Telescope & Eyepiece Calculator
Magnification, true field of view, exit pupil, Dawes limit & limiting magnitude — plus an eyepiece ladder planner, a will-it-fit target checker with FOV visual, and a reverse eyepiece solver. Built for visual observers, from 60 mm refractors to 12 ″ Dobsonians.
01Your telescope + eyepiece
02Results — this combination
True field of view — visual
03Eyepiece ladder planner
Enter every eyepiece you own (or plan to buy). A good set climbs in even magnification steps — roughly 1.4–1.7× per rung — from a low-power finder to just under your scope's max useful magnification.
04Will it fit? — target vs your field of view
Checks whether popular targets fit inside the true field of view of your current combination (). Tap a target to see it drawn against your field circle.
05Reverse solver — "what eyepiece for 200×?"
Pick the magnification you want (planets ≈ 150–250×, Moon ≈ 60–100×, wide-field DSO = your scope's minimum useful), and get the eyepiece focal length that delivers it.
06The math, in one screen
New to the arithmetic? Here is how to calculate telescope magnification — and every number that hangs off it — using the telescope magnification formula plus its companions: the telescope exit pupil formula, the true-field formula, and the resolution rules. The practical ceiling people call max magnification is the "max useful" rule at the bottom.
07What magnification do I need to see the planets?
If you are asking what magnification you need to see planets like Jupiter and Saturn sharply, start from the sweet spots below — then let seeing decide how far you can push on the night.
| Target | Sweet-spot magnification | Notes |
|---|---|---|
| Moon (whole disk) | 20–40× | Needs TFOV ≥ ~0.7°. Crater detail shines at 80–150×. |
| Moon (craters) | 80–150× | Push higher near the terminator where shadows pop. |
| Jupiter | 100–200× | Belts and Great Red Spot resolve around 150× on steady nights. |
| Saturn | 100–250× | Cassini Division typically needs 150×+ and calm air. |
| Mars | 150–300× | Only rewarding near opposition; small disk otherwise. |
| Venus / Mercury | 100–200× | Phases are the show — observe in twilight to cut glare. |
| Double stars | 150–300× | The one case where pushing past 2×/mm is sometimes justified. |
| Deep-sky (galaxies/nebulae) | 30–100× | Brightness beats power: exit pupil 2–4 mm is the sweet spot. |
08Eyepiece apparent fields — quick reference
| Eyepiece type | Typical AFOV | Character |
|---|---|---|
| Huygens / Ramsden / Kellner | 40° | Vintage narrow; common in cheap kits. |
| Orthoscopic | 45° | Sharp planetary classic, tight eye relief. |
| Plössl | 50–52° | The workhorse. Great value, comfortable to ~25 mm. |
| Erfle / wide-field | 60–65° | Roomy low-power views. |
| Super-wide (SWA) | 70–72° | "Spacewalk" feel starts here. |
| Ultra-wide (UWA) | 82° | Huge immersive field; pricier, heavier. |
| Hyper-wide (XWA) | 100–110° | Maximum immersion; check your focuser can take the weight. |
| Zoom eyepiece | 40–60° | AFOV narrows at the long end — enter the value at your setting. |
09Starter ladders that actually work
Three rungs cover most nights; four is luxury. Each rung ≈ 1.5–1.7× the last.
| Scope | Ladder (eyepieces) | Magnifications |
|---|---|---|
| 80 mm f/7.5 refractor | 24 mm 68° → 12 mm 60° → 7 mm 60° | 25× → 50× → 86× |
| 150 mm f/8 Newtonian | 32 mm 50° → 20 mm 68° → 12 mm 68° → 8 mm 60° | 38× → 60× → 100× → 150× |
| 200 mm f/6 Dobsonian | 30 mm 82° → 18 mm 82° → 11 mm 82° → 7 mm 82° | 40× → 67× → 109× → 171× |
| 203 mm f/10 SCT | 40 mm 43° → 25 mm 60° → 15 mm 68° → 10 mm 68° | 51× → 81× → 135× → 203× |
✦Frequently asked questions
ⓘHonest limits of the math
- Limiting magnitude assumes dark, transparent skies — suburban light pollution can cost 1–3 magnitudes.
- Dawes and Rayleigh limits are theoretical; atmospheric seeing usually caps resolution around 1–2 arcseconds.
- TFOV = AFOV ÷ M is an approximation that slightly overstates very wide fields; good enough for planning.
- Exit-pupil "ideal" ranges are guidance, not law — personal preference and sky conditions decide.