Telescope Magnification
Calculate telescope magnification, true field of view, exit pupil, and resolution from focal length, eyepiece, and apparent field of view.
Inputs
mm
mm
°
Results
Magnification
48×
True Field of View
1.083°
Exit Pupil
3.13 mm
True FOV65 arcmin
Dawes Limit0.77 arcsec
Max Useful Magnification300×
Min Useful Magnification21.4×
Light Gathering Power459.2× eye
Assumed Aperture (f/8)150 mm
How to Use This Calculator
- Enter your Telescope Focal Length (mm) — typically printed on the tube or in the manual.
- Enter the Eyepiece Focal Length (mm) of the eyepiece you are inserting — shorter eyepieces give higher power.
- Enter the Apparent Field of View (°) of the eyepiece — standard Plössl ~52°, wide-angle ~68°, ultra-wide ~82°.
- Read Magnification (×) and True Field of View (°) — aim for a magnification where Exit Pupil is 2–5 mm for planetary work.
- Compare with Max Useful Magnification (2× aperture in mm) and Min Useful Magnification to stay within the telescope's effective range.
Ad Placeholder
Formula
Mag = f_telescope / f_eyepiece; TFOV = AFOV / MagRelated Calculators
Telescope Limiting Magnitude
Calculate the faintest star visible through a telescope based on aperture and sky brightness conditions.
Astrophotography Exposure
Calculate the maximum untracked exposure time to avoid star trails using the NPF rule, 500 rule, and declination correction.
Star Magnitude Calculator
Calculate absolute magnitude from apparent magnitude and distance using the distance modulus formula. Includes luminosity ratio and parallax.
Light Travel Time
Calculate how long light takes to travel a given distance. Enter distance in AU or km and get travel time in seconds, minutes, hours, days, or years.
Ad Placeholder