Instructions: This calculator may be used to determine the length of star trails in an image due to field rotation which occurs with a given polar alignment error and exposure time. For more information on the mathematical model behind this calculator see my article on the subject. Alignment Error: Enter the total polar alignment error in arcminutes. If you know this value in arcminutes and arcseconds enter it as MM.SS and click "Convert MM.SS". Guide Star Angle: Enter the approximate worst case angle from the guide star to the farthest edge of the field of view in degrees. If you know this angle in arcminutes, enter the value and click "Convert MM". If you know the angle in degrees, arcminutes and arcseconds enter it as D.MMSS or D.MM and click "Convert D.MMSS". Focal length: Enter the optical focal length in millimeters. Time: Enter the total time of the exposure in decimal minutes. If you know the seconds enter that number and click "Convert Secconds" If you know the minutes and seconds enter it as MM.SS and click "Convert MM.SS". Declination: Enter the declination of the target in decimal degrees. If you know the degrees, arcminutes, and arcseconds enter it in the form DDD.MMSS or DD.MM and click "Convert DDD.MMSS". Calculate: When you have entered and converted all your inputs to the proper units, click "Calculate." The Field Rotation will be calculated and displayed in the field below.
