Oh no! The DPAC disease has spread to bino-viewers!
I made a new DPAC Ronchi screen and LED holder that works in 1.25" focuser. One thing I've always wanted to do was have a DPAC look at what happens to spherical and color corrections when a viewer is introduced into the light path. Visually, I've long noticed that with my faster "APO"s, like the TEC 140ED and, especially, the CFF 160 F6.5, most viewers degrade the color correction to some degree when used by themselves (no OCS, GPC or barlow in front of the viewer). This is easily seen by me at very high powers, particularly on stars, and especially, when a viewer is stacked with a prism diagonal. However, I've seen scant, if any really, changes in SA with the introduction of a viewer. Now one viewer in particular, seems rather free of color error at high power with my APOs, and that is the Zeiss Sharpest APO viewer sold by Denis Levatic. I also need to mention that I don't really see any color disturbances when using any of my viewers with my newtonians (and they always have a barlow device upfront too). The other thing to mention is that I also see tint differences between the right and left eyepieces. But, again, less so with the Zeiss Sharpest.
So I chose my TEC 160ED as my "test vehicle" due primarily to its wonderfully smooth figure, superb correction in green light and the fact it was already mounted on my test bench in DPAC. My reference 2" mirror diagonal was an AP Maxbright, the Baader Zeiss Spec prism was the prism diagonal reference. When testing the viewers, I tested them straight through with no diagonals.
So here are some of the results, first the viewers by themselves. The rows move from left to right with Base 160ED, Zeiss Sharpest, Denk II, then Zeiss "Biggest" (30mm CA prisms also previously sold by Denis). The top row is for the Right eyepiece collet, the bottom row for the Left collet. You'll notice right away that the Ziess Sharpest L&R are reversed relative to the other viewers. This is because you can rotate that viewer's halves all the way around to the "other side", which, for this sample viewer, gives excellent collimation.
My take on the color/tint differences will be in a later posting, but have at it. Click on the image to enlarge it.
Edited by Jeff B, 17 January 2021 - 01:00 PM.