Re: [APML]: Beginner Needing Advise

[Steve Bell <sb635@delphi.com>]

RLECKER@aol.com wrote:
> 
>     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.

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?

-- 
Clear skies,
Steve Bell
sb635@delphi.com
http://people.delphi.com/sb635 - Astrophoto page