The primary purpose of Sharp Stars is to determine the longest exposure time that will produce stars that appear sharp when printed.
When stars are photographed using either a telescope/camera or camera alone they appear to have a trail due to the Earth's rotation, unless a tracking device is used. Sharp Stars calculates the length of this trail in pixels.
Inputs to Sharp Stars include focal length, exposure time, camera type (i.e. Full Frame DSLR, m4/3, 1.5 Crop...), pixels, and optionally declination.
Sharp Stars calculations are based entirely on the rotation of the earth.They are not based on arbitrary rules such as the rule of 500. In addition to length of the star trail in pixels the length is also shown in mm. The program compares the length in mm to the camera's standard of blur (the "circle of confusion"). In this way the user can quickly determine if the exposure time is reasonable. Outputs of the program also include the horizontal and vertical angles and the circle of confusion (CoC) for the camera. When the star trail length is less than the CoC the result is shown in varying shades of green, and when greater in varying shades of red. Darker greens indicate shorter trails, and darker red longer trails.
Finally the Sharp Stars help file indicates some methods that can be used to make your trails shorter.
A secondary purpose is to display the horizontal and vertical angles of view for a given camera/lens combination.