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Old 03-07-2016, 12:56 PM
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Shiraz (Ray)
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Shiraz is offline
Join Date: Apr 2010
Location: ardrossan south australia
Posts: 4,820
Originally Posted by codemonkey View Post
Actually, having re-read your post, Ray, I'm guessing that 28mm eyepiece is too long. Any suggestions on what focal length might work? Would a 4mm plossl do the trick?

I'm considering buying a cheap, short focal length eyepiece, as if I can make that work, it'll save me hundreds.

I don't know anything about eyepieces, as I started out doing astrophotography and have never done visual.
I just pulled an old 9mm GSO Plossl to bits - got a nice doublet with a focal length of about 16mm. Guess a 4mm Plossl would yield similar with a fl of about 7-8mm and that would refocus your focal plane at roughly 32mm further out (or a bit more if you want).

Edit: been thinking about it and suggest that you use the whole eyepiece (eg GSO). Unscrew the chrome extension tube and then you would need to get someone to turn down the eyepiece main barrel so that it fitted in the 1.25 inch guider tube (also would probably need to cut off the threaded bit) - that would give you a way of securely mounting the transfer lens in the OAG. If you can find where the guider focal plane is (using a small scrap of baking paper with a daylight scene), you can work out roughly where the transfer lens has to be. For a 4mm eyepiece, the distance from the original focal plane to the eyepiece will need to be about 8mm and the new focal plane will be another 8mm or so out on the other side of the lens.

The measures that you will need in order to work out the appropriate eyepiece focal length is the distance of the CCD from the front of the camera, the location of the existing focal plane in the OAG tube and how far out along the OAG tube you can move the camera.

Last edited by Shiraz; 03-07-2016 at 01:55 PM.
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