Re: [APML]: ST-4 test first night

[Chris Vedeler <cvedeler@ix.netcom.com>]

The Astro-Photography Mailing List
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Chuck Vaughn wrote:

> The Astro-Photography Mailing List
> ------------------------------------
>
> Chris,
> >
> >I did get an E1 on one calibration, but all the others seemed to work fine
> >with the X Y values returning to within 1 or 2 values of where it started.
> >Thanks Mike Regish for explaining what the "E" means.  Geez, and I though
> >the Meade documentation was poor.
>
> It's on page 5 of my manual. "E1" means it didn't calibrate properly.
> No wonder it didn't track properly.

Thanks Chuck.  I only remember seeing that once out of perhaps 15 times so I
don't think it was a major problem.  I can't find in the documentation anywhere
where it talks about that "E" is a value greater than 9 when tracking.  To my
brain "E" would stand for "Error" which lead me to think that it was not working
properly.  It sounds like this is a normal and common thing to get.  It would
have been nice if it talked more about what the readout means while it is
tracking.  So far I am very impressed with the ST-4, but the documentation is
poor.

>
>
> >I just calculated out the size of a single pixel on the ST-4 at 2500 mm
> >focal length.  Its about 1.13 arc seconds per pixel.  Now if each correction
> >value of the ST-4 is .02 pixels that means that for each number of
> >correction, the guide star has only moved .022 arc seconds!
>
> It tracks to 0.2 pixels, not 0.02 pixels.

What's a factor of 10 among friends?  :-)   .2 pixels at about 1 arc second per
pixel is still  pretty darn sensitive for the kind of seeing I had last night.
(or almost any seeing for that matter).

>
>
> >Even in perfect
> >seeing and having perfect polar alignment, I know that my LX200 can't track
> >with 2 hundredths of an arc second accuracy.  I guess it's no big surprise
> >that I was getting AE and A9 guiding with the mediocre to poor seeing last
> >night.  It was probably  still guiding within 2 or 3 arc seconds, which
> >would be about the quality of the seeing here last night. Would it be
> >reasonable to assume that the ST-4 was trying to guide through the seeing?
>
> Taking the average pixel size at 15um, one pixel is 1.65" for a 2500mm
> focal length. So 3% of the CCD is either 9.5" or 8.2" depending on the axis.
> If you say you saw a change of 3% then that is bad.

3% was a guess.  I'll pay closer attention tonight.  After learning all this
stuff from you guys today, I kind of doubt anything was really wrong last night
(other than using 2500 mm focal length in poor seeing).  I just thought the "E"
meant it was erroring somehow, and couldn't figure out why it seemed to track so
well visually and yet show me such big errors.

>
>
> If your seeing was poor then you could be seeing E's but it sounds like there
> are other problems to address first.
>
> Chuck  <aa6g@aa6g.org>

Thanks for all the help!

Chris Vedeler
http://www.isomedia.com/homes/cvedeler/space.htm
Tucson, AZ