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Astrophotography - Calculator

Star Trails
The star trail method involves placing the camera onto a tripod and opening the shutter for an extended period of time. During the course of the exposure the Earth's rotation carries stars through the field of view causing them to trail on the film. The lens should be focused at infinity and the aperture set to its widest opening. This method works well for meteor shower photography as well as auroral photography. Where you aim the camera in the sky, and the focal length of the lens you use determines how soon the stars will begin to trail.

INPUT values
Focal Length*:  mm
Declination*:  deg

OUTPUT value
Stars will begin to trail in:  sec


Field of View
The field of view in an image is dependant upon the height and width of the film frame as well as the focal length of the lens or telescope used.
INPUT values
Focal Length*:  mm
Imaging Area*:

OUTPUT value
Field of View:  deg


Image Size at Prime Focus
When taking photographs of objects such as the Moon, Sun or planets it is useful to calculate the final image size of the object on the film. This is for the camera mounted at the prime focus of the optical system.
INPUT values
Focal Length*:  mm
Apparent diameter(avg)*:


OUTPUT value
Image Size(Prime Focus):  mm


Exposure Duration
The most important factor determining the exposure is the duration. The duration is dependant upon the focal ratio of the lens/telescope, the ISO rating of the film used, and the object which you are taking an image of. For deep sky objects (i.e. nebulae, star clusters, galaxies, etc.) exposure times vary significantly. It is also a good practice to "bracket" exposures around the recommended duration to ensure that at least one image will be properly exposed.
INPUT values
Focal Length*:  mm
ISO rating*:

Exposure factor*:


OUTPUT value
Exposure Duration:  sec


Eyepiece Projection
Eyepiece projection allows for larger image sizes recorded on the film. It is dependent upon the focal length of the projection eyepiece employed, the focal length of the telescope, and the projection distance (from the eyepiece to the film plane). Values obtained in this section may be used in the above sections.
INPUT values
Focal Length*:  mm
Aperture*:  mm
Focal length eyepiece*:  mm
Projection distance*:  mm

OUTPUT values
Effective focal length:  mm
Effective focal ratio:  
Magnification:  x

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