Comet Surface Brightness

Estimate how bright a comet will appear per square arcsecond so you can match optics, exposure length, and sensor sensitivity. The calculation scales the integrated magnitude by coma size to express surface brightness in mag/arcsec².

Photometric absolute magnitude H of the comet.
Distance from the comet to the Sun in astronomical units (AU).
Distance from the comet to Earth in astronomical units (AU).
Coma brightening exponent n; leave blank to assume 4, typical for long-period comets.
Angular diameter of the diffuse coma in arcminutes; default assumes 10′.

Examples

  • Long-period comet at 1.1 AU with a 12′ coma and n = 3.8 ⇒ 21.14 mag/arcsec²
  • Periodic comet at 1.8 AU sporting a compact 6′ coma ⇒ 26.32 mag/arcsec²

FAQ

What if the coma is asymmetric?

The model assumes a circular coma. If you see strong asymmetries, treat the number as an average and adjust your imaging plan based on the brighter quadrant.

How should I choose the activity index n?

Most dynamically new comets brighten with n between 3 and 4. Short-period comets with dust mantles trend closer to n = 2–3.

Does this include the central condensation or nucleus?

The calculation returns the mean coma surface brightness. The central condensation is typically a magnitude or more brighter within a small aperture, so plan exposures accordingly.

Additional Information

  • Total apparent magnitude follows m = H + 5 log₁₀(Δ) + 2.5 n log₁₀(r), letting you plug in published H and n values.
  • Surface brightness divides the integrated light by the coma's projected area in arcseconds squared.
  • A shrinking coma dramatically increases surface brightness even when the integrated magnitude barely changes.
  • Tail contributions, jets, and anisotropic outbursts are excluded—treat the output as an average coma surface brightness.

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