Astrophotography
Field of view and image scale calculator
Works out how much sky your camera and telescope actually see, how much sky one pixel covers, and whether that pixel size is well matched to a typical British night — the two numbers that decide whether a setup can photograph a given target at all.
The question this answers: “Will the Andromeda Galaxy fit in my frame with this telescope, and are my camera’s pixels the right size for it?”
Result
- Image scale
- 1.29″ per pixel
- How much sky one pixel covers. The number that decides how much detail is recordable.
- Field of view
- 2.24° × 1.5°
- That is 135′ × 90′. The full Moon is about half a degree across.
- Well-sampled range for this seeing
- 1–1.5″ per pixel
- Untracked exposure, 500 rule
- 0.8 s
- A rule of thumb, not a measurement. See the note below.
Worth knowing about this answer
- At 1.29″ per pixel this sits in the conventional well-sampled range of 1–1.5″ for 3″ seeing.
- Seeing is the limit far more often than the optics are, and it changes from night to night and hour to hour. A setup that is well sampled on a steady night is oversampled on a turbulent one.
- The untracked exposure figure is the “500 rule”, which is a rule of thumb with no authority behind it. It was calibrated in the era of prints, it takes no account of declination — stars near the celestial pole move far less than stars on the equator — and it is visibly optimistic when you examine a modern sensor at full magnification. Treat it as a starting point and halve it if the stars are not round.
Show the working
- Image scale
206.265 × 3.76 µm ÷ 600 mm1.293″/px - Field width
2 × atan(23.5 ÷ (2 × 600))2.244° - Field height
2 × atan(15.7 ÷ (2 × 600))1.499° - Untracked exposure
500 ÷ 6000.8 s
Image scale: 1.29″ per pixel. Field of view: 2.24° × 1.5°. Well-sampled range for this seeing: 1–1.5″ per pixel. Untracked exposure, 500 rule: 0.8 s
Image scale in arcseconds per pixel is 206.265 multiplied by the pixel size in micrometres and divided by the focal length in millimetres — 206,265 being the number of arcseconds in a radian, scaled for the change of units. Field of view is twice the arctangent of half the sensor dimension over the focal length. The well-sampled range is taken as a third to a half of the seeing disc per pixel.
- The sampling target of one third to one half of the seeing disc per pixel is a widely used convention approximating Nyquist sampling of a star profile. It is a rule of thumb, not a theorem, and reasonable people set it slightly differently.
- Seeing is treated as a single number for the night. In reality it varies hour to hour and with altitude above the horizon, so a setup that is well sampled at midnight may not be at three.
- Focal length is taken as the effective figure. Reducers and Barlow lenses change it, and entering the native value with a reducer fitted will give the wrong answer for everything.
- The field of view is the full sensor. Vignetting, an undersized field flattener or a small focuser can all mean the usable frame is smaller than the sensor.
- The untracked exposure figure is the “500 rule”, which is folklore with a real basis and no authority behind it. It ignores declination entirely — stars near the pole move far less than stars on the celestial equator — and it was calibrated when photographs were printed rather than examined at full magnification. Halve it if the stars are not round.
- It says nothing about whether a target is bright enough, whether the mount can track well enough, or whether the sky is dark enough. Those decide more images than framing does.
- Image scale is a property of the setup, not of the result. Poor focus, tracking error and turbulence all mean the delivered resolution is worse than the arithmetic suggests, and usually by a lot.
Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-14. More on Astrophotography.