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  #181  
Old 09-12-2018, 01:40 PM
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Originally Posted by Stefan Buda View Post
The front flange also has a silicone rubber O-ring that pushes the mirror gently into contact with the bare aluminium flange on its back side. I may add a very thin shock absorbing layer, but definitely not something as rough and agricultural as cork.
Just like in the case of the machined grooves in your corrector lenses arrangement how do you work out the gaps or compression zones dimensions needed in your mirror mounting when you have glass in direct contact with aluminium? When temperature changes, too much and it gets lose, too tight and the glass will be pinched.
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  #182  
Old 09-12-2018, 08:00 PM
Stefan Buda
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I made the diameter of the hub 0.05mm smaller than the hole in the mirror, at 20 degrees C. That should be good enough up to at least 80 degrees ambient. You can double check the math and let me know if I made a mistake - the diameter is 73mm and the alloy is 6061. I also have an O-ring on the hub to stop the mirror coming loose when the temperature drops. The amount of compression on the O-rings is based on nothing more than educated guesswork and it can be adjusted later, if needed. I made the axial gap also 0.05, a bit more than it is needed because the front flange is conical rather than spherical, so it does not conform very well to the front of the mirror.
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  #183  
Old 10-12-2018, 12:47 PM
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Originally Posted by Stefan Buda View Post
I made the diameter of the hub 0.05mm smaller than the hole in the mirror, at 20 degrees C. That should be good enough up to at least 80 degrees ambient. You can double check the math and let me know if I made a mistake - the diameter is 73mm and the alloy is 6061. I also have an O-ring on the hub to stop the mirror coming loose when the temperature drops. The amount of compression on the O-rings is based on nothing more than educated guesswork and it can be adjusted later, if needed. I made the axial gap also 0.05, a bit more than it is needed because the front flange is conical rather than spherical, so it does not conform very well to the front of the mirror.
Thanks for the explanation Stefan. Makes sense. I now realise precision machining and knowing exactly how much play you have goes a long way with potential thermal issues down the road. Packing with cork is a bandaid.
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  #184  
Old 11-12-2018, 08:05 PM
keller60 (Bill)
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I have been reading this in the background with great interest. The attention to detail has been remarkable and am looking very much forward to seeing the first light results
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  #185  
Old 12-12-2018, 07:24 AM
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Thanks for the explanation Stefan. Makes sense. I now realise precision machining and knowing exactly how much play you have goes a long way with potential thermal issues down the road. Packing with cork is a bandaid.
That's correct but don't get me wrong about cork. I think it has its place in ATM but for this particular job I don't think that it would be reliable enough.
If you remember, at the beginning of the thread, I said that there will be no collimation adjustment for the primary mirror, therefore its position must be very well defined/constrained.

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Originally Posted by keller60 View Post
I have been reading this in the background with great interest. The attention to detail has been remarkable and am looking very much forward to seeing the first light results
Thanks. The plan was to have some sort of first light by the end of the year and it looks like it may happen.
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  #186  
Old 12-12-2018, 07:29 AM
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Originally Posted by Stefan Buda View Post
That's correct but don't get me wrong about cork. I think it has its place in ATM but for this particular job I don't think that it would be reliable enough.
If you remember, at the beginning of the thread, I said that there will be no collimation adjustment for the primary mirror, therefore its position must be very well defined/constrained.
Understood.
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  #187  
Old 19-12-2018, 07:03 PM
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The backplate has been anodised very nicely at Elecrtromold and I was able to drill the electrical contacts, post anodising. I will use the backplate as common electrical ground for the fans and the heaters in order to simplify the wiring behind the primary mirror.
The heaters will be 3D printed with conductive filament. I have experimented with two different brands and the one from Jaycar turned out to be suitable, but it is so brittle that I can only print with it on a very hot day.
Apart from the heating elements and the dew shield; all mechanical parts are done and I will start assembly over the coming days.
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  #188  
Old 20-12-2018, 07:45 AM
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Wow - Looking forward to seeing pics of it all coming together.

