I recently purchased two Baader 2.25x Q-Barlows to use in my Zeiss binoviewer on my APM 152mm ED apochromat. My first tests were with a single barlow in my 63mm Zeiss Telemator and the results were disappointing. Nearly all eyepieces vignetted quite badly, some to the degree of being nearly useless. The images were very sharp, however, so I decided to give them another chance, after having conducted various tests to verify that it wasn't due to incorrect assembly.
Today I had a chance to try them out in their intended role, in the binoviewer on the APM 152 ED. It was early twilight and the Moon was high in the SW. The seeing was fairly good. First eyepieces was a pair of 25mm Zeiss OPMIs. The image was flat out shockingly good! No hint of any vignetting and sharp to the very edge! And SHARP! A pair of 20mm OPMIs behaved similarly. Elated, I began to test other eyepieces. A pair of 9mm UO orthos showed no vignetting and the 12.5mm's were also good to go. The 18mm's showed a bit of vignetting and the 25mm's showed quite a lot. I estimate that the AFOV of the 18mm was down to 40° and the 25mm perhaps 35°!
And that puzzles me to no end. The 18mm UO orthos have a SMALLER field stop than the 25mm and 20mm Zeiss OPMIs, so why do the UOs show vignetting, but the Zeiss'es don't???? That doesn't even begin to make sense. Oddly, a pair of 24mm ES68's also show less vignetting than the 18mm and 25mm UO's. Again, this makes no sense whatsoever.
The Q-barlows can also be used with their lens cells screwed directly into the eyepiece barrels, but the threads aren't flush with the upper end of the barrel, so this only works on some eyepieces with enough clearance inside the barrel. It works fine on the Zeiss OPMIs and the UO orthos, but the OPMIs show some field curvature, then, and the 18mm and 25mm UOs still vignette!
After seeing the fantastic lunar views with the 25mm OPMIs, I don't regret buying the Q-barlows, but I must say I'm quite puzzled by their odd behaviour with some eyepieces. A generic 2x Sky-Watcher barlow shows no such vignetting with any eyepiece.
Edited by Astrojensen, 27 June 2020 - 04:01 PM.