251 and would not fit into 
the ball bearings. Grrrrr. So I chucked the axels up in the lathe sanded it down with 1000 grit and WD-40. At 0.2499 
(on my uncalibrated micrometers) it would fit through the bearing with a stiff hold on the inner race. I tried for a 
couple of hours getting 2 bearings on the axle and the bearings in the beam races and never could get the beam 
to pivot freely. So, back to the lathe, and I took the axles down to 0.2496 and could get the beams to pivot with 
low friction. But now the lateral friction was so low, the beams would move on the axle. Grrrrr.

In each end of the axles I drilled a #1 center drill hole, and then I took six 4-40 socket head screws and put points 
(cones) on them to fit in the end of the axle. Now, finally, I have beams, properly balanced, in low friction pivots. 
Success like this deserves a couple of pictures::




Here we can see the beam with ball transfers at both ends, and small ball bearings in the middle. 
An axle runs through the bearings and is "caught" by two 4-40 screws. The beam will move when 
about 1/3rd of a grain (0.02 grams) is placed on either end--that is: low friction and good balance. 
The imbalance is also the maximum amount of force the beam cam impart into the back of the 
mirror at any elevation.

The beam is surprisingly stiff in deflection and in twist being supported only on the 1/8" axle and 
located by the 4-40 socket head screws.

In order to properly locate the beams, brass tubing was cut and filed to thickness. This gets rid of 
the lateral movement of the beams due to the sloppy fit of the axle to the inner races of the bearings, 
but hampers the ability to move the beams inward or outward finding the perfect support radius.

At the top of the image are two little brass tubes used to space the beams on the axle:




Originally the axle was supposed to hold the bearings stiffly enough that no spaces were required, 
and that the beams could be moved laterally with the 4-40 screws. But this is where this build ended 
up.

In order to attach the brackets to the Y-frame, I machined in some relief slots at precise angles on 
the bottom of the brackets so the brackets would practically grab the metal on the Y-frame. each 
slot was carefully positioned to pass through the center of the bracket. As to the brackets grabbing 
onto the Y-frame--yes they did.

With all of this machining and tight tolerances, at some point or another one has to question whether
 this whole thing can be assembled and have any fidelity to the original design point. In order to address 
the tolerance stackup, I built a wooden support plate with precisely located holes and machined up some 
collars to hold the beam ends at exactly the right positions. This plate eliminates tolerance stackup, only 
to expose your machining flaws.

Guess what? Remember that drills do not drill straight, well the beam axes are not exact, nor are the 4-40 
screws in the bracket exact. Grrrr.




Here we see all 3 brackets face forward on the alignment plate which <all but> eliminates tolerance 
stackup. The beam point positions are entirely determined by the support plate bushing locations. 
These can be measured (and have been) and are within about 0.002 or correct.

We can also see that the beams are in balance and that none are touching the support plate nor their 
bracket mid-section.

Now, I remove the ball transfers, flip the brackets over and press them onto the guide bushings:




At this point, the Y-frame can be dropped into the bracket slots which have been relieved so that 
the both the bracket and Y-frame can jiggle about 0.001-0.003". Just enough to prevent stress from 
<ahem> repositioning the support points on the ends of the beams.

When the upper part of the mirror cell is ready, the Y-frame will be trimmed to length, sanded, and 
painted (gloss black) and when dry, the brackets will be epoxyed onto the Y-frame while being held 
on the support plate. Now, the ball transfer roller balls will be precisely positioned.

You might notice that brackets and beams are labeled. Once the brackets are epoxyed to the Y-frame, 
each beam goes in exactly one direction, one other thing is that the ball transfers are also "indexed" 
nto a particular hole in a particular beam (they weigh differently--at the scale these beams can judge 
weight 0.3 grains).

So, a tale of woe is finally solved by a bit of cleverness.

Some observers might say "Why go to so much trouble". There are 2 responses to this: 1) It's my telescope, 
2) I am trying to build it more like a Swiss watch than a John Dobson Dobsonian. There is also a third reason: 
Mike Lockwood has been writing about telescope mirror cells stating that low friction is key to making thin 
light mirrors work.

This build is an experiment in how low a friction a mirror cell can have (thus ball transfers supporting the back 
and sides) and ball bearings supporting the beams. In the scale amateurs can achieve, this is pretty close to a 
little friction as reasonably achievable.

Some might ask if this is Overkill--Absolutely! it is!

But as an experiment in low friction cell design, in a low (wind) profile frame design that allow essentially unfettered 
air to the back and sides of the mirror itself. See later.

The mirror cell is a square with two 1/2×1 mild steel tubes 14.5" long and  two 1.25×1/8 flat steel silver brazed into a 
square. It was a conscious decision to position this frame fully surrounding the mirror itself. Each edge supports 
transfer their loads to a frame stiffening member. The frame stiffening member is then silver brazed to the mirror 
cell and the mirror cell is screwed directly onto the altitude bearings.

The Y-frame has been calculated to heave on-the-order of 0.001" from zenith to the horizon--well within the 
focusing tolerance of an F/3 scope. The frame stiffeners are considerably stiffer considering the 10.6 pound weight 
of the mirror.

The mirror cell will be directly attached to the altitude bearings leaving no intermediary way to lose stiffness.

