Telescope Light Grasp and Limiting Magnitude Calculator
Fill in the values and the answer updates as you type.
How the Telescope Light Grasp and Limiting Magnitude Calculator works
Light grasp scales with the area of the opening, so it equals (aperture ÷ pupil)². Each factor of 2.512 in light is one magnitude, so the gain in magnitudes = 5 × log10(aperture ÷ pupil). Telescope limiting magnitude = naked-eye limit + gain. The result is an ideal figure under steady, dark skies. Light pollution, optics quality, and observer experience lower it. Example: with Telescope aperture 200 mm, Dark-adapted pupil of your eye 7 mm, and Naked-eye limiting magnitude at your site 6 mag, the result is Light grasp compared with the eye (times) = 816; Magnitude gain = 7.3; Telescope limiting magnitude = 13.3.
Inputs and results
The Telescope Light Grasp and Limiting Magnitude Calculator takes 3 inputs and gives 3 results.
What you enter
- Telescope aperture (mm) — starts at 200.
- Dark-adapted pupil of your eye (mm) — starts at 7.
- Naked-eye limiting magnitude at your site (mag) — starts at 6.
What you get
- Light grasp compared with the eye (times)
- Magnitude gain
- Telescope limiting magnitude
Example: Telescope Light Grasp and Limiting Magnitude Calculator with the starting values
With telescope aperture 200 mm, dark-adapted pupil of your eye 7 mm and naked-eye limiting magnitude at your site 6 mag, the calculator returns:
| Light grasp compared with the eye (times) | 816 |
|---|---|
| Magnitude gain | 7.3 |
| Telescope limiting magnitude | 13.3 |
Change any value above and the results update straight away.
Telescope Light Grasp and Limiting Magnitude Calculator FAQ
How do I use the Telescope Light Grasp and Limiting Magnitude Calculator?
Enter telescope aperture, dark-adapted pupil of your eye and naked-eye limiting magnitude at your site in the fields, then read light grasp compared with the eye (times), magnitude gain and telescope limiting magnitude in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Telescope Light Grasp and Limiting Magnitude Calculator calculate?
How much more light a telescope gathers and the faintest star it can show. It returns light grasp compared with the eye (times), magnitude gain and telescope limiting magnitude from the 3 values you provide.
What values does the Telescope Light Grasp and Limiting Magnitude Calculator start with?
It opens with telescope aperture 200 mm, dark-adapted pupil of your eye 7 mm and naked-eye limiting magnitude at your site 6 mag so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Telescope Light Grasp and Limiting Magnitude Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Telescope Light Grasp and Limiting Magnitude Calculator for important decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm important decisions with a qualified professional. See the Disclaimer for more.