13" F/3 telescope build

[Mitch Alsup]

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.

 

[Mitch Alsup]

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.

 

[Mitch Alsup]

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.

 

[savarin]

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.

 

[jrkorman]

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!

 

[Bob Korves]

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.

 

[john.oliver35]

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!

 

[petertha]

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?

 

[Mitch Alsup]

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.

 

[petertha]

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?