Re: [APML] STV Guiding accuracy vs focus

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

Steve,

Thanks for the reply.
I just posted to the yahoo group.

I believe the count values were higher than 1500.  What I did not say is
that I started with the cheap lens and could not get excellent tracking.  It
was a non brand name old Japanese F4.5, 55mm-300 mm zoom lens.  Zoomed in
the stars looked a bit like seagulls.  Then when I switched to the Nikon,
perfect tracking.  Defocusing the Nikon would simulate the performance of
the cheap lens.

If I were imaging, this would be a no-brainier, use the good lens, but I
want to use a dedicated (cheap) lens for guiding and as a zoom finder.   At
a minimum I want to understand what is going on.

Rick

----- Original Message -----
From: "steve banbury" <banburys@sonic.net>
To: <astro-photo@seds.org>
Sent: Friday, April 19, 2002 5:54 PM
Subject: Re: [APML] STV Guiding accuracy vs focus


> HI Rick--you might ask the same question over on the Yahoo MI-250 group as
> I know there are a number of members using the STV and a MI-250.
> What star brightness values were you obtaining in the defocussed case?
> I have found greater error for brightnesses less than 1500 counts when
> using my STV and my MI-250.
>
> Steve
>
> At 04:47 PM 4/19/02 -0400, Rick Kellogg wrote:
> >
> >During the winter months, I've been trying to fine tune my mount/guiding
> >skills to the best possible.
> >
> >My mount is a MI-250 which has excellent tracking.
> >
> >I use an SBIG STV and have been experimenting with various lenses.
> >
> >The STV was connected to a Nikon 80-200 ED F2.8 lens, and a TC 14E 1.4
> >teleconverter, resulting in an F4, 112mm-280mm zoom lens.  With this
> >combination, and 3 second integration (to keep from chasing the seeing),
I
> >would get  ~.7 arc sec RMS on the STV display for RA tracking error.  But
If
> >I slightly defocused the lens, the RMS error would increase to about 1.5
arc
> >sec.  This confuses me, as I thought the STV uses the centroid of the
star
> >for tracking, which I would not expect to be a strong function of focus?
> >
> >Does anyone have an explanation?
>
>
>
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