From: stan_ccd 
Sent: Tuesday, June 11, 2002 1:06 PM
To: SBIG@yahoogroups.com
Subject: [SBIG] Re: True Color and Ratio

--- In SBIG@y..., Adam Block <ablock@n...> wrote:
<< ... My thought was to average the peak value for each star in each 
color and determine a pair of ratios ... >>

peak value is very unreliable.  The best approach is to measure the 
total flux for the star.  Unfortunately most popular CCD software 
does not have tools that directly measure flux (maybe AIP4win does?)  
But you can still do it, e.g. in MaxIm:

Activate the information box and set the cursor radius to maximum, 20 
pix.  This means that you will be measuring approx 314 pixels 
(PI*R^2).  

First measure avg adu for empty background near the calibration star, 
then subtract 100 (pedestal) and multiply by 314 to get the total 
background flux for your measurement aperture.

Next, with the measurement aperture centered on the calibration star, 
measure avg adu, then subtract 100 (pedestal) and multiply by 314. 
Subtracting the total background flux results in the total star flux.

Use these RGB total star fluxes to calculate the "solar white 
balance" RGB ratios.

Note that the calibration star should not have a max adu greater than 
about 3/4 of your full-well (usually about 30,000 for non-ABG and 
8,000 for ABG).  Also, the accuracy is improved if you take the 
images out-of-focus (but not so out-of-focus that the star-blob 
exceeds the 20-pix measurement aperture).

Stan