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  #41  
Old 02-03-2024, 01:02 AM
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Joshua Bunn (Joshua)
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So its a kind of astatic mirror support...?
Thanks for the explanation on the supports. Are the 9 points of floatation, fixed in the same horizontal plane, so each 3 do not pivot relative to the other 2?
Looks like a hefty mirror, whats its mass?
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  #42  
Old 02-03-2024, 09:04 AM
Stefan Buda
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Astatic is correct.
The supports are the classic pivoting triangles variety but the devil is in the details, as they say.
The cold plate has several layers. The main layer is a 3mm thick laser cut disc that is provided with 9 holes for the support pins to protrude. The holes are 10mm diameter, but the pins are only 6 and it is the second layer of the cold plate that stops the support triangles from rotating out of position. The second layer of the cold plate is a 1mm thick cork sheet that has 9 perforations of about 6.5mm fot the support pins.

The mirror is 38mm thick at the edge and started life as a classic full thickness (1 to 5 ratio) Pyrex blank that I sliced into two discs...but don't start me on that story.
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  #43  
Old 03-03-2024, 10:25 PM
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I opted for a hammered silver finish to reduce radiative cooling.
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  #44  
Old 05-03-2024, 05:06 PM
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I 3D printed a couple of custom plugs (grommets) for the wires that go to the Peltier devices and the temperature sensor. I used rubbery TPU filament.
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  #45  
Old 05-03-2024, 05:09 PM
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Thanks for clarifying the backplate configuration, Stefan.
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  #46  
Old 08-03-2024, 10:57 PM
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I think that the best use of 3D printing in ATM is for making baffles, using carbon filled filaments.

When I designed the 16" DK I decided not to use a secondary baffle, in order to keep the secondary obstruction as small as possible. As a result it was impossible to have a full cutoff of the sky background, which would be bad for an astrograph but not too bad for a planetary telescope.
Now, as an improvement, I 3D printed a couple of additions to the primary baffle for further reducing stray light reaching the camera. As a result, the fully illuminated field will be reduced to about 10mm diameter - more than enough for lunar and planetary imaging.
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  #47  
Old 13-03-2024, 09:02 PM
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With the heatwave gone, I was finally able to test the primary mirror with my Bath interferometer.
When I made this mirror, I relied on zonal measurements with a Coude mask and Ross Null testing, during figuring. I did not have an interferometer back then, so I was curious to see how close I got to the required conic constant and if there were any significant zonal errors.
I wasted half a day trying to get sufficiently stable interference fringes until I realized that the laptop was blowing a stream of warm air into the light path. After that I managed to capture a bunch of usable interferograms. The optical design calls for the primary to have a conic of -0.7298 and my test today came in at -0.731, which is a lot closer than what I expected.
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  #48  
Old 14-03-2024, 08:55 AM
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Congratulations Stefan.

Thatís an impressive result. How useful did you find the Ross null test?

Rod.
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  #49  
Old 14-03-2024, 02:00 PM
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Thanks Rod!

The Ross Null test is just about as useful as the Ronchi test would be if the mirror was spherical. In other words, it becomes difficult to interpret when you approach the required precision of better than 1/4 wave.
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  #50  
Old 19-03-2024, 09:31 PM
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At this point you can move to a knife edge , which will allow you to refocus the apparent shape to pick the best work line to work towards a plane wavefont in the null , which is not really a strategy available with a ronchi grating , which just displays the progression of relative slopes from centre to edge .



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Originally Posted by Stefan Buda View Post
Thanks Rod!

The Ross Null test is just about as useful as the Ronchi test would be if the mirror was spherical. In other words, it becomes difficult to interpret when you approach the required precision of better than 1/4 wave.
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  #51  
Old 21-03-2024, 08:17 AM
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Quote:
Originally Posted by Satchmo View Post
At this point you can move to a knife edge , which will allow you to refocus the apparent shape to pick the best work line to work towards a plane wavefont in the null , which is not really a strategy available with a ronchi grating , which just displays the progression of relative slopes from centre to edge .
Yes, but the knife edge test is much harder to combine with a Ross null lens.
The Ross null is very sensitive to misalignment and the light source offset needs to be kept very small, especially for fast mirrors.
In fact, it is mainly the residual misalignment, that shows up as astigmatism, is what mainly limits the usefulness of the test in the final stages of figuring.
However, the Ross null is very useful for evaluating the smoothness of the surface.
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  #52  
Old 22-03-2024, 08:04 PM
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I had to make small modifications to the gearboxes because I'm getting rid of the old unipolar stepper motors and replacing them with bipolar NEMA 17s.

