Astrophotography Exposure Calculator
Calculate the recommended sub-frame exposure time for deep-sky astrophotography based on your telescope, camera, and sky conditions. Ideal for planning imaging sessions targeting galaxies, nebulae, or star clusters.
About this calculator
The formula combines three independent factors: image scale, target brightness, and sky transparency. Image scale (arcsec/pixel) is given by the Dawes-based relation: scale = 206.3 × pixel_size / focal_length. A larger image scale means each pixel covers more sky, affecting how sky glow accumulates. The target brightness term 10^((magnitude − 15) / 2.5) scales exposure relative to a magnitude-15 reference object — each 2.5-magnitude step multiplies required time by 10×. Finally, the sky condition multiplier sets a base exposure cap: 600 s for excellent skies, 300 s for good, 120 s for moderate, and 60 s for poor. The combined formula is: t = round(scale × brightness_factor × sky_multiplier). This gives a starting sub-exposure in seconds; you would stack many such frames to build total integration time.
How to use
Suppose you have a 900 mm focal-length refractor, 3.76 μm pixel camera, targeting a magnitude-12 galaxy under good skies. Step 1 — image scale: 206.3 × 3.76 / 900 ≈ 0.862 arcsec/pixel. Step 2 — brightness factor: 10^((12 − 15) / 2.5) = 10^(−1.2) ≈ 0.0631. Step 3 — sky multiplier: 300 s (good conditions). Step 4 — multiply: 0.862 × 0.0631 × 300 ≈ 16 seconds per sub-frame. You would then capture many 16-second subs and stack them to achieve sufficient signal-to-noise on the galaxy's faint outer arms.
Frequently asked questions
How does focal length affect astrophotography exposure time?
Focal length determines your image scale — how many arcseconds of sky each pixel captures. A longer focal length yields a smaller image scale, meaning each pixel collects light from a narrower patch of sky. This reduces sky-background signal per pixel, which can allow longer individual exposures before the sky overwhelms faint details. However, it also magnifies atmospheric seeing, making high focal lengths better suited to steady skies and bright targets like planets or globular clusters.
What sky conditions should I look for when planning a deep-sky imaging session?
Excellent skies have Bortle class 1–3, with limiting visual magnitudes above 6.5 and no light pollution — think remote dark-sky sites. Good skies are Bortle 4–5, typical of rural areas. Moderate skies (Bortle 6–7) are suburban, with limiting magnitudes around 5. Poor skies are urban Bortle 8–9. Each step down roughly halves the practical sub-exposure length before sky glow saturates the sensor, so choosing the right site dramatically increases what you can image in a given night.
Why does target magnitude change the recommended exposure time so much?
The magnitude scale is logarithmic: a difference of 2.5 magnitudes represents a factor of 10 in brightness. A magnitude-17 galaxy is 100× fainter than a magnitude-12 galaxy, so it requires roughly 100× more total photons to detect — achieved through longer individual exposures, more stacked frames, or both. This exponential relationship means chasing very faint targets (magnitude 18+) demands exceptional skies, large apertures, and many hours of total integration time, which is why amateur astronomers plan sessions so meticulously.