[Aavso-photometry] Beginner CCD Questions..

[Wolfgang Renz]

Hi Derik

> how do I concentrate on areas of the sky with the
> least rotation from 38 degrees N Latitude?

See:
Doc G - Movement of a Star in the Image Field Due to Rotation
http://www.mailbag.com/users/ragreiner/rotation.html
Doc G - Rotation of Star Fields and the Use of  De-Rotators in
Alt/Azm Mounted Telescopes
http://www.mailbag.com/users/ragreiner/rotators.html
MAPUG - De-Rotator & Guiding
http://www.mapug-astronomy.net/astrodesigns/MAPUG/Derotatr.htm
http://www.mapug-astronomy.net/astrodesigns/MAPUG/images/Derotate.GIF

The rate of rotation of the star field is:
     rate = const * cos(latitude) * cos(azimuth) / sin(zenith distance)

The rate is always 0 when the scope is located at one of the poles
or when pointing it east or west.
Its as bad as it can get when the scope is located at the equator
and its worse when pointing it north or south.
But its alway worst when a star crosses exactly the zenith. There
it has a discontinuty where the Az drive should turn the scope by
180° in a moment and therefore the rotator should do it too.

So the answer is probably:
- east or west of the zenith its always OK
- in the south or north image at higher zenith angles (say above 30°)
- don't image when a star passes the meridian close to the zenith

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "Derek C Breit" <breit_ideas at hotmail.com>
To: <aavso-photometry at mira.aavso.org>
Sent: Thursday, August 02, 2007 11:37 PM
Subject: [Aavso-photometry] Beginner CCD Questions..

> 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..
> 
> Anywhere in the Eastern sky is good..
> 
> TIA..
> 
> Derek