The SMA panel jack is for the index pulse. It is connected to a microswitch, which is activated by a notch on the larger timing belt gear.
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  #53  
Old 07-04-2024, 09:14 PM
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I made a new power distribution box. The three switches are for the active cooling, the fans and the finder's heater (yet to be made).
The status LED for the active cooling is a colour changing one and it was a pain to get its current limiting resistor right. The cooler should never be on during observing so the changing colours should be a remainder to turn off the Peltiers.
Today I received some very fine Nichrome wire from Temu for making the finder's heating ring.
Also have ordered a new dual thermometer with humidity sensor.
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  #54  
Old 09-04-2024, 07:40 PM
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I added a 10kg counterweight to the equatorial wedge to bring the COG closer to the pier. The aluminium cylinder has room for another 20kg of lead.
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  #55  
Old 10-04-2024, 09:08 AM
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Very old school "Stellafane" like bit of kit, robust and capable looking and even the finder looks to be capable....of holding bird netting..?

I loved dreaming about having many of the masterpieces in the photos from that star party in the 80's

Mike

Last edited by strongmanmike; 10-04-2024 at 09:32 AM.
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  #56  
Old 10-04-2024, 09:42 AM
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Quote:
Originally Posted by strongmanmike View Post
Very old school "Stellafane" like bit of kit, robust and capable looking and even the finder looks to be capable....of holding bird netting..?

I loved dreaming about having many of the masterpieces in the photos from that star party in the 80's

Mike
Don't let looks fool you, this OTA contains hardly any metal, but if you refer to the mount, then yes, this rebuild looks like a step backward - going from AltAz to EQ. Also, this conversion has turned it into a bit of a Frankenscope even though it was never designed to be good looking. However, this is a special purpose scope and I think the EQ mount is a better option.
Regarding the bird netting, I did not notice the illusion until you pointed it out.
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  #57  
Old 10-04-2024, 10:05 AM
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Quote:
Originally Posted by Stefan Buda View Post
Don't let looks fool you, this OTA contains hardly any metal, but if you refer to the mount, then yes, this rebuild looks like a step backward - going from AltAz to EQ. Also, this conversion has turned it into a bit of a Frankenscope even though it was never designed to be good looking. However, this is a special purpose scope and I think the EQ mount is a better option.
Regarding the bird netting, I did not notice the illusion until you pointed it out.
This epic bit of work inspired my friend and I to construct a massive over engineered Hale 200"-esque Serrurier truss fork mounted 18" F5 Astrograph back in the 80's, even had the massive pillow block bearings....that we never finished, got about 3/4 done, before discovering wine, women and song... sad ....it really did resemble the Hale 200" except on a fork mount...so more like the 120" Lick perhaps?

Mike
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  #58  
Old 10-04-2024, 01:42 PM
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Thanks Mike for shearing your early ATM (mis)adventure.
That Stellafane scope looks quite a bit out of proportions though - a 12"-er should be a lot more compact, whatever the type of mount.
However, I remember 1982 very well, as it is the year of my escape across the Iron Curtain.
I built my first scope about two years later - just in time for Halley's comet.
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  #59  
Old 11-04-2024, 09:37 AM
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Hi Stefan

Iím enjoying reading about your progress. I like the shroud/tube the at sits inside the truss - looks very neat. Is it made of carbon fibre too? How important is it to baffling? Or is it mainly to keep the dust away?

Thanks

Rod.
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  #60  
Old 11-04-2024, 10:36 AM
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Quote:
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Hi Stefan

Iím enjoying reading about your progress. I like the shroud/tube the at sits inside the truss - looks very neat. Is it made of carbon fibre too? How important is it to baffling? Or is it mainly to keep the dust away?

Thanks

Rod.
Thanks Rod!

The shroud/tube is a single layer of carbon fibre laminated onto a 1m length of Formatube wrapped in aluminium foil as resin barrier. The Formatube was removed destructively. It weighs exactly 1kg and its role is to help with laminar air flow in the light path as the venting fans draw air throug it.
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