Hello:
I have a cassegrain, and it has small problem. Star images look sharp and I can see lots of detail in Jupiter, but the a star test shows either sphercial aberration or a turned edge. Thus, it's in the "tolerable but I'd like to do something about it" range.
The scope in question is a 12.5" classical cassegrain, about f/15 or so with an f/3.8 primary. I did the star test at 315x with a 15mm eyepiece, and defocused so that the star image was about 30" wide (a little less than the size of Jupiter with the same eyepiece). The test star was Spica, high in the SW, in moderately good seeing. Inside of focus, the image was brighter in the center (around the central obstruction) and fainter at the edge. Outside of focus, the image was brighter at the very edge and gradually became fainter towards the center.
I don't know the prescription for these optics, as they were given to me a while ago. It is possible that spherical aberration is being introduced because the primary to secondary mirror spacing is not correct. My questions are: does the scope have a turned edge or spherical aberration? If it's spherical aberration, how much does the secondary need to be moved to have a noticable affect (say, 1/2 wave) or in what direction?
Naturally, any help will be very much apperciated.
Cheers
Mike Connelley
I have a cassegrain, and it has small problem. Star images look sharp and I can see lots of detail in Jupiter, but the a star test shows either sphercial aberration or a turned edge. Thus, it's in the "tolerable but I'd like to do something about it" range.
The scope in question is a 12.5" classical cassegrain, about f/15 or so with an f/3.8 primary. I did the star test at 315x with a 15mm eyepiece, and defocused so that the star image was about 30" wide (a little less than the size of Jupiter with the same eyepiece). The test star was Spica, high in the SW, in moderately good seeing. Inside of focus, the image was brighter in the center (around the central obstruction) and fainter at the edge. Outside of focus, the image was brighter at the very edge and gradually became fainter towards the center.
I don't know the prescription for these optics, as they were given to me a while ago. It is possible that spherical aberration is being introduced because the primary to secondary mirror spacing is not correct. My questions are: does the scope have a turned edge or spherical aberration? If it's spherical aberration, how much does the secondary need to be moved to have a noticable affect (say, 1/2 wave) or in what direction?
Naturally, any help will be very much apperciated.
Cheers
Mike Connelley
