Re: [APML] Dynamic SCT Focusing?

["Steve Walters" <swalters@att.net>]

Jim,

I think that if the OTA is so unstable that your STV has trouble guiding,
then the film is in real trouble. Better to wait for the OTA to stabilize.
If your focuser can move without shifting the image, then possibly you could
do something like this but I would be there will be other problems showing
up like flexure in the focuser, etc.

It might be cheaper to have a carbon fiber tube made and address the problem
more directly. There are folks around who will make these for you, I have a
link someplace.

Steve...

swalters@att.net
http://astrophoto.home.att.net
----- Original Message -----
From: Jim Vineyard <sky4me@msn.com>
To: <astro-photo@seds.org>
Sent: Sunday, May 20, 2001 3:50 AM
Subject: [APML] Dynamic SCT Focusing?


> OK,
>   I've been wondering about this for quite some time now.  Can the STV
> provide a good sharp image of a guide star *during* actual guiding?  For
> those of us using SCT's with off-axis guiders, might it be possible to
> actually re-focus during an exposure since the film image forms so very
> slowly at f/10?  Both the film plane and guide star are both in focus (or
> out of focus) at the same time (parfocal).  Loss of focus is such a very
big
> problem due to tube contraction caused by falling ambient temperature and
> usually becomes a big limiting factor especially in the early hours of the
> night.  I think I've aborted exposures at 60 to 90 seconds into them at
f/10
> before and nothing has even registered on the film later after developing
> (with nothing really bright in the field though).
>   If one were to use an electric focusing motor (like the JMI unit which
can
> turn very slowly) and with a long cord the impact of vibrations would seem
> minimal.  I don't know if the STV could keep hold on the guide star during
> the re-focusing though due to mirror shift.  There would certainly be a
> large mirror shift if one were to want to finish with a counter-clockwise
> turn of the focusing knob, because any compensation for falling temp would
> require a clockwise turning of the knob (I think).  Perhaps if we went
> against the usual "correct focusing method" and always finished with a
> clockwise turning (even at the beginning focus too) then only a very
slight
> additional clockwise turn during the exposure (maybe an hour into it)
would
> be required and the mirror wouldn't shift much, unlike what it does when
> reversing the direction of focus knob turning.   Maybe this is simply an
> unworkable idea though.  Anybody tried it, or want to tell me why it
> categorically wouldn't work?  Any thoughts are welcomed, but please don't
> flame me too badly if this is an utterly stupid idea!
>
> Once Again, TIA,
> Jim Vineyard
>
> P.S.  Congrats on the acquisition Steve B!  I'm a still a wonder'en.
>
> Visit my Astro-photography website at:
> http://www.homestead.com/sky4me/astrophotography.html
>
>
>
>
>
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