Re: [APML] STV Guiding accuracy vs focus

["Richard Kellogg" <rkellog1@twcny.rr.com>]

Jim,

I think you have answered this.  (Along with Steve, in a parallel discussion
in the Yahoo MI-250 discussion group).

The STV tracking performance does appear to be sensitive to focus and star
shape.

Thanks for the feedback.

Rick

----- Original Message -----
From: "Jim Janusz" <jjanusz@dc.rr.com>
To: <astro-photo@seds.org>
Sent: Saturday, April 20, 2002 12:18 AM
Subject: Re: [APML] STV Guiding accuracy vs focus


> Hi Rick, see below.....
>
> snip
>
> > Jim,
> >
> >
> > I too have been using the eFinder successfully (1-2 arc sec is
> successful),
> > using a 200 mm finder scope.
>
> With my old GM200 and good seeing it would guide at .5 arc second average
on
> the ST-V.  My new AP1200 was guiding at .2 and .3 arc seconds during some
> testing last weekend.  Maybe your eFinder is not in perfect focus?
>
>
>   But I'm trying to improve it. And in
> > particular, I'm trying to get a zoom function, to allow both 50 mm "wide
> > angle" finder mode, and a ~300 mm zoom mode for guiding.
>
> I was thinking that an Olympus 135mm f2.8 that I have might have sharper
> stars the the little eFinder and be easier to focus.  I have seen others
> recommend short focal length refractors like the 80mm f4's that are
around.
>
> >
> > Many people use off axis guiders, and the guide stars in these often
look
> > like seaguls don't they? I only defocused to diagnose the problem, and
> then
> > only such that the star was about 1.5 times minimum size.  It was not
even
> > obvious that it was out of focus.
>
> I have a custom off axis guider for my 180 and it was a bear to use with
the
> ST-V because of focusing issues.  It has very slight star aberations
because
> it is before the field flattener and I think the -v is more critical the
the
> ST-4 which had no problems with them.
>
> One thing I will say is the the ST-V works easiest with short focal length
> guiding.  Longer fl's are much harder to settle down.
>
>
> >
> > I won't be using that old lens, but I'm not planning on using my
expensive
> > Nikon F2.8 80-200 mm lens for guiding either.  So I'm trying to
understand
> > what might be going on so that the next lens I acquire for the job will
> > work.
>
> I still say, use the best you have.
>
> Jim
>
> >
> >
> > Rick
> >
> > ----- Original Message -----
> > snip
>
>
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