The edge supports are ball transfers sitting in a fixture. On the top of the fixture is a threaded hole which will be 
used to hold the top edge restraint (so the mirror cannot fall forward out of the mirror cell.) At the back of the fixture 
is a 1/4-20 thread so the fixture can be bolted to the frame stiffener.  The height of the fixture above the ball transfer 
was sized so that when the mirror is touching the edge restraint the ball transfer should be exactly at the CoG of the 
mirror.

Here we see the bolt holding the fixture to the frame stiffener holding a ball transfer.


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## Mitch Alsup (Jan 20, 2018)

The ball transfer will be positioned at 0.481" from the back side of the mirror (CoG) in a future 
take (hopefully not one with so much woe involved.)

Two of these frame stiffeners are fitted to the mirror frame.




We see two of the stiffeners being held up on spacers touching the actual 13" mirror. This picture 
was taken before the stiffener tubes have been fitted. We see the square frame as described earlier. 
In the flat plates on the side, we can see some 1/4-20 holes in the plate where the altitude bearings 
will be bolted.

Any time I need to touch/move the actual mirror, my hands get washed, dried, and then I wait until 
the skin has no residual water remaining on it. The vast majority of the time the top cap (cardboard 
above) is left on the mirror to protect the surface. Only when the top edge of the surface is critical is 
that layer of protection removed, the top tissue paper remains.

Fitting of the stiffeners to the frame involves cutting material off of the ends of the stiffener such that 
the ball transfer remains in the center of the stiffener. This took a number of tries.

After some work all 4 frame stiffeners holding their ball transfers bolted to their tubes have been fitted 
to the mirror and to the frame.




At this point there is less than a piece of paper thickness between the mirror and any of the ball 
transfer--call this zero clearance. Later, I will adjust the clearance to more than 0.003 and less than 
0.010. 0.003 is the amount of dimensional change between the frame and the mirror between 100ºF 
and 30ºF. ) 0.010 is a small enough clearance that collimation will not shift as the scope moves in 
altitude.

Overnight I decided to use 0.008" thick card stock for my spacers--as it was easily available (at hands 
reach). and that I would space the ball transfers such that each was spaced 0.008" away from the edge 
of the mirror; 0.016" in total clearance. This is vastly more than thermal requirements, and still small 
enough that the mirror is accurately located. 0.008" clearance was obtained by machining 0.006" off 
the each end of each stiffening member and depending on a SQRT(2) = 1.414 multiplier due to the 
45º angle of the stiffener.

Having achieved this clearance, the frame stiffeners were silver brazed and test fitted against the actual 
mirror and a diametrical clearance of 0.016". These card stock shims fit with significant force required to 
push them between the mirror and the ball transfer. In order to achieve this       < ¿perfect?> fit I had to 
shave 0.003 off the back of the ball transfer fixture.  Apparently, the silver brazing allowed the stiffeners 
to end up 0.003 farther out on the mirror cell than the fitting procedure did. Oh well.

The mirror cell was then turned upside down with the mirror retainers attached at the bottom of the edge 
support fixture. The mirror was then carefully placed upside down touching it only by the edge and only on 
soft aluminum. The ball transfers were verified to be positioned at 0.481" from the bottom edge of the mirror; 
that is, at the CoG of the primary. Later on, the mirror cell retainers will be shaved 0.010" to provide the 
vertical clearance the mirror deserves and requires. (i.e., not and never touching the mirror)




The beams and brackets were taped onto the Y-frame and positioned over the inverted mirror 
and manipulated until the beams and balls were at their proper geometric point on the back side 
of the mirror.  In effect, the Y-frame is holding itself in the perfect position by using the actual 
beams, actual brackets, and actual ball transfers.

It is now a process of measuring and fitting some small posts to the mirror cell, then cutting and 
drilling a hole in the Y-frame so it can be semi-permanently attached to the mirror cell.

The astute observer will see 2 machinist triangles in the image. These are being used to prevent the 
Y-frame from moving under the slightest touch or vibration so that some measurements could be 
taken! Yes, Virginia, there is low friction, here.


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## Mitch Alsup (Jan 20, 2018)

At this point, measurements of the vertical space between the mirror cell and the Y-frame were taken, and card stock used to verify the fit prior to cutting steel.

Steel posts were cut and fitted between the mirror cell and the Y-frame. During the measurement and fitting one can measure that the right hand side of the Y-frame is about 0.002 taller from the back of the mirror than the left side of the y-Frame. The nose (0.850) to tail (1.115) dimensions are just about spot on the original drawings.

The mirror put back into safe keeping, and replaced by the support. The support plate is accurately centered and the Y-frame marked for cutting. Here we see that the support plate has been fitted to the edge supports on the mirror cell and sunk down to the same height that the Y-frame was found to be when supported on the back side of the mirror.




The Y-frame was cut and fitted to the posts. The posts had a press fit nut inserted and silver 
brazed into its top flush with the steel.

After cutting, the Y-frame is assembled on the support plate and the location of some holes determined 
for 1/4-20 socket head screws. I drilled the holes 7/32" to be smaller than 1/4" and used a round file to 
make the center of the hole match the center of the press in nut on the post. This fitment ensures low 
stress is induced bolting the cell to the frame.

