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Focal Reducers, how do they work?
"Richard Kenney" wrote in message thlink.net... How does a focal reducer work on a SCT? Does it shorten the efective focal lenght of the scope? In the case of the F6.3 reducer, a 2000mm focal length SCT becomes an effective 1260. The reducer screws on to the SCT rear port, and the visual back screws onto the reducer. And, as Roger points out, it also corrects curvature. Curvature is an anomaly in the focal plane, where focus is achieved at a greater depth at center, than at the edge. If you imagine the focal plane as a straight line drawn on paper, you can then imagine curvature by bending that line into an arc. When an eyepiece has curvature, depending on your eye's ability to accomodate (acommodation is our ability to focus both close and far from the same location), you may find that you can't bring the entire view into focus. Either the center or the edge is in focus, but not both. The general method to reduce the effect of this anomaly (in an eyepiece) is to focus the stars in the middle area, half way across the radius. For photography, there's no accomodation. A curved field results in the edge stars being somewhat elongated. The R/C reduces that to an acceptable level. The other issue with the R/C is that it will introduce vignetting. Increasing the field of view with the R/C effectively puts the focal plane further inward toward the primary mirror, and this must then be brought back out to reach the film plane of the camera, or the focal point of an eyepiece. To bring the focal plane out to where it is reachable, you crank the SCT focuser and move the primary forward toward the secondary. Because of the baffle design in the SCT, this results in the inability to see the entire primary mirror from the edge of the focal plane. The result is that the edges of the focal plane experience a loss of illumination, by percentage of visible mirror. With the R/C in place, the area of the focal plane that is available without vignetting is large enough to accomodate a CCD, but is borderline for 35mm film. Meade makes an F3.3 R/C which can be used for CCD without vignetting, but unlike the F6.3 R/C, this one cannot be used with eyepieces. In practice, the maximum focal length of the eyepiece you can use with the F6.3 before vignetting is a factor is 30mm. The main point of that last tidbit is that large 2" eyepieces like the Pentax 40mm or the Panoptic 35mm and 41mm will show vignetting with the R/C in place. The technical reason for that is that the field stop of these eyepieces is large enough, and the focal length puts the eye far enough back, to allow the primary mirror edges to hide behind the SCTs internal baffles. -Stephen |
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