Patrick - I think the topic you are looking for is G2V Calibration (color balancing). G2V calibration is a technique that uses your filter set and images of class G2V stars to compute standard color correction calculators for your camera and filter sets. Chapter 20 in AIP4WIN contains discussion of the process and AIP4WIN has G2V color balancing tool. (Color | Color calculator). I haven't used it yet. For Maxim DL users, DL also has a G2V Calibration tool. It is demonstrated at this webpage: http://www.mistisoftware.com/astronomy/Process_m8.htm There is also a DVD specifically for Maxim DL and Photoshop illustrating how to use the DL color calibration tool. http://www.caelumobservatory.com/video/dvd2.shtml The calculators also account for extinction based on the altitude of the object at the time of imaging. Extinction effects each color wavelength differently. Berry's book (at 533) states for most CCDs and filters, the balance weights are 1:00.93:0.78. Happy imaging - Kurt _______________________________________________ Sent via CSolutions - http://www.csolutions.net