After cleaning up the welding detritus, the mirror cell was fully assembled for the first time.




In this form, the astute reader will recognize the almost unfettered access of air flow across the 
mirror when in use. The back side of the telescope will have nothing to prevent airflow from occurring 
naturally, and will include a fan to help airflow early in the night.

Here is a look at the 30º angle cut on the Y-Frame giving unfettered access to the socked head screw, 
and a pleasing look.




A bit of painting and presto:


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## Mitch Alsup (Jan 20, 2018)

The assembled strut system is now fitted to the mirror cell:




The two back truss pole attachments will couple to the mirror cell through the altitude 
bearing, giving a stiff vibration dampening attachment. The single front truss pole attachment 
couple the mirror cell through the stiffener directly to the top of the mirror cell where it is 
attached to the Y-Frame. By these means the <removable> truss is stiffly attached to the 
mirror cell without a mirror box getting in the way.

The mirror cell is not under the mirror as it is in most Dobsonian designs, only the Y-frame is. 
The mirror box has absolutely no structural loads, the mirror box is present merely for weather 
protection of the mirror. The slightly higher placement of the mirror frame enables the altitude
bearings to be slightly smaller and thereby slightly less bulky in this F/3 design.

And I found out that the nose of the mirror frame needed a stiffener to pick up the loads from 
the isolated truss attachment at the top of the truss system. I knew this stiffener was needed all 
along. I was originally going to make this out of aluminum and bolt it to the wooden (ahem) 
mirror box, but I decided to braze it to the steel frame and prevent the compression of wood 
from altering the stiffness of this critical strut attachment.




The stiffener is carefully fitted to the mirror cell and to the truss pick up point. Inside the stiffener is a 
fitted press in nut which  is also silver brazed. This nut is rotationally positioned such that the thumb 
screw going through the truss pick up point threads right  into the stiffener, joining them as if one. 
Later on I will use some Muggy-Weld stuff that can solder aluminum (truss pick up) to the steel stiffener.

The stiffener is silver brazed onto the mirror cell and also onto the post holding the Y-frame. During the 
brazing, a vertical c-clamp is used to make sure the post does not move and the stiffener is clamped to 
the post.

Yes this did cause me to have to repaint the mirror cell. That is one of the costs of getting your telescope 
build out of order.


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## Mitch Alsup (Jan 20, 2018)

Here I am cutting out the material that will become the vanes of the spider.




The material is 0.008" thick galvanized roof flashing that comes in a roll at the local Home Despot. 
$10.00 worth will provide for a lifetime of spider vanes of the typical ATM.


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## Mitch Alsup (Jan 20, 2018)

The strip is folded in  the middle and then the middle is bent back out to 90º.




Before painting the seam will be silver soldered to retain a nice crisp 90º bend under tension.

Finally, the taper on the vanes is cut, the adjustment ends fitted and this is placed on a setup jig 
for measurements and sanity checks. As imaged here, the vanes need to be shortened by about 
0.500.




And this is about where the whole kit and caboodle is right now.


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## savarin (Jan 20, 2018)

Thanks Mitch, an absolutely superb write up and resulting project.
Heres my two projects, as an ex chef I only use rough guestimates.
https://www.hobby-machinist.com/threads/the-giant-binocular.55688/
https://www.hobby-machinist.com/threads/80mm-long-focal-length-refractor.26212/
If I could have planned them in the detail you use so effortlessly I think I could have avoided a heap of problems before they reared their ugly heads.
Looking forward to see the final result.


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## jrkorman (Jan 21, 2018)

Fantastic - Going to have to reread this a couple of times to get it all in - Love the 'scope - I've only a small refractor and binoculars to use here! And since we have quite dark skies here, I have been thinking about buying/building something a bit better!


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## Bob Korves (Jan 21, 2018)

savarin said:


> as an ex chef I only use rough guestimates.


I am an ex chef as well.  That is the ultimate improvise, use your imagination, and git-er-done job, more so than machining.  It is also a good way to work strange hours, and every weekend and holiday.


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## john.oliver35 (Jan 21, 2018)

Mitch - Very nice work!  I am not a telescope guy, but I bet your record of this build will be valuable to a lot of people!


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## petertha (Jan 21, 2018)

Very impressive. I can see that hobby makes a lot of demands to many aspects of machining, materials types & especially fitting.
Do you integrate a digital camera into the assembly to record images?


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## Mitch Alsup (Jan 21, 2018)

petertha said:


> Do you integrate a digital camera into the assembly to record images?



At my current level of machining, I typically make a part 3 or more times before it is good enough to go on my telescope.
So, under these circumstances, I just take pictures when I have something to show.


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## petertha (Jan 21, 2018)

No, your build pictures are excellent! I mean do you have provisions for a camera hooked up to the telescope to take pictures of distant galaxies?


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## Mitch Alsup (Jan 21, 2018)

petertha said:


> No, your build pictures are excellent! I mean do you have provisions for a camera hooked up to the telescope to take pictures of distant galaxies?



Sort of:: I have an adapter and t-ring that attached to my Canon dSLR; that goes in the focuser and enabled astrophoto.
But 99.9994% of my telescope use is visual.

