Calculate Maximum Focal Length for Polar Alignment Error

Alignment Error arcmins
Guide Star Angle degrees
Time minutes
Declination degrees
Field Rotation microns

Maximum Focal Length millimeters

This calculator may be used to determine the maximum focal length 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 focal length of the instrument is within limits for a chosen guide star. 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".
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 Focal Length will be calculated and displayed in the field below.