Planner · Astrophotography
Will your camera and lens actually frame the target — and for how long?
Pick a camera and focal length, choose a nebula or galaxy, and see the real field of view, pixel scale and the longest exposure before stars trail. Clear framing maths, sampling verdict and export — no guesswork, no account.
Camera & optics
sensor + lensTarget & mount
what you shootSensor frame & target
to scaleNumbers in detail
geometry| Quantity | Value | Note |
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What to do next
How the numbers are worked out
Every figure below is arithmetic you can check by hand. Nothing is fetched from a server.
Field of view
FOV = 2 · atan(sensor ÷ (2 × effective FL)) in degrees, done separately for width and height. Effective FL is focal length × reducer (0.8) or Barlow (2×). Diagonal is from the sensor diagonal √(w²+h²).
Pixel scale
Pixel pitch = sensor width ÷ image width × 1000 (µm). Image width is √(MP·10⁶·aspect). Pixel scale = 206.265 × pitch (µm) ÷ FL (mm) arcsec/px. Image scale = FOV ÷ image width °/px.
Framing & sampling
Framing = target ÷ FOV; target pixel width = target (arcsec) ÷ pixel scale. Sampling bands: 1–2″/px ideal deep-sky; below 0.5 oversampled; above 3 undersampled for planetary.
Trailing limits
Crop = 43.3 ÷ sensor diagonal. 500 rule (untracked) = 500 ÷ (FL × crop). NPF = (35 × f/ + 30 × pitch) ÷ FL. For fixed tripods the 500 value is the limit; for trackers/GoTo the NPF value is used and compared to your exposure.
What this is not: a geometry and sampling estimate; real results depend on seeing, guiding accuracy, mount quality and processing.
Questions
How do I calculate field of view for astrophotography?
FOV is 2 times arctan(sensor dimension divided by twice the focal length). Use the sensor width and height separately, so a full-frame 36×24 mm sensor at 500 mm gives about 4.1° by 2.7°. A reducer or Barlow changes the effective focal length first.
What is pixel scale and why does it matter?
Pixel scale is 206.265 times pixel pitch in microns divided by focal length in millimetres, in arcseconds per pixel. Around 1–2 arcsec/px suits most deep-sky work; below 0.5 is oversampled unless seeing is excellent, above 3 is undersampled for planetary.
What is the difference between the 500 rule and the NPF rule?
The 500 rule gives a quick untracked limit as 500 divided by focal length times crop factor. The NPF rule is (35×aperture + 30×pixel pitch) divided by focal length for a one-pixel trail and is a better guide for tracked shots.
How long should I expose each subframe?
Keep single exposures under the trailing limit for your mount — use the 500 rule on a fixed tripod and the NPF rule on a tracker or GoTo. Then stack many frames: about 1–4 hours total for bright nebulae, longer for faint galaxies.
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