Deep-Sky Exposure Planner

Translate your detector characteristics and sky brightness into a practical deep-sky imaging plan. The calculator selects a sub-exposure that swamps read noise, then estimates the light, dark, and flat frame counts required to hit your target stacked SNR.

Expected photo-electrons per second from your target object.
Photo-electrons per second contributed by sky glow at your site.
Root-mean-square read noise per frame in electrons (e⁻ RMS).
Leave blank to target an SNR of 30 in the stacked image.
Multiplier describing how strongly sky noise should swamp read noise (5× is a common target).

Examples

  • Bright emission nebula in Bortle 5 skies aiming for SNR 40 with a 6× swamp factor ⇒ Sub-exposure 68 s, Lights 62
  • Faint galactic dust under dark skies targeting SNR 25 ⇒ Sub-exposure 34 s, Lights 78

FAQ

What if my camera saturates before the suggested sub-exposure time?

Lower the swamp factor or desired SNR until the histogram peak sits comfortably below saturation—full-well limits are outside this simplified model.

How do I estimate signal and sky rates?

Capture short test exposures, convert ADU per second to electrons using your camera gain, or pull the values straight from acquisition software that reports e⁻/s.

Why cap the sub-exposure at 15 minutes?

Most mounts, filters, and guiding setups struggle beyond that duration, and bright stars begin to clip. The cap keeps the guidance within realistic tracking limits.

Can I change the number of flats?

Absolutely—treat 25 as a starting point. Increase the count if your workflow benefits from lower flat-field noise.

Additional Information

  • Sub-exposure times are clamped between 30 seconds and 15 minutes to prevent impractical recommendations.
  • Calibration frames are assumed to match the light-frame exposure length so dark current scales correctly.
  • Dark frames default to roughly 20% of the light count, while flats stay fixed at 25 for a balanced master flat.
  • Stacked SNR assumes uncorrelated noise and ideal calibration; guiding drift or gradients will lower real-world results.

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