Re: [APML]: Beginner Needing Advise

["Philip Perkins" <philip.perkins@pop3.hiway.co.uk>]

Steve

> Thanks for this info. I've asked the "guide box size" question a few times, and you are 
>the only one that has 
> given me an answer. Can you change the size of the guide box to something smaller, say 
>10 by 10 pixels? A 36 by 
> 36 pixel box seems a bit large to me.  And I don't quite understand how a star outside 
>the box could cause the 
> 201XT to shift to that star. I know that the SBIG autoguiders don't even digitize 
>what's outside the guide box, 
> so a star of near equal brightness, but outside of the guide box, does not confuse the 
>SBIG autoguiders.

No, you can't adjust the size of the guide box. There are 6 custom changes
- all clearly listed in the manual, and this aint one of them!  Neither do I
understand why they should want you to be cautious about stars outside the
guide box, however the manual is very explicit on this point. Maybe they're
just advising caution in case drift should bring a star in close proximity
into the guide box. I've noticed that after centering a star in the reticle
eyepiece and getting a nice 4,4 reading on the Pictor, it can be offset by
quite a bit (up to 30 arc seconds) after the calibration process is
complete. This could be enough to bring another star into the guide box, and
I suspect this is what's happened on some occassions. I suppose it's best to
always re-check the eyepiece FOV after doing a calibration.

> What I was actually referring to by the words "guidance exposure" was how long of an 
>exposure do you use on the 
> 201XT. This sets how frequently the 201XT issues guidance corrections. Given that, how 
>long of a guidance 
> exposure do you use? 1 sec, 5 sec, 10 sec, etc?

Sorry!  It's like being back at school - my teacher always used to complain
that I didn't read the question :-)  I used exposures between 3 and 20
seconds - I suppose the average was somewhere between 5-10. I got stars
bright enough to use 3 secs on a couple of occassions. I only tried 20 secs
once and then aborted the guide - it was just taking *too* long between
exposures (this was when shooting M51 - very difficult to find a decent
guide star). Personally, 15 secs is just about my tolerance limit. However
the knowledge that the 201XT can guide successfully on brightness levels as
low as 5+ is very useful. Several times, I've only been able to find a very
faint guide star, and thought that I'd need to use an exposure of 15 secs,
but tried it on 10. The brightness reading was perhaps only 6 or 7, however
it guided happily for 1 hour and the results were every bit as good as with
a high brightness reading. 

Dark skies, beautiful images

Philip Perkins  --  philip.perkins@pop3.hiway.co.uk
51 27'N 1 36'W  --  dark skies, dark clouds (mostly)