Re: [APML]: Anyone Autoguiding a G-11 with a Pictor XT?

[Wil Milan <wmilan@airdigital.com>]

The Astro-Photography Mailing List
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Robin Casady wrote:
> 
> The difference between the 208XT and the 201XT is that the 208XT is cooled.
> I had assumed that this brought it into par with the ST-4 as to how faint a
> guide star can be. Is this false?

I have no experience with the 208xt, only the 201xt. The cooling
would help the 208xt in allowing it to acquire fainter stars and
stay locked on longer. Whether it brings it to par with the ST-4
I can't say.

There are other differences, however, which I consider very
important, and as far as I know these apply to both the 201xt and
208xt:

- The Meade guiders guide by sensing when the brightest point of
a star moves from one pixel to the next. The ST-4, however, uses
a much more accurate method: It samples an area of pixels around
the guide-star location, then does a centroid calculation to
determine the geometric center of the star image to within 1/5 of
a pixel. The ST-4 can therefore detect guide-star motion to a
much finer degree than the Meade guiders, and indeed the ST-4
reports guiding errors in units of 1/5 of a pixel. This finer
detection of drift allows the ST-4 to guide more accurately. It
also allows the ST-4 to guide successfully with guide scopes much
shorter than the photographic scope, which means one does not
have to mount as large a guide scope.

- The ST-4 tunes itself to the mount as it guides. It continually
evaluates the response of the mount to guiding inputs, and it
adjusts its calibration parameters on the fly to optimize guiding
accuracy. This is very useful and can make a very significant
difference. When I start a photo I always allow the ST-4 to guide
for at least a couple of minutes before starting the exposure.
During that time I usually see the reported guiding errors go
down as the ST-4 tunes itself to the mount. After the errors have
been reduced to minimum then I start the exposure and get optimum
guiding performance.

- The ST-4 has a "faint star" mode in which it averages a large
number of pixels around the guide star, allowing it to detect
much fainter guide stars. The area averaging of the Faint mode is
also very good for dampening out the effects of poor seeing, and
that too is very useful.

- The ST-4 has user-settable backlash-compensation settings that
allow it to work with mounts which have considerable backlash.
One of the biggest problems I had with the 201xt, that it was
very intolerant of declination backlash and therefore didn't like
an old mount with which I tried to use it. At the time someone
from Meade told me they were considering adding backlash
compensation as a future enhancement. If they have that would be
a good thing; I don't know if they have.

- The user interface of the ST-4 is *much* easier to use. It does
involve an external control box, but that control box brings with
it a much more informative display and clearly labeled individual
buttons for every function. There are even direction keys which
allow you to slew the mount from the ST-4 control panel while
viewing how the guide star positing is being shifted across the
CCD. Compared to this the single button on the Meade guiders with
all the multi-count pushes required and the cryptic 2-digit
display are very primitive and comparatively much harder to use.

I could go on with more, but those alone I consider enough of an
advantage to decide the matter, or at least they were for me.
Some of the others include true relay output (works with any kind
of mount), does not try to slew both axes at once (you've already
encountered that problem), and nice little touches such as an
alarm relay output (I've been meaning to connect that to a
shutter to suspend the exposure if the guide star is lost --
wouldn't that be nice?). All those things and a few more are why
I say that even against the cooled 208xt the ST-4 is still a much
nicer and much more capable guider.



Wil M.      
mailto:wmilan@airdigital.com
Astrophoto web site: http://www.airdigital.com/astrophoto.html
---
"When I consider your heavens, the work of your fingers, 
the moon and the stars which You have set in place, 
what is Man that you are mindful of him, 
or the son of Man that you care for him?"  -- Psalm 8