I will never make a mirror or grind a lens, nor will I win an award for any photograph that I take through a telescope. The reason being is that the quality that I require exceeds my ability to deliver. So, sometimes the best tool to reach for in cases like this is my credit card...
One aspect of the hobby I really enjoy is testing. It boils down to two reasons:
(1) to understand how flawed my telescope is and how these imperfections degrade the view.
(2) to verify claims made by manufacturers.
So, I've had my CFF92 since October of 2016. Its been a fantastic companion. And, because its so short, with the balance point almost directly in the middle of the OTA, the mount requirements are minimal. Build quality is very nice, and optics seem to be first class.
I have star tested the CFF dozens of times both indoors on a collimator (parallel point source) and outside on a real star. As many of you know, the star test is easy to do but a PITA to interpret mainly because there are several hard to control environmental issues to account for that really have nothing to do with the figure of the optic like atmospheric turbulence and the temperature delta between the lens/mirror and the night sky. I much prefer indoors.
As sensitive as it is, the star test is a qualitative measure (except when using Roddier). Its always bothered me to hear that an optic has been pronounced good/bad based on a quick rack in/rack out of the focuser. This occurs at star parties ALL of the time.
For the last 5 years or so on the CN ATM forum there has been a lot of chatter on the pros of using Double Pass Autocollimation. This is another qualitative test, but when combined with the star test, really gives great confidence on the overall condition of the optic. I've always intuitively known that its better when two test methods agree with each other. In fact, I have never seen a mirror or lens that does well in DPAC do poorly in use. Roland Christen figured his lenses using DPAC prior to building his Cerevolo spherical wave interferometer. And, TEC mentions DPAC use in their test methods for their compound telescopes.
So I purchased back my Lawrence Livermore National Laboratory 10" optical flat (~1/20 wave) from a friend - I sold it to him years ago. And, while expensive, this was the easy part. I still had to design/build a stand for the flat so it could be leveled to the telescope as well as an eyepiece containing both a Ronchi grating and an LED light source - still leaving enough space in the eyepiece to view the test and take photos.
Don't scoff at the the use of a Ronchi. In DPAC its a very powerful test - giving you an indication of the spherical correction, edge condition as well as an overall picture of the zoniness/smoothness of an optic. DPAC/Ronchi is not be confused with a "single pass" Ronchi test when you throw one of the commercially available Ronchi testers into a focuser of your sub F/4 Newt and you see straight lines. Testing a fast optic in this single pass manner is quite insensitive to errors. Double pass is very sensitive - its as the name says - the light used in the test traverses the optic twice (through the lens bounced off of a flat and back through the lens to your eye or camera) so the errors noted are doubled. Aberrations are much easier to see. Conversely, if no errors are seen - bands are "jail bar" straight - you have an excellent optic.
Here are the results for my CFF92. Ignore the minor thermal anamolies within the star test.
Edited by peleuba, 13 April 2018 - 12:11 PM.