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.

Aperture = inches
Telescope Power =
Visual Magnitude Limit =
Your Age =
    Telescope Type
  • Reflector
  • Refractor
  • Schmidt Cassegrain
Color Index of Star =
Zenith Distance = degrees
Extinction Coefficient =
Seeing Disk Diameter = arc seconds


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)
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
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 in seeing dim stars helps, but this is a qualitative parameter.