Re: [APML] NGC7000 - Still Trying

["westergren" <westergren@netzero.net>]

Steve,

I think you still have FR.  I see elongated stars at a 45° angle in the
upper left and vertical elongation at the lower left.  The stars over on the
right are points (within the resolution of your picture).  My guess is that
your guide star was near the bottom of the Pelican.  If I'm right, then you
have FR. Going to longer shots will just make it worse.

To get a 60 minute shot with a 300 mm lens and have less than 2 arc sec
elongation, you need to have polar alignment drift less than 1 arc sec in 4
minutes, both axis.  I use the 2 arc sec criteria as my threshold for seeing
star elongation due to ADR or FR.  Looking at you image, it looks like FR =
6 to 8 arc sec.  Can you drift longer or use a higher magnification to see
the small amount of drift?

Don

----- Original Message -----
From: "Steve Lindsey" <stevelindsey@earthlink.net>
To: "APML" <astro-photo@seds.org>
Sent: Friday, July 04, 2003 11:36
Subject: [APML] NGC7000 - Still Trying


> I redid this shot last weekend with the 300mm lens wide open again at
f4.5.
> Made sure the polar alignment was great and pushed the Provia 400F only 1
> setting vs. 2 the last time.  Also got my friend to help mount a ring for
> the front of the lens to ensure no flexure in the lens mount.
>
> Still got arc'd stars in the corner areas but they were more symmetrical
and
> I think this time were just optical from vignetting vs. field rotation.  I
> thought this lens could do wide open but maybe not.  This was a 60 and 45
> min stack and much more cropped than the last post.
>
> http://home.earthlink.net/%7estevelindsey/NGC7000.html
>
> I think my options are as follows but would like your advise:
>
> 1. Stop lens to f5.6 and go 1 or 2x120 min with same film and push or
> 2. Stop lens to f5.6 and switch to "old Supra" and do 2x60 min shots
>
> What do you folks think?
>
> Steve
>
>
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