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Does this mirror cell cause Astigmatism?

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#1 calypsob

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Posted 20 August 2018 - 11:57 PM

I have a 6" F4 GSO newtonian that I plan to pair with a small pixel camera 2.3um pixels its 20mp total and samples at about .83 arc seconds per pixel.

I know right off the bat that its going to show every defect in the world and that any mis collimation is going to be a pain to deal with. 

 

After reading some complaints about how these scopes keep collimation, I have thought about redesigning the primary mirror cell so that it is a bit more robust. I have been reading an old build tutorial by Gary Seronik  https://garyseronik....your-reflector/  And it seems interesting because he has done away with springs and mirror clips.  The center of the primary rests on a threaded acorn looking nut.  I am curious though to hear feedback from the experienced mirror making folks.  Will this central nut cause astigmatism in a 6" mirror?  How about if I beefed up the central nut and used a roller bearing with a floor flange mount? https://www.amazon.com/TruePower-10-9111-Roller-Transfer-Bearings/dp/B01BV9ZUSU and I was thinking about replacing the washer with a  cupped magnet

http://www.leevalley...=1,42363,42348 

 

Before I buy anything Im curious to hear what others have to say about Gary's cell especially if you have feedback on the astigmatism aspect.  Has anyone built their own?  Thanks! 


Edited by calypsob, 21 August 2018 - 12:00 AM.


#2 J A VOLK

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Posted 21 August 2018 - 12:33 AM

the acorn nut is on the mirror mounting plate, not the mirror. This is basically similar to the way secondaries are mounted on commerical SCTs. It should work just fine. The stiff mounting plate simply pivots about the acorn nut. Usually the cause of astigmatism from convention mirror mounts is from allowing the clips to contact the mirror surface.

Edited by J A VOLK, 21 August 2018 - 12:36 AM.


#3 Darren Drake

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Posted 21 August 2018 - 06:57 AM

A 6 inch mirror is not big and or heavy enough to distort under it's own weight so support is not really an issue.  It's more collimation than anything at this level...


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#4 calypsob

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Posted 22 August 2018 - 09:04 AM

the acorn nut is on the mirror mounting plate, not the mirror. This is basically similar to the way secondaries are mounted on commerical SCTs. It should work just fine. The stiff mounting plate simply pivots about the acorn nut. Usually the cause of astigmatism from convention mirror mounts is from allowing the clips to contact the mirror surface.

Thanks, I realize that the nut is not affixed to the cell itself.  I am still questioning however if it will create astigmatism.  It is creating a central pressure point on the plate that the primary rests on. 



#5 calypsob

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Posted 22 August 2018 - 09:06 AM

A 6 inch mirror is not big and or heavy enough to distort under it's own weight so support is not really an issue.  It's more collimation than anything at this level...

I believe that is a generalization. When you are sampling below 1 arc second, any imperfection is going to be magnified.  

I suppose this will boil down to trial and error. Luckily its not very expensive to build a mirror cell so I went ahead and ordered some components. 



#6 X3782

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Posted 22 August 2018 - 10:05 AM

I have a 6" F4 GSO newtonian that I plan to pair with a small pixel camera 2.3um pixels its 20mp total and samples at about .83 arc seconds per pixel.

I know right off the bat that its going to show every defect in the world and that any mis collimation is going to be a pain to deal with. 

 

After reading some complaints about how these scopes keep collimation, I have thought about redesigning the primary mirror cell so that it is a bit more robust. I have been reading an old build tutorial by Gary Seronik  https://garyseronik....your-reflector/  And it seems interesting because he has done away with springs and mirror clips.  The center of the primary rests on a threaded acorn looking nut.  I am curious though to hear feedback from the experienced mirror making folks.  Will this central nut cause astigmatism in a 6" mirror?  How about if I beefed up the central nut and used a roller bearing with a floor flange mount? https://www.amazon.com/TruePower-10-9111-Roller-Transfer-Bearings/dp/B01BV9ZUSU and I was thinking about replacing the washer with a  cupped magnet

http://www.leevalley...=1,42363,42348 

 

Before I buy anything Im curious to hear what others have to say about Gary's cell especially if you have feedback on the astigmatism aspect.  Has anyone built their own?  Thanks! 

 

Astigmatism means e.g. the mirror loses strict rotational symmetry in some way.

 

Collimation errors usually mean the mirror is tilted or translated with respect to the camera or coma corrector lens axis, to first approximation these introduce coma rather than astigmatism. This extra aberration (misalignment-induced coma) cannot be corrected by the coma corrector to first approximation, and remains in the final photo. The misalignment can be caused by play in the mirror cell springs or flexture in the focuser or OTA tube if it is not thick enough. But since the users want light OTA's or smaller diffraction spikes, many commercial products are made in a flimsy way it seems. At F4, the collimation error has to be within a few hundred microns if one pretends to want "diffraction-limited" performance (rarely achieved in practice I think), this is likely to be more serious than the imperfections in the mirror like astigmatism. At this level the play in the focuser or camera attachment become problematic too (a 1.25" barrel eyepiece attached to an an adapter placed in an expensive 2" focuser has a repeatability of axis of 0.15 mm or worse as I saw). This is part of the reason why astrophotographers opt for very rigid and massive OTA's weighing 10 kg or more for a 6 inch. These collimation errors and rigidity become more confounding for an equatorially mounted OTA, because of the way it rotates as well as tilts. Things like the thickness of the OTA holding rings start to matter as I have heard.

