Many magnitude estimates may have been made from plates taken specifically in blue light, such as those of the earlier Polomar survey. (Just a guess...) -----Original Message----- From: James Cobb [mailto:james@cobb.name] Sent: Wednesday, December 17, 2003 5:37 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] In the blue I believe he is referring to magnitudes obtained from black and white photography; it is biased toward blue light. Jim ---- Jim Cobb james@cobb.name On Dec 17, 2003, at 5:12 PM, Gibson, Jim wrote: I was just reading an article about the Herschel 400. The author Jay Reynolds Freeman made this statement: I have informally known for a long time that many older cataloged magnitudes of deep-sky objects systematically underestimate their brightness to the visual observer--the fact that many magnitudes were obtained in blue wavelengths guarantees that result... What does it mean "obtained in blue wavelengths "? How are magnitudes obtained in the blue wavelengths? Jim _______________________________________________ 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
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/
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