Calculate Maximum Exposure Time for Polar Alignment Error

Alignment Error arcmins
Guide Star Angle degrees
Focal Length millimeters
Declination degrees
Field Rotation microns

Maximum Exposure Time minutes

This calculator may be used to determine the longest exposure time which can be use with a given polar alignment error while keeping field rotation within the tolerance specified. 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.
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 Exposure Time will be calculated and displayed in the field below.