How do I estimate the equivalent exposure time for a series of planetary filters in a filter wheel. Assume as a hypothetical, that one has taken a satisfactory luminosity image and would like to cycle three visual-photo planetary filters through the wheel - and make an equivalent exposure: Filter Transmission Filter Factor (1/Trans) 08 Yellow, Light 0.83 1.2 21 Orange 0.46 2.7 25A Red 0.14 7.2 The following is what I think the answer is, but would like some confirmation from more experienced amateurs. Using the basic exposure equation: t1 = F1_ratio^2 / ( ISO_speed * Brightness_B ) and Covington's _Astrophotography for Amateurs_ Appendix A modification applying a filter factor representing the optical density of a filter: t1 = F1_ratio^2 / ( ISO_speed * Brightness_B / Filter factor1 ) and then dividing two equations, one for t1-Filter factor1 and a second for t2-Filter factor2, you can simplify to: t2 = F1_ratio^2 / ( ISO_speed * Brightness_B / Filter factor2 ) t1 = F1_ratio^2 / ( ISO_speed * Brightness_B / Filter factor1 ) t2/t1 = 1 / ( (1/Filter factor2) / (1/Filter factor1) ) = 1 / ( Transmission%2 / Transmission%1 ) t2 = t1 * ( Transmission%1 / Transmission%2 ) or alternatively, t2 = t1 * ( Filter factor2) / Filter factor1 ) So, for example, if I take a correctly exposed luminosity image (Filter-factor1 = 100%) at 100 secs, and then turn the filter wheel, the estimated equivalent exposure times should be: Filter Filter Factor t1 t2 No filter 1.0 100 100 08 Yellow, Light 1.2 100 120 21 Orange 2.7 100 270 25A Red 7.2 100 720 Does this look right? - Canopus56 (Kurt) __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com