--- Canopus56 <canopus56@yahoo.com> wrote: <snip all> P.S. - In the back of Appendix B, Covington also includes some java code for his declination drift equations and for field rotation. The java rountine in the book is distributed through his website with a note that it can be freely redistributed. Here it is: // Polar.java - M. Covington 1997 /* To accompany Astrophotography for the Amateur, 1998 edition. You may freely redistribute this program. */ /* Snip input output routines */ // THE COMPUTATION ITSELF // Library functions and constants double cos(double x) { return Math.cos(x); } double sin(double x) { return Math.sin(x); } double acos(double x) { return Math.acos(x); } double asin(double x) { return Math.asin(x); } double square(double x) { return x*x; } double sqrt(double x) { return Math.sqrt(x); } double abs(double x) { return Math.abs(x); } double DR = 180/Math.PI; // degrees-to-radians factor, ~57.3 double decdrift( // Declination drift, in arc-seconds, computed from: double e, // Polar misalignment, in degrees double h, // Hr angle of star - hr angle of false pole, degrees double delta, // Declination of star, in degrees double t) // Duration of exposure, in minutes of time { double x, y, z; // local variables e = e/DR; // convert to radians h = h/DR; delta = delta/DR; t = (t/4)/DR; x = sin(delta)*cos(e); // core of computation y = cos(delta)*sin(e); z = acos(cos(h+t)*y + x) - acos(cos(h)*y + x); z = z * DR * 3600; // convert to arc-seconds if (abs(z) < 0.0001) z = 0.0; // discard if too small (imprecise) return z; } double rotation( // Field rotation, in degrees double e, // Polar misalignment, in degrees double h, // Hr angle of star - hr angle of false pole, degrees double delta, // Declination of star, in degrees double t) // Duration of exposure, in minutes of time { double w, x, y, z; // local variables e = e/DR; // convert to radians h = h/DR; delta = delta/DR; t = (t/4)/DR; w = sin(e); // core of computation x = sin(delta)*cos(e); y = cos(delta)*w; z = asin(w*sin(h+t) / sqrt(1 - square(cos(h+t)*y + x))) - asin(w*sin(h) / sqrt(1 - square(cos(h)*y + x))); z = z * DR; // convert to degrees if (abs(z) < 0.0001) z = 0.0; // discard if too small (imprecise) return z; } - Best of luck - Canopus56(Kurt) __________________________________ Yahoo! Mail - PC Magazine Editors' Choice 2005 http://mail.yahoo.com