Daniel wrote:
The short answer is "they don't work". . . . According to Harrold Sutter, modern refractors are corrected for color focus only and two or three points in the color spectrum. All colors beyond and between those colors are just plain out of focus.
Your point goes to refractors but what about reflectors? I've used them with reflectors and have seen subtle but demonstrable effects on Jupiter, Mars and Saturn. Having flipped through my astro library and filter product sheets, my main problem has been finding a consistent, no nonsense organized list of which filter to use on which planet to what effect and a no nonsense description of why a particular filter works on a particular planet in a particular way. This tends to slow me down using planetary filters. I can see what you are saying about sharpening by removing the other refracted light wavelengths, but what about color mixing and contrast effects?
Plot that versus the V value and you have a crude but reliable approximation of the H-R diagram . . . The B-V value is listed in the Observer's Handbook as standard data for the list of bright stars. Study that number along side the spectral class and you will see how they vary together.
Garrison's (300) Brightest Star list in the Handbook is a list of nearby stars with highest precision known absolute magnitudes, where the absolute magnitude could be found from good Hipparcus parallaxes or the mass of a primary could be ascertained from binary star mass studies. I have most of Garrison's table imported as part of my Excel observing spreadsheet primarily to be able to call up reliable targets for use with my Rainbow Optics broadband eyepiece spectrometer. When you plot Garrison's stars in an H-R B-V format - they look like this: http://members.csolutions.net/fisherka/astronote/Clarkxref/img/HRDiagram.JPG Clear Skies - Kurt - and they do really appear to be clear tonight -:) _______________________________________________ Sent via CSolutions - http://www.csolutions.net