I thought about building a fork mount for the 13 (maybe after I get the other telescopes done) that would be good enough for long exposure astrophoto. But its in a not yet category.


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## Mitch Alsup (Nov 7, 2018)

An update: I finally got a working version of the MIPS plate I talked about 1/2 way up this thread.

As you recall the purpose is to lower the cantilevering forces on the focuser and the base plate mounting it to the upper assembly.

But let us start with the process of discovery:: the following is a Paracorr 2 with the inner (focusing) barrel removed.


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## Mitch Alsup (Nov 7, 2018)

The outer barrel on the Paracorr 2 is threaded onto the optics barrel.

For a long time, I wanted to machine a thread in a part and simply screw the P2 optics housing onto some plate. But making 2.78" M0.75 threads to a blind stop inside a bored hole 1mm deep is "beyond" my abilities to machine. My guess is that the manufacture made a bottom tapping die and uses it to perform the threading.

So rats.......That is, until I realized threads go the same direction backwards and forwards! So, I use a piece of the P2 flipped it over and I have my threaded adapter--and it already had a thread (cam) machined around its periphery. So this next picture shows the outer barrel flipped.


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## Mitch Alsup (Nov 7, 2018)

I machined up a barrel that is a nice fit to the outer housing, threaded a hole, turned a thumbscrew to fit the cam profile and put dimples on th ends where set screws hold the barrel to the base plate (later).


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## Mitch Alsup (Nov 7, 2018)

The first image shows the P2 out as far as the cam allows:



While this next picture shows the P2 in as far as the cam allows.


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## Mitch Alsup (Nov 7, 2018)

This mechanism was designed to allow the Paracorr 2 optics barrel to be installed from the back (inside) of the upper assembly.

The base plate started life as a 9" by 4" by 1" piece of 6061T6 aluminum. The first operation was boring a hole at an offset but centered just smaller than the outer size of the barrel machined to hold the Paracorr 2 optics (2.700"). Then 1/2" from the front was faced off, and 1/4" from the rear was faced off so that there was enough projection on the front side to pass through the upper housing and allow installation of the focuser.




After being faced to size, the plate was mounted on an arbor on the rotary table where it was lightweighted.
A picture of the side profile:


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## Mitch Alsup (Nov 7, 2018)

Next up was a means to focus the Paracorr 2. In effect one has to hang a transparent film 52mm above the top surface of the last lens in theP2 optical barrel.




The inside was cut with a taper such that the outer edge has a snug fit to the threads at the end of the P2 optics barrel. Then after this end was fit to the P2, measurements were performed to determine how deep to bore a recess and ledge. Then a small ring exactly fitting this bore and lege were machined.


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## Mitch Alsup (Nov 7, 2018)

The outer barrel was turned to 1.998,5 to fit easily in a 2" focuser barrel.

A piece of scotch tape is tapped to the ring, the ring inserted face forward to the ledge, and we have a device that will hold the transparent plane at the correct distance to th optics. The telescope is pointed at a bright object, and th P2 is rotated until the object reached sharp focus (zero power observation.)

This picture shows the focuser assembly covering the front lens element and ready to hold the scotch tape at the correct distance.




More later.


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## Mitch Alsup (Nov 8, 2018)

Here is a picture of the plate mounted to the upper assembly, face first:




and from the side:




And from the inside"


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## StevSmar (Sep 21, 2019)

Mitch Alsup said:


> ... I invented a new mirror cell architecture which one can puruse at
> https://www.cloudynights.com /topic/547689-mitchs-mirror-cell-architecture/


Nice project! I enjoyed seeing how you modelled the mirror support.

I’ve been wondering, when mirrors are ground and lapped they appear to be fully supported. Does the mirror support you’re using introduce some distortion, or are the mirrors lapped using the same support points as your mirror support has?


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## Mitch Alsup (Sep 21, 2019)

There is a freely downloadable program called PLOP (Plate Optimizer). Plop has been designed for parabolic mirrors and models the surface distortions that the <point load> supports will imposed on the mirror.

In my case, PLOP suggests that the support is better than 1/128 (4.5E-6 m) wave of surface distortion--a figure so much better than the expected surface that it is essentially invisible.




I have also seen that several professional mirror makers serving the amateur market support their mirrors on mirror cells on the polishing machines.


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## StevSmar (Sep 21, 2019)

Mitch Alsup said:


> ...I have also seen that several professional mirror makers serving the amateur market support their mirrors on mirror cells on the polishing machines.


Thanks for the reply. That’s good to know that some mirror makers take the intended support method into account when polishing their mirrors.

I’ve enjoyed reading this thread, and the others you linked to.


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## pdentrem (Sep 21, 2019)

I was following your cell build on cloudynights until the take over by a couple well respected guys got into it. Any ways I made my own cell for my 13” Zambuto but I am not happy with the upper structure and it’s support system. Keep this going, I hope to learn a few ideas for my rebuild.
Pierre


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## savarin (Sep 21, 2019)

I'm well jealous of your Zambuto mirror.
My financial controller would have a fit If I wanted to get one of those.


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## pdentrem (Sep 22, 2019)

savarin said:


> I'm well jealous of your Zambuto mirror.
> My financial controller would have a fit If I wanted to get one of those.



