Re: [APML] STV Guiding accuracy vs focus

[steve banbury <banburys@sonic.net>]

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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