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
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.