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#1
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SCT, FR and wide field 2" eyepieces
What 2" wide field eyepiece works on SCT telescopes using focus reducers?
I have a 2" Pentax 40 mm 65 FOV and it don't work with SCT focus reducers installed. I had the C.6 FR lens before, now I have the Denkmier Starsweaper. Later don't have vignetting problem, but stars badly distorted. JS |
#2
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If you had read the manual (Celestron's) you'd know that it expressly
says not to use wideangle 2" EPs with the FR installed, due to vignetting and field correction issues. You can still use shorter focal length 2" EPs, from 16mm downward they're little affected. Andrea T. |
#3
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#4
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Apparently the original poster wasn't too happy of the combo 2" EP
(long focal length) and SCT+FR. The manual says why and off the top of my head I seem to remember that from 32mm 2" EPs shouldn't be used with FR, thereby confirm the "experimental" result of te original poster. IOW, you're just wasting glass, costly glass. Let's be frank, SCTs field correction is poor as they are, don't need a FR to reduce it further. Cheers Andrea T. |
#5
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A F/10 to F/6.3 focal reducer takes a 2" preimage from the optical
path and makes a 2"/f/10*f/6.3 = 2" * 6.3/10 = 1.26" circle of illumination! In effect the FR takes a 2" FoV and smashes it down to a 1.25" FoV which coincidentally is the size of the typical eyepiece (will wonders never cease). Any eyepiece that has a wider field stop than 1.26" will vignette. Therefore, a Focal Reducer is a device that enables one to avoid buying 2" eyepieces. The old Lumicon BIG reducer was/is a device that takes a ~3" image and creates an ~2" image and can be used with big eyepieces with the 10" and larger SCTs. However, the focus sharpness of the extended field from the main optics is "not all that good", and you end up with a big field where the center is acceptably focused and the outer ring of fuzz is anoying. |
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