Took some dollars to get it and time but well worth it. 
I currently have a two truss system but may return to a 6-8 truss scope.
Pierre


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## Mitch Alsup (Sep 22, 2019)

A 2 truss has vibration written all over it. It is just not stiff.

But note, It took me about 2 years to find the one <pair of> connection that was allowing flex into my 6-pole truss.
I beefed it up and the scope works like binoculars (i.e., without vibration.)

Zambuto makes fine mirrors, but stops at F/4. I am using a Lockwood mirror (who is willing to make <about> anything you are willing to pay for.) Your financial controller would have a hissy fit instead of just a fit..


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## Mitch Alsup (Oct 3, 2019)

I thought you guys would enjoy the means by which I made the vanes for the upper assembly.

{After 4 unsuccessful attempts over 1.5days, I gave up on making the triangles and trying to adapt them to the upper assembly at hand.}

So, after thinking about is over night I came up with a plan whereby I would hold the secondary frame in place, and also hole the upper assembly in place so that I could cut, cut, fold, mark and drill the holes in the vanes with the rest of the structure not moving.

The secondary frame is raised off of the table saw bed by a 1/4" thick 45 degree triangle. The frame is clamped to a 1-2-3 block with a setup bolt and nut. The frame is then held down with setup clamps and the tension adjusted so the frame was perfectly square to the bed of the table saw.

The pickup point beams were then placed at mid adjustment and bolted to the upper assembly so that they were stiff for the duration. One side of the beam was removed for easier access.

Then one by one each vane was constructed, cut out of raw stock (roofing flashing material--lifetime supply for $8.00). After being cut two 0.111 holes (4-40 thread diameter) at the proper separation. after the holes are drilled, the vane is screwed and clamped to the bending jig and bent to 45 degrees. Back at the assembly table the vane is screwed into the secondary frame, a corner is colored with dikem and marked with a scribe from teh other side. this hold is then located and drilled 0.111. brought back and tested. Repeat 3 more times.



Then, finally, after all for vanes are fitted and brought into tension, the entire setup jig is disassembled and the upper assembly stuffed with fire proof blanket then the vanes are silver soldered in situ. This produces vanes that are absolutely square to the optical axis.

Here is an image of the upper assembly assembled onto the scope with the secondary (less optics).




I am very happy with this spider assembly. It is stiff enough one can grab the upper assembly from the secondary frame and hoist the upper assembly around with little chance of even harming  the collimation! It is stiff. And to the extent I can measure it, it is perfectly square.

Other than the somewhat cramped quarters of the assembly jig, this means worked quite well.


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## Mitch Alsup (Oct 3, 2019)

savarin said:


> I'm well jealous of your Zambuto mirror.
> My financial controller would have a fit If I wanted to get one of those.



My optics came from Mike Lockwood:: Set of 3: 13" F/3 for wide fields, a 30" F/3 for deep space, and a 20" F/3 for when I'm too tired for setup the 30".

My financial controller has no observations into spending from my slush fund, nor I to hers. It is better that way--for both of us.


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## Mitch Alsup (Oct 14, 2019)

Introducing Margot::



Margot is my Grand Daughter, and grand indeed; but that is not the reason I reintroduced this thread.....


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## Mitch Alsup (Oct 14, 2019)

Today I dedicate this telescope to Margot, and naming it Margo::
But this thread resurrection is about the assembly  of the telescope.

So we start by installing the primary mirror in the mirror cell  (frame):



You will notice the measurement devices, in order to place the primary in the middle of the mirror cell one has to be able to measure where that middle is, and when the mirror cell will move under any touching, one has to know when the cell has moved and when it has not.

There are 4 ball transfers in 4 machined cylinders, and each BT is being positioned such that the top of mirror to top of frame and bottom of mirror to bottom of frame are equal (similar side to side--except different measurements) with the bottom of the mirror touching BT on the frame. The BTs on the top of the frame are used to create space so the mirror is not pinched in any way.

The BT outward distance is adjusted by a 1/4-20 screw and bolt pushing on the back of the BT with the nut pulling on the block the cylinder was machined into. In any event, after fiddling with it for a while, I was able to get 0.004" clearance at both top BTs with the mirror positioned within 0.002" of where I wanted it.

By the way, the "top" of the frame is the far side in these images.


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## Mitch Alsup (Oct 14, 2019)

Next, we flip the mirror over and attach the mirror "clips". These clipt prevent the mirror from falling forward out the frame when the frame is at any angle (including pointing down.)




The clips were "adjusted" until a sheet of paper could clear from the top and rotate over the edge of the mirror, simply from the stiffness of the paper itself. The paper measures 0.004,5" thick. THe overhang is very tiny compared to mirror cells that allow the mirror to move to a greater degree.

By the way:: 0.004" was chosen as the clearance of the mirror at the BTs because the differential expansion/contraction of the mild steel frame and the sperMax glass is 0.003" from 100º F to 0º F. I, personally, will not be observing at less than 0º F !


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## Mitch Alsup (Oct 14, 2019)

Next up is the attachment of the altitude bearings::




Those of you who have build Dobsonians are going to be scratching your heads thinking "where is the mirror box" ? We will get to that a bit later !

