Re: [APML] Dumb question time...

[Jerry Lodriguss <lists4@astropix.com>]

Hi George,

Or you could just not worry about it!  That's what I did in my scope for 
more than  ten years!

Sometimes we can get a little anal-compulsive around here... :-)

Jerry


At 10:26 PM 8/27/2005, you wrote:
>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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