There is good information here and since collimation issues have recently generated another thread under the refractors topic I'd like to re-open this issue as it relates to the Tele Vue NP series of scopes.
I have an NP127is that is about three years old and while it has taken some good pictures I've always wondered whether I was getting all that I could from this particular design. I've had some brief discussions with a vendor who has told me that ALL Petzval scopes suffer from some degree of astigmatism (including, from his direct experience, Takahashi) so based upon that bit of "advice" I've just assumed that I'm never going to get perfectly round stars in all areas of an APS-C frame with my NP127is. One caveat here, I'm imaging at 1.5 arc seconds per pixel with a 16 megapixel DSLR-type camera and I suspect that if I were using a camera with larger pixels (say 6 to 8 microns) then I might not even notice the star distortions that I get with my NP127is. Truth be told, I've never used a scope that produced perfectly round stars right out to the corners of an APS-C field (even with field correctors, although my Edge HD seems to do pretty well at prime focus), so maybe the high-resolution DSLR sensors that we have now are just too "good" for even the best scopes.
Okay, given the above how should one approach the lens collimation issue on the NP series of scopes? I'm doing tests to help determine where any POTENTIAL problems may exist (camera tilt, focuser tilt, collimation, or maybe as good as it's going to get -- that being residual aberrations that can't be fixed).
One issue that really worries me about the design of the NP scopes is that it SEEMS that the front lens collimation relies totally on the friction between a set of three screws and the main tube wall of the scope itself. Furthermore, it would SEEM that the first time that these screws are tightened (at the factor) that the screws will become more-or-less self-set into depressions in the tube wall. Thus, if the latter is true it could mean once a scope goes out of collimation from movement of the lens cell that you'll never be able to get the collimation to hold again after adjustment because the tube wall is already deformed (dimpled) and any re-tightening of the screws will just be working against a "slippery slope." So, you may think you can get it right after re-adjustment but then comes the first little shock and it's easily out of adjustment once again.
I guess the question here is what have users found concerning the stability of collimation after they have adjusted the front lens cell? Does it really hold, or are you back at it again in a few months or do you finally just have to give up?
A related question is this, can the NP127is really produce round stars right into the corners of an APS-C field (when working at under 2 arc seconds per pixel)? Since a picture is often worth a thousand words, here is the kind of results I'm getting now with my NP127is (reproduced here at 1.5 arc seconds per pixel and with using the aberration inspector script in PixInsight, there is some non-uniformity between the four corners and while the center is okay the edges are suffering a bit). This is also from a 36 image stack, so the eccentricity is probably a little better than for a single image and the seeing conditions on this night were only so-so which makes the stars a bit large and thus perhaps a little more round looking. At prime focus the NP127is has a 660mm focal length at f/5.2 and the angular field of coverage (diagonal) is about 2.4 degrees on my APS-C camera.
Edited by james7ca, 24 August 2015 - 12:10 AM.