Although I haven't commented up until now I've really enjoyed following this build thread (and of course your Riccardi Honders build thread as well).
Great to see someone building more complicated optical configurations with such craftsmanship.
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  #189  
Old 20-12-2018, 08:40 AM
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Looks very good. Is that bore (threaded?) conical in the middle?
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  #190  
Old 20-12-2018, 11:44 AM
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Very cool!
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  #191  
Old 21-12-2018, 01:09 PM
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Quote:
Originally Posted by Retrograde View Post
Wow - Looking forward to seeing pics of it all coming together.

Although I haven't commented up until now I've really enjoyed following this build thread (and of course your Riccardi Honders build thread as well).
Great to see someone building more complicated optical configurations with such craftsmanship.
Thanks Pete.
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Looks very good. Is that bore (threaded?) conical in the middle?
The bore is threaded on the other side. The conical bit is just a stray light trap.
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  #192  
Old 22-12-2018, 09:01 PM
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The tricky operation of bonding the back plate retainer pads is done.
I had to throw into battle my Taylor Hobson micro alignment telescope that I bought years ago at a Thales auction and never used until now.

It required the making of a precision adaptor that is bolted to the back plate.
I removed the back plate and put it back a couple of times, after the epoxy had set, and the repeatability is very good. The line of sight always hits the centre of the secondary hub within 0.001" .
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  #193  
Old 22-12-2018, 09:39 PM
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Originally Posted by Stefan Buda View Post
The tricky operation of bonding the back plate retainer pads is gone.
I had to throw into battle my Taylor Hobson micro alignment telescope that I bought years ago at a Thales auction and never used until now.

It required the making of a precision adaptor that is bolted to the back plate.
I removed the back plate and put it back a couple of times, after the epoxy had set, and the repeatability is very good. The line of sight always hits the centre of the secondary hub within 0.001" .
I was watching the video about the various applications of the micro alignment telescope and read the brochure on the Taylor Hobson website. What did you exactly use it for and what was the procedure? Squaring the back plate? Centering the secondary?
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  #194  
Old 22-12-2018, 09:47 PM
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I was watching the video about the various applications of the micro alignment telescope and read the brochure on the Taylor Hobson website. What did you exactly use it for and what was the procedure? Squaring the back plate? Centering the secondary?
I used it to make sure that the axis of the back plate was pointing exactly to the centre of the secondary hub while the epoxy bonded the retainers into their exact locations.
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  #195  
Old 23-12-2018, 07:17 AM
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I used it to make sure that the axis of the back plate was pointing exactly to the centre of the secondary hub while the epoxy bonded the retainers into their exact locations.
Was that with the center spotted secondary in place or did you have a very small centered marker at the front to achieve that level of precision?
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  #196  
Old 23-12-2018, 08:42 AM
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The shaft of the stepper motor has a tiny dimple in the centre and I aimed at that.
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  #197  
Old 23-12-2018, 11:57 AM
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I did another test today, with excellent result. I wanted to see just how rigid the backplate was with the four attachment points, so I mounted the tube on my EQ6 and did a horizon to horizon sweep. This is a severely overdone test, considering the weight and length of the alignment telescope, which is about 5Kg. It is like attaching a 10Kg camera at the right spacing. Well, the line of sight did not deviate more than 0.002", and that is better than what I was hoping to achieve.

Next I will test the rigidity of the spider assembly, and for that I will have to make an accurate crosshair target.
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  #198  
Old 23-12-2018, 12:28 PM
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The shaft of the stepper motor has a tiny dimple in the centre and I aimed at that.
The stepper motor at the front that is part of the mechanism to focus the secondary?
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  #199  
Old 23-12-2018, 12:40 PM
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Yes.
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  #200  
Old 23-12-2018, 08:44 PM
sharpiel
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Impressive workmanship down to minutiae. Love your OCD attention to detail.
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