I purchased the AT92 new from Astronomics on impulse after reading about it in this thread: https://www.cloudyni...84898-the-at92/
The optical quality, on paper, looked impressive. And, I really like high quality, small APO's.
This is my first foray into an OTA from China. I own AP and TAK, and previously have owned TEC's, CFF etc. so I did not know what to expect. The fit/finish is excellent, exceeding my expectations. The paint is a textured powdercoat very similar to the CFF line of APO's. The focuser is outstanding - FAR better then I would have expected at this price point - there is no need to upgrade it to any of the U.S. made boutique focusers for visual use. I am not into astrophotography so cannot comment with any authority on its ability to hold an imaging train steady over a period of time. Overall, I am struck by the value proposition that this telescope represents.
Below are some images taken last evening of the AT92 in double pass autocollimation (DPAC) from my test bench.
Some points to note about DPAC... First, the test cannot assign a wavefront or Strehl value to any optic with any sort of accuracy. However, it can tell you - in seconds - if your telescope is closer to a 1/4 wave or closer to a 1/10 wave. The test can also give you a reasonable picture of the optical surface(s) - how smooth/rough it is or if zones exist. But, probabaly, the biggest advantage of DPAC testing is that there are few sources of test-method induced error. It's EXTREMELY difficult to get a false positive. In other words, its nearly impossible for a bad telescope to test out as "good". With other test methods, this sometimes happens because of a lack of understanding on how to perform the test then not knowing how to interpret type and severity of errors in the result.
Additionally, when testing optics using Double Pass Autocollimation, the error uncovered is "double" what actually exists in the glass. Part of the test equipment is an optical flat which reflects the light back through the telescope traversing the optical path twice - so any error found is double what actually exists in the lens. This is the reason the test is so sensitive; making it easier to see small figuring errors. Finally, I have never seen a telescope do well in DPAC testing and perform poorly in the field. I also verify the DPAC results with an indoor artificial star test using a collimator to generate a parallel beam that I use to create an artificial star. DPAC and star test tell me all I need to know about the relative "goodness" of an optic.
A few final thoughts on this test... (1) the telescope rested in the v-blocks on the bench for more then 24 hours so that the lens was in thermal equilibrium with the test area. (2) I test at 3 wavelengths: Green, Blue and Red. This telescope has best correction in Green; its nearly perfect at this wavelength. In Blue its slightly overcorrected. In Red its slightly undercorrected. Its fairly typical to see the red and blue have "opposite signs" as these colors are on opposite sides of the spectrum. The lens also has a very slight hill (or depression) in the center. This is very common in machine polished lenses and will have negligible impact on the view.
My camera is not as sensitive in Red so the DPAC images in red are not nearly as good. This was a quick test last evening. If I was documenting it, I would take more time to capture better photos.
Overall, this lens looks to be a terrific performer.
(edited for grammar)
Attached Thumbnails
Edited by peleuba, 25 April 2019 - 12:31 PM.
