Thanks Chuck. -----Original Message----- From: Chuck Hards [mailto:chuckhards@yahoo.com] Sent: Wednesday, December 17, 2003 6:10 PM To: Utah Astronomy Subject: RE: [Utah-astronomy] In the blue Ok, here it is from memory. Early plates were taken twice, once with a blue-sensitive emulsion, once with a yellow (or red, not sure after all these years) emulsion. Subtracting one value from the other yields a "color index" for the star, helping place it on the H-R diagram. When the visual values were written-down, typically only one exposure was consulted, the blue one. Some objects are MUCH brighter to the eye than they appear on a blue-sensitive plate. I may not have recalled it 100% accurately, but I think that's the gist of it. C. __________________________________ Do you Yahoo!? New Yahoo! Photos - easier uploading and sharing. http://photos.yahoo.com/ _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.utahastronomy.com