From: Milton Aupperle <milton@outcastsoft.com>

Date: July 23, 2009 5:12:38 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] Photometry on an asteroid 790 Pretoria

Hi Terrence;

It depends on how fast Pretoria rotates,  how long your exposure time has to be to get a good signal to noise ratio for a mag ~13.5 object and what aperture / focal length of scope your using too.

From what the curves at

http://epmac.lpl.arizona.edu/asteroid.php?Des=790

show, it looks like it's about a 260 hour rotational period and has a magnitude variance of of about 0.2 Mag low to high. So you likely won't see a a detectable amount of brightness change except over a 24 hour period of monitoring (0.20 mags / 130 hours is about 0.0015 magnitudes change per hour). The Ephemeris shows it's a fairly slow mover to of about 9 arc minutes per day

I'd probably shoot a few minutes of images, stack them and see what sort of signal to noise ratio Astro IIDC gives you for a single frame. You can do either differential or absolute measurments

Don't forget that to get good magnitude precision you need to shoot flats and darks too, along with the other suggestions described on  page 52 and page 64 of the Astro IIDC manual for doing photometric measurements.

HTH..

Milton J. Aupperle

On 23-Jul-09, at 3:41 PM, Terrence Redding wrote:

July 19th occultation of the subject asteroid has resulted in the discovery of a new binary star.  IOTA has asked for photometry of the asteroid to aid them in determining a period of rotation.  I have an occultation at 9:30 PM local, and the asteroid rises at about 10:30 PM.  So, eager to play with the new DMK21 and Astro-IIDC this seems like a good opportunity to reconfigure the scope for imaging Jupiter, and then switch over and capture some video of the asteroid for Photometry.

The question in my mind is, how much video should I capture?  Should it be continuous for a few minutes, or a series of frames periodically?

All suggestions welcome.