[Aavso-photometry] Beginner CCD Questions..

[arne]

Derek C Breit wrote:
> Can some one answer a question or two..
>  
> I will be using a 12" LX200 in alt az mode, and an ST7 ABG camera..
>  
> Any rough ideas of a general rule for my maximum useable exposure time, what
> mag I can reach, what would be the minimum exposure time recommended to be
> useful in regards to noise overwhelming the data, how do I concentrate on
> areas of the sky with the least rotation from 38 degrees N Latitude?
>  
> Any ideas as to what magnitudes an ST7 can reach in 10,15,20,30,45, 60s .
> or more?
>  
> I intend on doing NGC 7990 standards in V for starters.
>  
> I am very used to video when it comes to finding targets. My primary
> question is what exposure time should I start out with to display mag 12
> stars in an Auto Dark subtracted image, or a raw image,  with enough signal
> to see the star above the noise, but not go too much deeper than that so as
> to not be overwhelmed by field stars when I am still locating the target? 
>  
> Also, if anyone would like suggest any other targets to try (as a beginner)
> feel free.. I already know I can put any star down to mag 12.5 on a CCD
> Chip..
>  
The major problem with alt-az is of course field rotation.  You can
get an idea of how bad the problem is with the spreadsheet from
Harry Kambanis, obtained at
http://users.altecnet.gr/harryk/
Usually, you can expose for ten seconds or so without significant rotation,
depending on where you are pointed - near the meridian but away from the
zenith is usually pretty good.  You just have to experiment to see what
errors you are willing to accept.

The usual rule of thumb is that a 16" telescope can reach mag 15 with
signal/noise of 100 in 60 seconds.  You can scale from there.  A good
starting point is the signal/noise calculator from Michael Richmond:
http://www.tass-survey.org/richmond/signal.shtml
(Michael Newbury has a spreadsheet that does this)
You can determine your limits for your telescope yourself.  However,
12th magnitude detections are easy.
Arne