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Old January 24th 04, 08:35 AM
Michael
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Default SCT Focal Reducers and Vignetting (explained)


"Stephen Paul" wrote in message
om...
I was trying to find some information on the workings of the Meade
F/3.3 Reducer for CCD imaging and I came across the web page linked
below. Interestingly, as a side effect of the infomration, I finally
understood the vignetting problem with R/C's. It was good to see a
graphic that depicts the light cones exiting the rear cell of the SCT
at F/10, F/6.3 and F/3.3. The key thing to note is the reduction in
the diameter of the light cone that hits the focal plane.

Consider that the inside diameter of the primary baffle tube of a C8
is 38mm. This is the maximum diameter that the light cone exiting the
back end of the C8 can reach (25mm in a C5, 47mm in a C9.25, and 52mm
in C11/C14's, IIRC).

Conveniently, 38mm matches closely the field stop diameter of the 35mm
Panoptic eyepiece, which in turn, at 2000mm focal length and F10
yields (for all intents and purposes) a fully illuminated field of
1.1 degrees.

an eyepiece with a field stop that is larger than the 38mm, means that
less to no light is reaching the edge of the eyepiece field. That's
vignetting.

Introducing a reducer, doesn't do anything to improve the situation.
All it does is reduce the diameter of the light cone at the focal
plane to something less than 38mm, so that all the light of the
objective fits within a _smaller_ field stop. As a result, increasing
the field of view within that smaller field stop.

Hence, introducing a reducer before an eyepiece with a field stop of
38mm (or greater, given the C8), will cause vignetting, where it
didn't exist before. You _will_ still get 1.1 degrees of fully
illuminated field, but that 1.1 degrees now fits in a smaller circle
at the center of the eyepiece, beyond which there is less and less
light striking the field lens of the eyepiece, going out to the edge.



Ya got me baffled