Instructions: This calculator may be used to determine the maximum guide star angle which can be use with a given polar alignment error while keeping field rotation within the tolerance specified. It is particularly useful when a guidescope is being used to test if the guide star chosen will cause any noticeable field rotation for the given 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". 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". Field Rotation: Enter the maximum star trail length tolerable in microns. Rule of thumb: 1/3rd the smallest star size produced by your optics, or just use the pixel size of your camera. If you know the angle in degrees, arcminutes and arcseconds enter it as D.MMSS and click "Convert D.MMSS". Calculate: When you have entered and converted all your inputs to the proper units, click "Calculate." The Maximum Guide Star Angle will be calculated and displayed in the field below.
