Binoviewer Vignetting

Hi All,

I've read that binoviewers with smaller prisms can vignette when using long focal length and/or wide field eyepieces. But doesn't this also depend on the scope being used? For example, a F15 Mak-Cass scope will have a narrower light cone 5-6 inches (typical binoviewer position) inside it focal plane whereas a F6.3 SCT will have a wider light cone at that same position. So wouldn't it be possible for the SCT to vignette while the Mak wouldn't using the same binoviewer and eyepieces?

Thanks, Tom

There are two types of vignetting to
consider -- vignetting at the center of
the field, and vignetting at the edge of
the field. For planetary work you only
care about the field center, but for other
stuff you care about both.

Vignetting at the field center does not
depend on the eyepiece focal length / field
size. It only depends on the scopes F ratio
and the size of the prisms. Most of the binoviewers
add about 5 inches to the optical path. So a
unit with say one inch prisms should be OK
with focal ratio 5" / 1" = F5 or slower.
For one of the binoviewers with small
prisms its 5" / (1/2") = F10 or slower.

Vignetting at the field edge is more complex.
It depends on the scope light cone, the size of
prisms in the binoviewer, and the size of the
eyepiece field stop. If the binoviewer is one of the
microscope derived units with 12mm prisms, and
you use an eyepiece with a 27mm field stop,
the field edges are going to be completely
vignetted (dark) no matter what the
scopes focal ratio is. Fast scopes will probably
have some vignetting at the field edge, even
with large prisms -- maybe 50% light loss at the
edge -- detectable but not terrible.