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SCT Focal Reducers and Vignetting (explained)
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. Now, keep in mind that the field of view will never be larger than what the field stop of the eyepiece will allow. BUT, you can surely stuff a 55mm Plossl with a 47mm field stop into your 2" diagonal. The result? You will still only have 1.1 degrees of full illumination, because the light cone coming out of the back end of the C8 at F10, is at best 38mm in diameter. Exceeding the diameter of the light cone at the focal plane by using 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. So, just like stuffing the 55mm Plossl into the scope at F10, stuffing a 35mm Panoptic into the scope at F6.3 will give you a wider field, but the 100% illumination of the field will be something less than the field stop of the eyepiece will allow, and no greater than the central baffle will allow at F10. It has been presented that beyond 75% illumination, one really begins to notice the dimming of the field. So, the maximum useful gain of the F/6.3 R/C with a 35mm Panoptic (38mm field stop), is that point approaching the field stop at which only 75% of the objective is visible through the 38mm primary baffle. This might be calculated if all the values are known, or it might be carefully measured by projecting light through the OTA, but in the end the best way is probably by experiencing the effect in the eyepiece, and doing some timings of objects as they pass through the acceptably illuminated area (a subjective measure for sure). Since I just so happen to have a 35mm Pan and an F/6.3 R/C, guess I need to get a 2" Diagonal and do some timings. g Seems so simple and obvious, now. I definitely got my picture's worth. I hope you do too. http://benedict.isomedia.com/homes/c...l_reducers.htm -Stephen ("Steve") Paul |
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