Since there are a number of prisms in the field, the tests will be done in both short and fast focal ratio refractors (f/8 APO and f/6.25 APO)
Just a comment:
F/8 is still quite fast and only marginally different from F/6.25. Something in the F/12-F/15 range would seem reasonable for comparison purposes and it could be a CAT...
Curious...If things look excellent in f/8, would you expect them to get worse in an f/15?
btw, f/6.25 & f/8 is all I have that can take both 1.25 and 2" diagonals...unless of course you want me to put a diagonal in the focuser of my Newt FWIW, Baader states that f/7 or longer is where their prisms work best. Most modern APOs fall in the f/6 to f/8 range, so IMO these two are good representatives for 90% of what's out there that would be using these items. Exceptions being of course CAT systems. But for those I would hazzard that the primary interest areas would be threshold brightness and planetary performance in the f/8 as it should be reliably extensible to what outcomes to expect with a CAT. I do have an older Meade 8" SCT, but it only has a 1.25" visual back. If I have time, I can spot check some of the 1.25" diagonals on that for planetary to see if they react the same after done with the APOs. But of CATs are not my cup of tea so never had what I would term as exacting planetary performance from mine due to their thermal gremlins. So not holding out much hope that I'll see any difference in that beast.
By trade, I am an experimental materials scientist. I have learned that when making measurements, it is good to be guided by analysis, not so good to be guided by word of mouth (that's a good way to have a sample mashed beyond recognition) and it is good to make as many measurements over as wide a range of the variable(s) of interest as is possible.
I have not seen the analysis that suggests that F/7 is a cut off though I imagine that someone like Vlad could provide that if they were to happen along here. Refractors are designed by analysis, this analysis would seem to be comparatively easy.
But, in any event, my point was that I think it's important to recognize that in general terms, F/8 is not particularly slow, I would classify it a moderate focal ratio, definitely not fast but still fast enough that many simpler eyepieces will still have issues with off-axis astigmatism.
From my point of view, it seems primary question concerning the use a prism diagonal in a faster scope is the dispersion and the resulting chromatic aberration. This probably depends to some extent on the type of glass but little else. So if one limited their observations to chromatic aberration, rather than testing a number of diagonals, testing a number of focal ratios with few diagonals could provide better understanding of the relationship between chromatic aberration of the prism and the focal ratio of the telescope.
This approach need not only include refractors, in fact, because of their complete lack of chromatic aberration, a reflector might be a better test platform because of the availability in faster focal ratios as well as complete lack of chromatic aberration, any chromatic aberration observed could be ascribed to the prism. If one is using a refractor, one would have to keep in mind that individual refractors have differing residual chromatic aberrations and so the chromatic aberration one is seeing is the result of both the prism and that particular refractor's residual color correction.
With all this in mind, my approach would to begin by looking for chromatic aberration using a single prism diagonal in as many focal ratios as possible... If I could get it to focus, my 12.5 inch F/4.06 Newonian would provide an interesting data point.
As far as the performance in a slow (>F/12) refractor, I think one might find that a prism diagonal has less scatter than a mirror diagonal and so the actual performance of the diagonal as a function of focal ratio is a combination of scatter versus chromatic aberration. Since in terms of raw numbers, I have to think most prism diagonals are used in CATs, the relative performance would be of interest...
Anyway, I have outlined a much different way to approach this question.. Isolate the variable(s) of interest and investigate it over a wide range. For me, now that Venus is gone from the evening sky, the starting point would be to just put a prism diagonal in my NP-101 and look at Rigel or Sirius for signs of chromatic aberration. That would provide one point that is, in practical terms, about as fastest focal ratio that as is possible with a refractor that is well corrected for color.