Determination of the Limiting Magnitude of a Telescope

The parameters below all effect the limiting magnitude of the dimmest star that you can see through your telescope. Enter the values apropos to your situation to get the value for your telescope. Change the values of each to learn the effect of each parameter.

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Aperture = inches Telescope Power = Visual Magnitude Limit = Your Age = Telescope Type Reflector Refractor Schmidt Cassegrain Cleanliness DIRTY CLEAN Color Index of Star = Zenith Distance = degrees Extinction Coefficient = Seeing Disk Diameter = arc seconds Experience NOVICE EXPERT LIMITING MAGNITUDE = EXIT PUPIL DIAMETERS(in mm) TELESCOPE:______ EYE: This program by Bradley Schaefer calculates the limiting stellar magnitude an observer can expect to see with various types and sizes of telescopes, and under various conditions. It is fully discussed in SKY & TELESCOPE magazine, November, 1989, page 522. The program was originally written in Basic and has been converted to Java Script here by Larry Bogan. © May, 1998

Explanation of Parameters Telescope Aperture The diameter of the objective lens or mirror. (This is the most important determining parameter) Power The magnifying power of the telescope. This changes with the focal length of the eyepiece you use. Power = (focal length of the objective)/(focal length of the eyepiece) Visual Limiting Magnitude This is the magnitude of the dimmest star you can see near the zenith. Use the link to vislimit.html page to calculate this for you OR: See the section called Sky Transparency in "The Observer's Handbook" published by the Royal Astronomical Society of Canada for a guide to determining this using the stars around Polaris. Type of telescope Reflector has a mirror as its objective Refractor has a lens as its objective The Schmidt-Cassegrain has a lens and a mirror Cleanliness This is a qualitative evaluation - check the relative value for your telescope Color Index of the star This is the blue minus the visual magnitude. The bluer a star, the more negative is this value. You can find the CI of stars in atlas handbooks The Sun: yellow type G2: CI = 0.63 Betelgeuse: very red: M2 type star: CI = 1.85 Sirius: whitish' star: A0 type:CI = 0.0 Regulus: bluish B7 type: CI=-0.11 Zenith Distance The angle in degrees from the vertical direction Extinction Coefficient Typically 0.2-0.6 mag/airmass, but use the Vislimit.html Javascript page to calculate the extinction coefficient for your situation. Seeing Disk in arc seconds. The diameter of the image of a star. This varies on the turbulence of the atmosphere and can vary from 0.5 to several seconds of arc. You have to judge this at the telescope. Experience Experience in seeing dim stars helps, but this is a qualitative parameter.