The short answer is "they don't work". The standard accessory filter set has been sold to amateurs for a very long time, dating back to the age of the refractor. Such telescopes have chromatic aberration that causes different color light to focus and different focal length, you can focus on one color but the rest of the spectrum is out of focus and thus is noise. You can get a sharp image by filtering out all but a single dominant wavelength and that's what the filter set allowed you to do. Of course the different filters sort for different colors so you can see different features in different filters, or you could use a telescope whose primary optics is a mirror. 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. The modern Apochromat does a better job than the traditional Achromat, but still has the problem. I finally found a use for my color filter set, or at least the red and green ones. I use them for those "three D" photographs. Hold one over the right eye and the other over the left eye. On a more serious note, the Hubble Space telescope carries a large array of filters for an entirely different reason than amateur visual astronomers. By photographing a cluster of stars through a B filter and then through a V filter you can reduce the data and have the B-V value for each star in the cluster. Plot that versus the V value and you have a crude but reliable approximation of the H-R diagram for the entire cluster of stars without having to do spectroscopic analysis on each individual member of the cluster. 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. DT __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com