You can see a bit of the inletting I did in order to fit the truss pole end supports. The end support points attach 2 truss pole ends (1" aluminum ball) between two 1" machined races and held together with a thumb screw. THe thumb screw screws onto the stud coming out of the <now> attached altitude bearing.


----------



## Mitch Alsup (Oct 14, 2019)

Next up is the other altitude bearing:


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## Mitch Alsup (Oct 14, 2019)

Next we attach the top ring::




And secure it with 3 bolts.


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## Mitch Alsup (Oct 14, 2019)

Now, finally it is time to take a look at the "mirror box" 




Yes, two pieces of cylindrical wood cut and inletted to fit the appropriate gaps in the mirror frame are pushed into place and held with pure spring tension !


and


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## Mitch Alsup (Oct 14, 2019)

At this point the lower assembly is completely assembled, in storage and in transit, the upper assembly rides on top for a compact unit:




There is a single top that can be used to cover and protect the "in transit" assemblage:




Or it can be used in the field to protect the mirror when the scope is not in use:




knob to be added later


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## Mitch Alsup (Oct 14, 2019)

In order to point a telescope one must have <at least> two bearings that allow the scope to move in altitude and azimuth (or declination and right ascension). 

Classical Dobsonians use another large box shape to carry the altitude bearing.

This design used the innovative "flex rocker":




Focused on the far side we see 2 strips of teflon contact cemented to the light wooden frame.




At the top of the post in the flex rocker is a strip (each) of teflon screwed into a retaining block of 6061. Par of this was that I bobbed the riser too far when I cut it from rectangular to having the shape of the altitude bearing.




There is a finger reaching out from the riser to position the flex rocker onto its "ground board". The finger itself is just 6061 aluminum with a 1/4-20 bolt tightened securely. 

Since I am using 1/4-20 stainless bolts; and the threaded diameter is 0.246, AND I just happen to have a 0.246 drill. I drilled some 6061 stock and found out about all I never wanted to know about drills walking. Eventually, I figured out that if I drilled the hole first, and them turned the outside on centers, I could get the rollers to run true. At this point is was simply an issue of figuring out the correct diameter for the rollers (and we will come back to this point)


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## Mitch Alsup (Oct 14, 2019)

The flex rocker needs a "Ground Board" in order for the rollers to define a circle and operate as if it were a set of bearings:




The ground board was created with a router then attached to my rotary table and milled to 0.003" tolerance. THen about a billion coats of tung oil was applied and allowed to cure for months.

As one can see, the ground board is not anywhere close to the ground--this is to provide a comfortable viewing sitting height when the scope points at the horizon, and a comfortable viewing standing height when the scope is pointing at zenith.




I made a mechanism to hold the ball ends of the poles in the sockets I machined spo the poles can be removed for transport and storage.




There are 3 feet which capture the 6 pole ends and are clamped with another machined part.

The legs are angled such that the planes they define coincide at the CoG of the fully assembled telescope. 

Currently I am using high tensile twine to hold the legs at proper angles--these will be changed to "bow string" after the telescope trip in 2 weeks.


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## Mitch Alsup (Oct 14, 2019)

The rocker box (sic) is placed on top of the "ground board:




Then the lower assembly is placed in the rocker box:


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## Mitch Alsup (Oct 14, 2019)

The truss poles are installed onto the studs and will hold their position while being tightened:





This is one of the reasons I chose the lower ball race to be under the wooden positioning ring.


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## Mitch Alsup (Oct 14, 2019)

The upper assembly is attached:




And the telescope can now point in any "upward" angle:


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## Mitch Alsup (Oct 14, 2019)

The secondary lives in a box reminiscent of a box for a large eyepiece, 




The secondary is inserted into a <semi> kinematic mounting on the nose of the secondary <frame> holder, and can be adjusted in tip and tilt from there. 




The image shows my laser collimation reticle device in the focuser after squaring the focuser base plate to the upper assembly.




And an image of the not quite collimated reticle on the primary.


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## Mitch Alsup (Oct 14, 2019)

There are ground spikes to hold the feet of the ground board in position (against the flow of stumbling feet over the course of a night.)




The recessed portion of the head is to allow a high tensile rope to be attached making removal easier.


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## Mitch Alsup (Oct 14, 2019)

And now a few looks at various other parts of the scope (not covered way up above):




Spider vane outer attachment. I wanted a way to adjust tension without imposing any twist on the vanes. I milled some Al and milled of the end of some threaded rod (1/4-20 brass) to just fit in the slot with essentially zero clearance. All three attachments are done with 4-40 stainless cap head screws.




The secondary frame. One can see how the inner portion of the vanes are attached and where the secondary is attached to its kinematic mount.




The Paracorr 2 (coma corrector is MANDATORY at F/3) is inserted from the inside of the upper assembly.


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## savarin (Oct 14, 2019)

Hi Mitch, what adhesive did you use to hold the secondary to its bracket?
Thanks


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## Mitch Alsup (Oct 14, 2019)

The pictures show double sided foam tape.

The real mounting will be done with silicone at 1mm thickness (after washing both sides with lacquer thinner, and dabbing all 6 points (3 each) with silicone and mounting both in a jig for squareness.)


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## savarin (Oct 14, 2019)

Thanks Mitch, ai thought it was double sided tape and wondered about its strength.
Thanks for the idea of a jig to get it all aligned correctly. 
Thats a bit I'm finding difficult.


