Re: [APML] Dumb question time...

[Jerry Lodriguss <lists4@astropix.com>]

Hi George,

It sounds like astigmatism to me.

If it is symmetrical, and the same amount at the same distance is all four 
corners, then it probably is that.

If it is random, sometimes its there, sometimes it not, sometimes its worse 
here and not there, it's probably the film emulsion absorbing moisture 
during the exposure and swelling and moving.

Your's sounds like astigmatism though.

You can see what astigmatism from a curved focal plane in a refractor looks 
like here:

http://www.astropix.com/Roland/FF.HTM

With a telescope you can use a field flatner. With a camera lens you can 
stop it down.

Try stopping the lens down to see if it gets better.

Jerry


At 02:08 AM 8/27/2005, you wrote:
>I think that I finally have the alignment on my mount nailed.
>But when I look at my negatives (25 minute HA on hypered TP with a 135mm
>lens) I find the stars are elongated by about 75% in the outer corners.
>This elongation is in an increasing radial pattern from the center, not
>a rotation about the center of the frame. Almost like the FL of lens was
>changing (zoom effect), but it is a fixed FL lens.
>The camera is solidly mounted with 1/4 thick aluminum angle stock for
>portrait orientation, but the lens itself is not supported.
>Any suggestions?


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