Re: [APML] Focusing an ST-4

[Emmanuele Sordini <vega@bloomingstars.com>]

At 10.39 23/05/2003 -0700, you wrote:
>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).

Unfortunately I don't have an el-cheapo refractor of that class to devote 
to that for good.
Well, that's not completely true: perhaps I could use an old 60 mm (700 mm 
FL) refractor of mine, but I think it's too small an aperture to be useable 
with the average random guidestar I could run into, at least when doing 
deep-sky AP with FL's of 1000 mm or more. Or am I wrong perhaps? How dim a 
star could such a refractor allow to guide on?

And I wonder: is a star absolutely necessary to to the tuning? Wouldn't a 
distant (i.e. a few miles) streetlight be enough?

>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.

My mount happens to be a GM-8, so it is (or at least should be) sturdy 
enough for that purpose. But my primary goal was that of optimizing times 
by using some workday evenings to set up for the real observing 
expeditions, that take place normally on weekends.

Looks like mine is an hopeless task and I'll have to devote some night-long 
sessions for preparation... :-((

Emmanuele

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