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Re: [APML] Dumb question time...
Many thanks for the replies Richard and Jerry
I would seem to have an astigmatism problem with the lens since it is
just as Jerry described and is very similar (I have slightly more
astigmatism) to the problems he is seeing with his scope.
I'll try closing down incrementally the next time we have clear skies
and see how far I have to go to eliminate the problem. If it is too many
stops I'll have to purchase a different lens for my Pentax.
George
Clear skies and good health
Jerry Lodriguss wrote:
>
> 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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