Telescope Eyepiece Calculator
Pick your telescope (or enter its aperture and focal length), add the eyepieces you own or are considering, and see the magnification, exit pupil, and true field of view for each. Add a Barlow or the f/6.3 reducer to see every combination at once. The table flags duplicates (a 25mm with a 2x Barlow is about the same as a 12mm) and gaps in your set. Your settings are saved in the page address, so you can bookmark or share them.
- Telescope
- Celestron NexStar 6SE: 150mm aperture, 1500mm focal length, f/10
- Max useful magnification
- ~300x (2 × aperture in mm). Celestron lists 354x. Most nights the atmosphere allows ~200–250x.
- Min useful magnification
- ~21x (7mm exit pupil)
- Widest true field
- 1.03° with 1.25" eyepieces
Choosing eyepieces? See the best eyepieces for the 6SE.
| Eyepiece | Magnification | Exit pupil | True field | Moon widths | Best for | Notes |
|---|---|---|---|---|---|---|
| 32mm (50°) | 47x | 3.2mm | 1.03° | 2.1 | Deep sky, Moon | Field limited by the 1.25" barrel (~27mm field stop). A longer eyepiece won't show more sky, just a smaller image of the same field. |
| 25mm (50°) | 60x | 2.5mm | 0.83° | 1.7 | Deep sky, Moon | – |
| 12mm (60°) | 125x | 1.2mm | 0.48° | 1 | Moon, Planets | – |
| 9mm (60°) | 167x | 0.9mm | 0.36° | 0.7 | Planets, Double stars | – |
Gaps in your set
- Gap between 60x and 125x: an eyepiece around 17mm (~87x) would fill it.
The formulas
- Magnification = telescope focal length ÷ eyepiece focal length
- Effective focal length = telescope focal length × Barlow (or × 0.63 with the f/6.3 reducer)
- Exit pupil (mm) = aperture ÷ magnification
- True field ≈ apparent field ÷ magnification, or field stop ÷ focal length × 57.3 when the field stop is known
- Max useful magnification ≈ 2 × aperture in mm; min useful ≈ aperture ÷ 7
For a full walk-through with worked examples, see how to calculate telescope magnification.
What the warnings mean
- Above max useful magnification: the image gets bigger but not more detailed, and looks dim and soft.
- Seeing limit: most nights, turbulent air limits useful power to about 200–250x whatever the telescope.
- Exit pupil over 7mm: the beam is wider than a dark-adapted pupil, so some light is wasted.
- Exit pupil under 0.5mm: very dim, and floaters in your eye become distracting.
- Field limited by the barrel: a 1.25" eyepiece can’t have a field stop much over 27mm, so longer eyepieces don’t show more sky.
- Baffle vignetting: Schmidt-Cassegrains have a rear baffle tube (about 27mm on a C6, 37mm on a C8). Wider fields get darker edges.
Telescope presets
Key numbers for each preset. Values are from the makers’ spec pages; the Dobsonian rows are typical of popular 8" and 10" models.
| Telescope | Aperture | Focal length | Focal ratio | Max useful (2x rule / maker) | Widest 1.25" field |
|---|---|---|---|---|---|
| Celestron NexStar 4SE | 102mm | 1325mm | f/13 | 204x / 241x | 1.17° |
| Celestron NexStar 5SE | 125mm | 1250mm | f/10 | 250x / 295x | 1.24° |
| Celestron NexStar 6SE | 150mm | 1500mm | f/10 | 300x / 354x | 1.03° |
| Celestron NexStar 8SE | 203mm | 2032mm | f/10 | 406x / 480x | 0.76° |
| Celestron NexStar Evolution 6 | 150mm | 1500mm | f/10 | 300x / 354x | 1.03° |
| Celestron NexStar Evolution 8 | 203mm | 2032mm | f/10 | 406x / 480x | 0.76° |
| Celestron NexStar Evolution 9.25 | 235mm | 2350mm | f/10 | 470x / 555x | 0.66° |
| Sky-Watcher Skymax 127 | 127mm | 1500mm | f/11.8 | 254x / 254x | 1.03° |
| Celestron StarSense Explorer DX 130AZ | 130mm | 650mm | f/5 | 260x / 307x | 2.38° |
| 8" Dobsonian (typical f/5.9) | 203mm | 1200mm | f/5.9 | 406x | 1.29° |
| 10" Dobsonian (typical f/4.7) | 254mm | 1200mm | f/4.7 | 508x | 1.29° |
FAQ
How do I calculate telescope magnification?
Divide the telescope’s focal length by the eyepiece’s focal length. A 1500mm telescope with a 25mm eyepiece gives 1500 ÷ 25 = 60x. A Barlow multiplies the telescope’s focal length first: with a 2x Barlow it becomes 3000 ÷ 25 = 120x.
What is exit pupil?
The width of the beam of light leaving the eyepiece: aperture ÷ magnification. Between about 2mm and 4mm images look bright and comfortable. Above 7mm some light is wasted; below about 0.5mm images get dim and floaters show.
How is true field of view calculated?
Approximately, apparent field ÷ magnification. If the eyepiece’s field stop is published, field stop ÷ focal length × 57.3 is more exact. The calculator caps 1.25-inch eyepieces at a 27mm field stop and 2-inch at 46mm.
What is the maximum useful magnification of a telescope?
About 2x the aperture in millimeters (300x for a 150mm scope). Some makers quote higher figures. In practice the atmosphere limits most nights to around 200–250x.