Re: [APML]: Beginner Needing Advise

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

> >     According to John Hoot who is the guy that redesigned the Pictor 201 XT,
> >  the cooling won,t get you much in the way for guiding as the cooling does
> > for imaging.  He explained it to me, something about the dark current and
> > dark frame and mag. that make the SBIG and the Pictor very similar in what
> > they can guide on.   
> 
> Now that I find surprising. As I understand it, cooling a CCD dramatically reduces the 
>background noise in the 
> chip and enhances its sensitivity, which then allows fainter stars to stand out much 
>more so than if the chip 
> wasn't cooled. I would think that, for a given brand of CCD, said CCD would be able to 
>guide on much fainter 
> stars if it were cooled. And with regards to dark frames, I've read that the new 201XT 
>uses a dark frame to 
> enable it to guide on dim stars (and therefore corrects for the renowned problems of 
>the 201).  I also find 
> that surprising.  I've read that subtracting a dark frame from a light frame when both 
>were taken using short  
> exposures (5 sec or less) is reasonably useless for enhancing the visibility of the 
>stars in the light frame.  
> Has to do with the fact that for short exposures, each pixel is not that much more 
>exposed (excited) than in 
> the dark frame, I think. And remember, the advise given on this list by people who know 
>is to never go much 
> over 5 secs for a guidance exposure due to atmospheric perturbances introduced into the 
>image.

The dark frame on the 201XT makes a tremendous difference to sensitivity
(over the 201) but I believe that cooling would give it more still.
Everything I've read about the ST4 (from users) leads me to believe that it
is more sensitive than the 201XT. I believe that they both use the same
chip. That's not to say I'm unhappy with the XT - it's sensitive enough for
my needs so far, and I value the easy setup. 

> I would be very interested in seeing some photos that were autoguided with
> the 201 or the 201XT. Anybody know where some are on the net? 

I have a lot of photos, but unfortunately I have no means of scanning them 
(yet).

Dark skies, beautiful images

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