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## brave_ulysses (Oct 15, 2019)

excellent craftsmanship. looking forward to the first light report


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## kb58 (Oct 15, 2019)

You reminded me to check my records, as I, too, am waiting for a Zambuto mirror, which is due this December if he sticks to his delivery schedule. As for where I'm going to store the scope once it's complete, that's a different story!

Anyway, how are you planning to attach the secondary to its support? I real of all kinds of methods, and most work, and a few don't! I haven't decided on the spider design. At first I was heading down the wire rabbit hole but may end up doing very much the same as you for rigidity and simplicity.

Regarding the ball ends on your trusses, did you fabricate them or buy knobs from McMaster? Of the latter, are they aluminum?

When complete, mine will be a 16" f/4.3 6-truss design. The details of its aesthetics is still up in the air, but will probably end up more functional than artistic. I don't plan on transporting it much and want it stout and not losing collimation too easily.


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## Mitch Alsup (Oct 15, 2019)

kb58 said:


> Anyway, how are you planning to attach the secondary to its support? I real of all kinds of methods, and most work, and a few don't! I haven't decided on the spider design. At first I was heading down the wire rabbit hole but may end up doing very much the same as you for rigidity and simplicity.



The secondary is attached to the secondary frame by means of the 1/4-20 threaded rod in the center, there are 3 points from the frame that hit the center drilled holes in the secondary holder. These 3 (4-40 screws with pointed ends) are used to point the secondary when collimating the scope.



> Regarding the ball ends on your trusses, did you fabricate them or buy knobs from McMaster? Of the latter, are they aluminum?



I got them with 1/4-20 threads and polished them with 1000 then 1500 grit paper on my lathe. They have a setscrew drilled in so the ball won't unthread itself from the 1/4-20 4140 threaded studs. The threaded studs are held in the poles with a 1/4-20 star nut and the pole ends are also threaded 1/4-20. So collimation consists of loosening the pole end and the opposite pole clamp, then turning both the pole and pole end, then tightening the pole end. Almost all collimation is translation of the upper assembly, and this is performed by adjusting the lengths of the poles that are parallel with each other.


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## kb58 (Oct 15, 2019)

Mitch Alsup said:


> The secondary is attached to the secondary frame by means of the 1/4-20 threaded rod in the center, there are 3 points from the frame that hit the center drilled holes in the secondary holder. These 3 (4-40 screws with pointed ends) are used to point the secondary when collimating the scope.


Thanks for the reply! I wasn't clear in my first question: what adhesive did you use to attach the glass secondary to its holder?


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## Mitch Alsup (Oct 15, 2019)

kb58 said:


> Thanks for the reply! I wasn't clear in my first question: what adhesive did you use to attach the glass secondary to its holder?



post #56


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## kb58 (Oct 15, 2019)

I used silicone on a 10" mirror and it was a real ***** to remove. That's why I was asking, and surprised that some people said their mirrors were dropping off.


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## Mitch Alsup (Oct 15, 2019)

Removal requires a razor blade (and it better be single edged!) and some acetone.

I have used the 3.8" on my 20" F/4 DOB for 19 years without any hint of failure. I'm just glad the coating is holding up as well as it is.

The mirror is currently sitting on 3 sheets of tissue paper, a wooden log held in the vicce on my mill with the aforeseen part held square at 0.038" above the secondary with silicone between the two parts curing.


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## Mitch Alsup (Oct 16, 2019)

I got out and had first <day>light with the scope this afternoon.
I can reach focus on objects that are under 200-odd feet away but not farther.
This tells me the truss poles are too long. So I bobbed them by 1/2".

But what I thought you guys might like to see is:: "How does one get 6 poles to the same length, within a couple of thou, when one does not have a measuring device that long?

I came up with this::




Edge to edge of my tables aw is within spitting distance of 27", and the newly bobbed poles are trying to be 26.5" (first try).

So I got out my mill setup stuff and a couple of 1-2-3 blocks and clamped them to the table saw bed.
Then I took a random piece of 1/2"×1" mild steel tubing and clamped it to a 1-2-3 block.
Now I can set the pole lengths the same way one feels a telescoping gauge in the micrometer.
The poles are in a circular chain, so one simply takes the one on top as the next one, does the feel test,
and when all 6 touch both the 1-2-3 block and the steel tube, with that micrometer feel, then they are all the same length.


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## Mitch Alsup (Oct 24, 2019)

Here is the packaged scope fitting the the back of my Merc S-600. And if anyone knows about the size of  the trunk in this car, you will be amazed at how SMALL it is, especially with the refrigerator in the back.


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## Mitch Alsup (Oct 24, 2019)

Someone ask about the scale of this "thing", so here is a picture with a 24" ruler/level::




It is actually 21" tall in storage, 16.5" if you don't count the altitude bearings. And, yes, that is the whole scope (i.e., all the necessary pieces.)


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## Mitch Alsup (Oct 24, 2019)

kb58 said:


> Anyway, how are you planning to attach the secondary to its support? I real of all kinds of methods, and most work, and a few don't! I haven't decided on the spider design. At first I was heading down the wire rabbit hole but may end up doing very much the same as you for rigidity and simplicity.



