Telescope calculators
Every one of these runs in the browser, updates as you type, and shows the formula it used. Astronomy is one of the few hobbies where the maths genuinely changes what you buy, because aperture, focal length and focal ratio set hard limits that no amount of magnification can move.
7 pages in this section
- Focal Ratio CalculatorCalculate the focal ratio of any telescope from focal length and aperture, see what a Barlow or reducer changes, and understand... Open →
- NPF Rule Exposure CalculatorCalculate the longest untracked exposure before stars trail, using the NPF rule and the older 500 rule, with pixel pitch, aperture... Open →
- Telescope Eyepiece CalculatorPlan a complete eyepiece set for your telescope using exit pupil rather than guesswork, with magnification, true field and the... Open →
- Telescope Field of View CalculatorCalculate true field of view for any eyepiece and telescope, or the sky coverage of any camera sensor, in degrees and arcminutes... Open →
- Telescope Magnification CalculatorWork out the magnification, exit pupil and true field of any telescope and eyepiece pairing, and see the maximum useful... Open →
- Telescope Mount Payload CalculatorWork out whether a mount can actually carry your telescope, with separate visual and imaging limits, accessory weight and a... Open →
- Telescope Size CalculatorWork out what aperture your targets actually need, what it gains you over what you have, and what that size costs in weight... Open →
Start here if you are new
- How to choose a telescope, the whole decision in one page
- Complete builds at three budgets, with running totals
- Magnification calculator, the number everyone gets wrong
- What you can see by aperture, honestly