Re: [APML] Focusing an ST-4

["Michael Stecker" <mstecker@dslextreme.com>]

Emmanuele:

If you use a guide scope like the 80 mm Astro-Physics, you only have to
focus it once on a star and then lock the focus with set screws (also works
for some off-axis guiders when used with a set ring sold by SBIG).  I did
this and photographed for three years without ever refocusing the ST-4.  If
you use this technique, then you will also need an extension tube so the
ST-4 and eyepieces for your guide scope are roughly parafocal.  By sliding
the eyepieces in and out of the extension/draw tubes I could also achieve
accurate focus with 40 mm and 12 mm illuminated reticule eyepieces for
centering the guide star.  This technique takes some time and
experimentation, a good mount and accurate polar alignment (you do not need
to see Polaris in order to polar align) to focus the ST-4, but you only have
to do it ONCE.

Michael Stecker
mstecker@dslextreme.com

My Web Site:
http://www.homestead.com/mstecker/index.html (INDEX)
http://www.homestead.com/mstecker/astrogallery.html (Astroimage Index)
http://www.homestead.com/mstecker/placesindex.html (Places Index)



----- Original Message ----- 
From: "Emmanuele Sordini" <vega@bloomingstars.com>
To: <astro-photo@seds.org>
Sent: Friday, May 23, 2003 8:36 AM
Subject: [APML] Focusing an ST-4


> Hi everybody,
> having acquired an ST-4 recently, I was trying to reach proper focusing it
> with my different scopes. I'm faced with the following problems:
>
> 1) I am doing some trials from a small terrace in a building downtown
quite
> a big city, from which the north pole is not visible.
> 2) I wanted to use my "downtown trials" to get all my rig up and running,
so
> as to not waste my time when in the field under good skies.
>
> Problem #1 is a pain in the butt since mine is not a permanent setup, and
> whatever chosen star is chosen always drifts out of the field of view very
> very quickly. So, focusing by the only means of the ST4's brightess
readout
> is very difficult and doing a drift alignment can take quite a
considerable
> time.
> Thinking about alternative ways of nailing good and consistent focus, I
was
> wondering if the following could work:
>
> 1) Focusing onto an artificial bright object (such as a street lamp or an
> artificial star) lying far enough away (HOW far away is another matter).
No
> more ages spent aligning, putting together and taking apart the whole
> equipment.
> 2) Focusing with a PC. Trouble is, the ST-4 is not a webcam so one can't
> have a continuous, real-time flow of frames. Perhaps, a hartmann mask
could
> be of some help here, and the software used could make a big difference as
> well.
>
> Anybody have any suggestions/tips on the above? Are those ideas sensible
or
> should I just take the pain of squandering a whole observing night on
good,
> ol' techniques?
>
> Thanks in advance,
> Emmanuele Sordini
>
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