But the field stop is 29 mm on a 27-mm baffle? Does that mean it would be vignetted?
I assume this in reference to my earlier statement:
Yes, a C5 with F/6.3 R/C will vignette with a crop sensor camera, but it's not like it is dark in the corners. You may also want to crop the photos square. Either way, you may still appreciate the extra space from a compositional standpoint. And visually, for night time astronomy, I was surprised how well the combo works. Yes there is detectable falloff at the field edges with a max-field 1.25" eyepiece, but there is useful illumination to the edge. In the day time, I could see how the 5.6mm exit pupil would cause problems for people (your day-time pupil can't accommodate the exit pupil, so it effectively stops down the scope, making the secondary even harder to ignore), but I find the combo usable at night with even my 35mm Ultima. Different people have different sensitivities to the shadow of the secondary, though.
Vignetting happens even before you get to a field width the size of the rear baffle. I doubt a C5 fully illuminates even a 10mm image circle. After that it vary gradually gets a little dimmer. And once you pass the 27mm point or so, it dims faster, and eventually faster still.
But remember, human beings don't sense brightness linearly. We are relatively insensitive to this sort of fall off. Without a reducer, the 35mm Ultima is no issue at all with a C5, And with a 0.63X reducer, your effective baffle size is reduced by that same 0.63X factor, so 27mm becomes 17mm. And yet the C5 still provides some illumination all the way to the edge of an eyepiece with a 27mm (or even 29mm) fieldstop. I'd rather have the extra field with all its flaws (vignetting, coma, field curvature) than not.
With a camera, vignetting is more obvious, but can be adjusted with post processing. Or you can crop. And again, if you crop square, you may get rid of most of that vignetting.
Still not sure I understand the exit pupil thing, but on paper, 32 mm looks like it works out really well with 1250 mm of focal length and a Barlow. A 2X Barlow on a 32-mm eyepiece with a 52-degree AFOV provides a TFOV of 1.2 Lunar diameters, which is the exact same true field of view I would get on a Canon APS-C sensor at prime focus. So the 32-mm eyepiece basically works as a DSLR simulator, so I can go back and forth between visual observing and prime-focus imaging always seeing the same true field of view (with the 2X Barlow).
>>which is the exact same true field of view I would get on a Canon APS-C sensor at prime focus.
I'm a Nikon guy, but last time I checked, Canon DSLR sensors were rectangular.
At any rate, that's a pretty tight composition, which is why I might be tempted to use the R/C.
Visually, I wouldn't use a 16mm plossl with a C5 to look at the full lunar disk-- I'd use something longer.
A 32mm eyepiece with a barlow is often not a good idea. As I've mentioned before, barlows work better with shorter focal length eyepieces.