So last night I had my old 6" F10 AP triplet out and was comparing diagonals (specifically my AP maxbright and Baader Zeiss spec prism diagonals) with my Zeiss 30mm viewers from Denis. This is a really special, one-of-a-kind objective that Roland kindly refigured for me about six or seven years ago. Being an older pre-ED oiled triplet there is indeed a little out of focus blue/violet visible at high power, above say 150X. I used the moon, Vega, Arcturus, and the doubles Izar and the double-double for most of the evaluation and just plain observing. I spent a bunch of time on Vega and the moon as they are tough targets when it comes to color errors in refractors. Specifically, Vega is a very bright blue/white and lunar crater rims and mountain peaks can be intensely bright as well and I find they are very good at revealing color fringing in the system.
Seeing was not all that good, especially with the moon in the south but overhead was not at all bad sometimes.
In mono-vision, it was tough for me to tell any differences in sharpness or color fidelity between the diagonals though I thought there was a little extra bit of red splashing around Vega and, at times, a very, very subtle reddish hue bordering crater rims against the shadow blackness when using the prism diagonal. It took me several back and forths to identify it. Vega's stellar core was nicely white with both diagonals but especially with the AP diagonal. But both views with both diagonals were very sharp.
However, even when using the Baader 1.25X GPC between the Zeiss 30mm viewer and the diagonals, the viewer did add a little bit more blue and red splashing around Vega, with a slightly more yellow stellar core. This slight color degradation has been consistent for me with other observations with other refractors and viewers where I stack the prisms in a typical viewer, on top of the prism in the Baader diagonal.
Then I had a hunch, went inside and got one of my sample of the Zeiss Sharpest "APO" viewer and let it warm up outside for about 45 minutes.
I'll tell you what, the Sharpest is indeed sharp... and very color free. Other than a reduction in brightness when using the viewer, I could detect NO, differences in sharpness or color fidelity compared to mono-vision with either diagonal. In fact, with the 1.25X GPC in the light path and the AP diagonal, I'd almost swear the view via the Sharpest was, subjectively, very slightly sharper with less blue error than in mono-vision. For example, I was really hard pressed to see any fringing or hues against the bright crater rims and Vega's core was very white looking with a reduced blue/violet halo that was a bit more tight to the core image. Izar was just beautiful too and both pairs of the double double were hard, white, little airy disks with no fringing that I could see. Very, very nice.
I had a similar experience early last year with my CFF 160 F6.5 triplet. Using this viewer, a 1.7X GPC, and an AP or Baader BBHS silver diagonal was consistently more color free than any of my other viewer diagonal/OCS combinations with this particular scope.
You might chuckle a little with the term "APO" applied to a bino-viewer, but in this case, well, maybe it's not so far off the mark.
Anyone else have similar experiences?