Telescope magnification and resolution calculator
Aperture sets what a telescope can resolve and how faint it can see. Magnification is just eyepiece choice.
degrees
Magnification
120.0
- Focal ratio
- 6.00
- Exit pupil Above about 7 mm you waste light; below 0.5 mm the image dims and blurs.
- 1.667
- True field of view
- 0.4333 degrees
- Dawes resolving limit
- 0.580 arcseconds
- Rayleigh criterion
- 0.690 arcseconds
- Limiting visual magnitude
- 14.21
- Maximum useful magnification About two times the aperture in mm; the atmosphere usually stops you first.
- 400
- Light grasp against the naked eye
- 816
Show the working
- Aperture = 200
- Telescope focal length = 1200
- Eyepiece focal length = 10
- Eyepiece apparent field = 52
- Magnification = 1200 / 10 = 120
- Focal ratio = 1200 / 200 = 6
- Exit pupil = 200 / 120 = 1.666 67
- True field of view = 120 > 0 ? 52 / 120 : 0 = 0.433 333
- Dawes resolving limit = 200 > 0 ? 116 / 200 : 0 = 0.58
- Rayleigh criterion = 200 > 0 ? 138 / 200 : 0 = 0.69
- Limiting visual magnitude = 2.7 + 5 * log10(200) = 14.2051
- Maximum useful magnification = 200 * 2 = 400
- Light grasp against the naked eye = (200 / 7)^2 = 816.327
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
Scenarios & local history
Compare a saved scenario
Recent calculations
The formula
- magnification = telescope focal length / eyepiece focal length
- exit pupil = aperture / magnification
- Dawes limit in arcseconds = 116 / aperture in mm
- true field = apparent field / magnification
On most nights atmospheric seeing limits resolution to 1 to 2 arcseconds regardless of aperture.
Limiting magnitude also depends heavily on sky brightness; the formula assumes a dark site.
Related calculators
- Field of view
- Thin lens
- Tethys gravity and escape velocity
- the Moon gravity and escape velocity
- the Sun gravity and escape velocity
- Titan gravity and escape velocity
- Titania gravity and escape velocity
- Triton gravity and escape velocity
- Uranus gravity and escape velocity
- Venus gravity and escape velocity
- Vesta gravity and escape velocity
- Weight and gravity anywhere
- All astronomy & space calculators →