 

Compared to the above major problem of collimation and rigidity and stability of the optical mount, mirror deformation caused by some acorn pieces at the back of the mirror not in direct contact with the mirror has almost no effect. The risk in this design is that the mirror is rigidly held by silicon adhesives from all sides, back as well

as side, on a rigid support plate (I think this support method is not of "floating type"), so it is important to avoid that the mirror gets deformed this way, not to apply too much stress when mounting it or using too much adhesive, I have seen that adhesive applied on a large surface can cause deformation. The article recognizes this issue and recommends to use adhesives sparingly and the design for thickness-to-diameter ratios=1:6 mirrors or more, whereas the GSO mirror is closer to 1:8 classified as a "thin mirror" (so strictly speaking it it outside the author-recommended application?). This might need some iteration but the article tells you what to avoid. I think this three-point adhesive-non-floating design would clearly not be recommended for a 10 inch thin mirror of 1:8 or 1:10, but I think the author does not recommend that either.

 

I'm not sure that I agree spring-loaded mounts are inherently unstable, the author does not explain why he thinks so. Springs are widely used in most mirror mounts not only in astronomy, and if the springs are tight enough I often do not see much problems at least at the micron level. The springs prevent play when materials with different thermal expansion coefficients contract in different ways. Springs can be skipped in some cases if the mount is monolithic, but this is not the case here; the main parts of the mount are made of wood screwed together with metal.


Edited by X3782, 22 August 2018 - 10:59 AM.

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#7 stargazer193857

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Posted 24 August 2018 - 08:02 PM

Supports behind a mirror can affect the focus quality. The question is how much. It depends how much weight is where. A 6 inch mirror is not likely to be affected much, especially at low power. That said, a single support point is about the worst. Use a high power eyepiece on a star on his. You'll see.


Edited by stargazer193857, 24 August 2018 - 08:18 PM.


#8 stargazer193857

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Posted 24 August 2018 - 08:20 PM

the acorn nut is on the mirror mounting plate, not the mirror. This is basically similar to the way secondaries are mounted on commerical SCTs. It should work just fine. The stiff mounting plate simply pivots about the acorn nut. Usually the cause of astigmatism from convention mirror mounts is from allowing the clips to contact the mirror surface.

 

Then it depends how the plate is set up, but my guess is that fixes the issue if there is an air gap between the two in the center.



#9 dave brock

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Posted 25 August 2018 - 04:57 AM

Stargazer,

did you even look at the link?



#10 dave brock

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Posted 25 August 2018 - 05:04 AM

I think this cell could cause astigmatism if an adjustment screw is tightened too much without

loosening the other two. Especially if the plate is too thin.

I think the spring loaded design is better if the springs are strong enough as there is nothing really

trying to bend the plate.


Edited by dave brock, 25 August 2018 - 05:04 AM.


#11 dave brock

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Posted 25 August 2018 - 05:12 AM

I have a 6" F4 GSO newtonian that I plan to pair with a small pixel camera 2.3um pixels its 20mp total and samples at about .83 arc seconds per pixel.

I know right off the bat that its going to show every defect in the world and that any mis collimation is going to be a pain to deal with. 

 

After reading some complaints about how these scopes keep collimation, I have thought about redesigning the primary mirror cell so that it is a bit more robust. I have been reading an old build tutorial by Gary Seronik  https://garyseronik....your-reflector/  And it seems interesting because he has done away with springs and mirror clips.  The center of the primary rests on a threaded acorn looking nut.  I am curious though to hear feedback from the experienced mirror making folks.  Will this central nut cause astigmatism in a 6" mirror?  How about if I beefed up the central nut and used a roller bearing with a floor flange mount? https://www.amazon.com/TruePower-10-9111-Roller-Transfer-Bearings/dp/B01BV9ZUSU and I was thinking about replacing the washer with a  cupped magnet

http://www.leevalley...=1,42363,42348 

 

Before I buy anything Im curious to hear what others have to say about Gary's cell especially if you have feedback on the astigmatism aspect.  Has anyone built their own?  Thanks! 

Your roller bearing link has the word "mount?" added at the start by mistake. Should be

   https://www.amazon.c...s/dp/B01BV9ZUSU


Edited by dave brock, 25 August 2018 - 05:13 AM.


#12 stargazer193857

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Posted 25 August 2018 - 02:18 PM

Ok, I now looked at the link. Looks similar but not the same as how a secondary mirror is mounted.

I doubt it will cause astigmatism. I also doubt collimation will be easier.


The trade off with gluing the primary is that if the glue is soft, the primary can tilt, and if it is hard (or soft) the thermal expansion of the glue and underlying support can warp the glass. But that applies more to the thin glass of a secondary.

Edited by stargazer193857, 25 August 2018 - 02:19 PM.


#13 stargazer193857

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Posted 25 August 2018 - 02:22 PM

A nice thing about glue is it adapts and can distribute loads better than a hard surface that always has just 3 points of contact. However, the same softness can allow tilt and collimation drift, if of course the mirror is tipped upside down so force is applied.

#14 davejlec

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Posted 25 August 2018 - 05:18 PM

I've incorporated the central pivot design from the Sky and Telescope article with mirrors from 4-1/2 to 12-1/2" d.  When silicone is applied to the cell it's important to heed the articles instruction to use spacers, about 1/8 to 3/16" thick that remain in place until the silicone fully cures- this isolates any bending of the cell from the mirror.  1/2 thick Baltic birch would be fine for a 6 inch mirror, or 3/4 thick if using a lesser quality plywood. Lots to like about the design - simple to build and works beautifully. The 3 holes on the bottom plate of the cell should be slightly larger than the threaded studs used to collimate, this will prevent any binding. A ball bearing can be used in place of an acorn nut- wouldn't recommend a cupped magnet because it will likely shatter but thick washers with an appropriate inside diameter recessed w/ forstner bits into both top and bottom plates with the bearing sandwiched between makes adjustments smooth and precise.    


Edited by davejlec, 26 August 2018 - 12:33 PM.

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