I know a lot of people dismiss the star test, but when you have done a star test for a perfect telescope, it really burns into your memory.
DPAC Is better, but the analysis is not as easy (or at least not to me). I can see errors like zones quite easily but quantifying errors shown in DPAC are beyond my ability. I can see a less than great lens, but I can't tell how much less than great it is...
Still, I am hugely enjoying these tests and hope we see many more...
If you search on these 3 words: "Bench Test of", you will come across 4 (or more) topics I have started that include DPAC analysis of various telescopes. Jeff has a bunch, too.
I also want to point out that both the star test and autocollimation are qualitative and are not a reliable means of giving an exacting wave front rating to an optic - estimates only.
To me - for refractors - the star test, performed outdoors, is an order of magnitude more difficult to interpret then the test results I glean from autocollimation - for two primary reasons:
(2) Atmospheric instability
To reliably star test outdoors you must have longish periods of good seeing which is not a common phenomena in my area or most of the U.S. - Southern CA, Hawaii, Gulf Coast and Florida are three three major exceptions. To combat the seeing issues, I built a collimator using an LED, a 5 micron precision pin hole and some aspherics which generates parallel light for an artificial star. Indoor star testing is SO much easier to perform reliably and the results are repeatable - there are no thermal gremlins. And, the star test views closely match what Aberrator displays.
I am a true believer of testing in double pass. Employing DPAC has helped me become a better star tester. For DPAC, try using these two software programs: Diffract and Ronchi for Windows to estimate the correction similar to what Aberrator does for the star test.
I have never once seen a telescope perform poorly in DPAC and be excellent outdoors in the field. I have seen several telescopes that on a given night had crappy star tests but do well at other times. Moreover, its not easy for even the best star testers to differentiate high order SA from a turned edge.
I have put together two interferometers - a Bath and a variant of the Michelson which itself is a variant of the Twyman-Green. Each is not currently on my bench. Rather, right now, they are on the shelf because they are finicky beasts and have been replaced by a DPAC test stand/flat combo and a collimator for star testing. As I was telling Roland in a private email conversation a few months back, I am just glad when I actually get fringes to analyze regardless of the quality of the wave front . I have come to the conclusion that testing in DPAC and then using the indoor star test is all I really need. Interferomtry, while fun to play with can be a time-suck and a frustrating endeavor.
A good friend and professional astronomer/optics guru has created a star test device that utilizes a 5 micron precision pinhole, a beam-splitter and some auxiliary optics that, when placed into the focuser of the telescope under test, outputs a point source. When placed in front of the optical flat, and viewed with an eyepiece through the beam splitter, you are able to star test in double pass. I have the device and its really cool to use.
At the end of the day, while both autocollimation and the star test are qualitative, when combined they are powerful tools that compliment each other. I perform both tests - almost exclusively indoors and results of both tests always agree. The star test is best for astigmatism/coma/tilt/pinch/wedge and is OK for spherical. But in an imported ED refractor, pure SA is often overshadowed by edge issues and spherochromatism. Have you ever tried to determine whether an edge is turned or if the lens is suffering from high order SA? In real world star testing to inexperienced eyes, they look nearly identical. So, what happens if both aberrations are present? How do you sort it all out when star testing in typical nighttime conditions? (rhetorical)
Ed - I really appreciate your dedication to the star test. I have learned a lot from your years of posts going back to the AstroMart forums especially pertaining to star testing with obstructions. And, I really enjoyed reading about your restoration of Buffy and your progression to using a Dob. Fast forward to today you've almost got me convinced to apply for a HELOC and buy night vision gear .
Edited by peleuba, 12 February 2020 - 08:52 PM.