80mm long focal length refractor

It came out parallel ok which now let me mark it out for the next three sides so back onto the tool post to remove the waste.
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I must have listened to you guys too much and thought I could do no wrong. I got a tad impatient and attempted to speed it up a bit.
ARGHHHHHHHHHHHHHHHHHHHHHHHH disaster
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It started chattering and pulled the tool out of the chuck and I took a huge divot out before I could stop. I will have to place one on the other side and call it a feature.
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I think that part of the problem was not just the extra speed but the tool bit wasnt quite right so I ground the back of a drill bit so I could turn the cutting edge to the correct orientation (as I saw it) to hopefully not do the same muck up again. This cut very smoothly.
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Now all 5 sides are cut I can drill the holes for the bearings and bolts.
Clamped at 45', first holes at 3.3 mm then 10mm clearance for the 9mm dia bearings.

Then back onto the lathe to bore the 53mm dia hole for the focusser tube.
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Once that was bored I could cut the apex of the triangle off
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The design is not my idea its from this clever gentleman http://jimshomeplanet.com/CrayFocus/CrayFocus.html so credit where it is due.
Thats it for a couple of days as I have to lay a heap of pavers in the back yard. (a large honey do)
 
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Charles

I was looking at your bino-dobsonian project. I would suggest adding two controls for collimating the mirrors. one mirror moves around virtical axis and second moves on horizontal axis. The movement needed is very miute but without this adjustment your eyes will see different views which becomes confusing to the brain. I have viewed through similar bino and I have seen the problem.

-Prasad

savarin said:
Hmmm Tony, then I guess you want to see how this one is going eh?
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The truss tubes are 12mm square steel tube. They are made but I still have to weld the diagonals in.
I have the two lazy susan bearings for the top cages.
I have the glass for the two mirrors plus the grinding and polishing machine ready.
They will be two f6 12" mirrors where the back face will be ground to the same radius of curvature as the fronts.
They will be polished to a sphere then flexed to the parabolic shape required. I have made one 8" f3.7 spherical mirror flexed to a perfect mirror to check the process.
When I finish the refractor I will be starting on these again and will start a project blog.
Click to expand...
 
Its already in the plan but not on that drawing. However, when I was discussing this with the Bolton group who had a similar project they said they never had a problem with that and that the mirrors remained collimated. We will see. My thoughts are to ensure they can be co-collimated, better safe than sorry.
Also as they will be flexed then they wont slide around in the mirror cells so that may help.
 
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Wow! time flies.
At last the focusser unit is finished.
all-parts.jpg
Moving downwards:- main block, 4 bearings-screws-spacers, then plastic washers from milk bottle, brass plungers that hold the shaft, then bolts with pressure springs to hold the shaft against the focus tube, brass pusher with spring and adjustment knob for tension and locking, large knurled knob with smaller knob bolted to it, smaller knurled knob for other side.

I wound the springs on this mandrel but a lot shorter than the test in the photo.
The first two coils were wound with no travel, then the threading was engaged at 3mm pitch, then stopped to wind the last two coils with no travel.
spring-winding.jpg

The four bearings are held in the block and just protrude into the bore for the focus tube to ride upon. Yes, thats two spacers on that bolt, my drill wobbled a tad out of alignment and it needs a tad more clearance.
I still have to make the dust shield to cover the top 3 faces of the block. I cant decide upon ally or brass, guess it will be whatever I find first.

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The shaft then bears against the other side of the tube, friction alone moves the tube in and out as the shaft is turned.

The small knobs are for coarse feed with the large knob for fine feed/focus.
The brass knob is an extra pressure point onto the centre of the shaft as a lock.

assembled.jpg

I am most pleased as it moves very smoothly with no grabbing or jerking. Just as a focusser should.
Its bolted to the brass ring that screws onto the last tube of the telescope.
Tomorrow is testing time to check how short the focus tube really needs to be (or longer if my calcs are way out)
 
Charles, fantastic work as always.

"Billy G"
 
Thanks Matthew.
I just realised I never showed the bearing holes
bearing-side.jpg
Pretty ugly and 4 large holes for dirt etc to enter so I covered them with a thin plate
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That funny twisted loop is actually a length of cotton reflected in the polished cover.
 
The scope is finished and assembled. It took long enough.
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The end cap has a small brass loop for some stainless chain that will be bolted to the bottom of the flange so as not to loose the cap.
Its held in the cell via an "O" ring.
I wont collimate the lens until its in the cradle on the mount as it will be easier then.
Now started on polishing all the cradle parts.
I cant look through it yet as its too cumbersome to hold still and I dont want to scratch the tubes by resting it on the fence.
 
Very nice Savarin. You are a craftsman at his best.

"Billy G"
 
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