See #37 in this thread for the vanes and secondary frame.

In my 20" F/4 I spent 2 years fighting 3 different wire spider designs never getting rid of vibrations or collimation issues. Then I gave up and built a 4-vane spider using 0.007,5 galvanized steel (roof flashing; $10 at Home Despot for a lifetime supply). This thing worked so well I never needed anything better. The new 13" F/3 has a similar 4-vane spider (the design shown in the link below (which is just page 1 of this thread). It is strong enough you can grad the whole upper assembly by the secondary frame and pick up the upper assembly forcefully without harming collimation.

Here is a picture of the secondary attachment double sticky taped to the secondary mirror:




This bracket has:
a) slots cut into it to relieve some of the thermal differences in expansion between AL and Glass
b) kinematic mounting points on the secondary frame to preserve most of the collimation from mounting and dismounting
c) articulates all forces through the CoG of the secondary (see:: https://www.hobby-machinist.com/threads/13-f-3-telescope-build.65805)
d) the secondary lives in a box (like an EP box) while the scope is not being used (for protection of both the secondary and primary)

I used a similar model for my 3.8" secondary of my 20" F/4 that I have used for 20-odd years.

Here is a picture of the secondary sitting on 5 sheets of tissue paper, sitting on a board in the milling vise as the silicone cures. I am using the quill and spindle to hold the secondary bracket square to the secondary as the silicone cures, and using the quill height to set the proper thickness of the silicone blob. Whilecuring I wrapped this whole thing with 5 layers of more tissue paper to prevent crap.....






> Regarding the ball ends on your trusses, did you fabricate them or buy knobs from McMaster? Of the latter, are they aluminum?



If I recall correctly, I got them from "cheap balls" 1" 6061 balls with 1/4-20 threaded holes. 
All I did was spin them up on the lathe with 1500 grit paper to polish them.



> When complete, mine will be a 16" f/4.3 6-truss design. The details of its aesthetics is still up in the air, but will probably end up more functional than artistic. I don't plan on transporting it much and want it stout and not losing collimation too easily.



I am hoping the mechanism I built will hold collimation through disassembly and reassembly!

I collimate the scope by changing the length of the poles. The balls were drilled and tapped for 4-40 set screw that holds the 1/4-20 threaded rod (4140) in the ball. The pole end is threaded 1/4-20, and the tube has a 1/4-20 star nut inside. 

A 6-pole truss with adjustable pole lengths is known as a "Stewart Platform" and has several important properties:
a) 6 degrees of freedom supported by exactly 6 points (the balls in the ball races.)
b) adjusting pole lengths allows the upper assembly to translate and tip/tilt/droop in 3 dimensions.
b.1) I collimate with a laser by translating the upper assembly so the laser comes back to the secondary where it originally left the secondary.
b.2) this is why the poles all start off the same length (no tip/tilt/droop)
c) once collimated, the pole end is tightened and the ball race clamp is tightened and the scope remains collimated
c.1) while at the same time the truss is converted from pivoting ends to fixed ends (for evenmore stiffness.)
c.2) once tightened all one has to do is put the poles back where they were and it will be "close" probably very close (we shall see.)


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## Mitch Alsup (Oct 24, 2019)

A dupe of the previous.


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## Mitch Alsup (Nov 2, 2019)

I thought some might want to see the arrangement I used to square the upper assembly to the lower assembly::

The first thing to do is to square the lower assembly to gravity. So I took the scope off its base and place it on plywood on top of the table saw. Then I shimmed this until it was square to gravity in 2 orthogonal directions.

Next the poles and the upper assembly are assembled. On the 4 spider vane attachments, 4-plumb bobs (darts) are positioned to almost touch the lower assembly.

Then the upper assembly is translated using pairs of poles until the plumb bobs indicate the upper assembly is square with the lower assembly. At which point I remembered to take a picture::




Then individual poles were lengthened until the upper assembly was square to gravity using a level in a 24" rule.




At this point the upper assembly is square to the base. I expect to have to lengthen the poles (end to end) but I am rebuilding the "ground board" for more comfortable use of the scope (the poles need to be longer--which I did today.)


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## Mitch Alsup (Nov 5, 2019)

Well, I finally got the secondary properly positioned.


Here one can see (on the primary with tissue paper) that the secondary is perfectly centered over the focuser and perfectly pointing at the primary.
{You can tell this by the 2 dots beyond the line at all 4 corners) The second set of dots in the upper left is from a second sheet of tissue paper.

One can also see the primary is being hit in its center.


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## Mitch Alsup (Nov 5, 2019)

At this point the focuser is pointing square at the secondary and the secondary is pointing at the center of the primary. But due to very small build errors, the primary is not pointing at the focuser.


So we make a couple of translations of the upper assembly (by lengthening pairs of poles equally. And Presto:


It is close enough do do fine collimation on stars themselves.


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## Mitch Alsup (Nov 5, 2019)

I decided to change the "ground board" support to make the scope taller. I decided (this go around) to make the scope comfortable viewing at Zenith::


Thus, the "ground board" has become a "waist board".


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## Neptune (Dec 11, 2020)

Love this scope.  Ascetically pleasing to look at as well as to look through!


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