Astrophotography Bortle Exposure Equivalency Calculator

What this estimate does and does not do

This calculator estimates how much total integration time you may need when moving an astrophotography project between two sky-brightness levels. You can compare Bortle classes or enter measured SQM values in mag/arcsec². The result is most useful for broadband deep-sky imaging where sky background noise is the main limitation.

It does not guarantee that two images will look the same. Light pollution changes gradients, colour balance, contrast, and processing difficulty. Moon phase, haze, target altitude, transparency, local lights, optics, f-ratio, camera gain, filters, calibration frames, dithering, and processing can overwhelm a simple Bortle comparison.

Formula / method used

The calculator converts SQM magnitude difference into a linear sky-brightness ratio. Astronomical magnitudes are logarithmic, so a 1 mag/arcsec² brighter sky is about 2.512 times brighter.

If the target sky has a lower SQM number than the reference sky, it is brighter and the ratio is greater than 1. If the target sky is darker, the ratio is less than 1.

For sky-background-limited imaging of the same target with the same setup, SNR scales approximately like:

So the integration time for equivalent SNR scales approximately with the sky brightness:

The filter profile modifies the raw sky ratio with a simple sensitivity blend:

This is intentionally conservative and approximate. Broadband uses 100% sensitivity. Narrowband profiles use lower sensitivity because they reject much of the skyglow, but the adjustment depends heavily on the target, filter bandwidth, spectrum of the local light pollution, and moonlight.

Worked example

Suppose you have a 4 hour broadband image from a Bortle 4 site and want to estimate the broadband time needed from a Bortle 7 site using the built-in SQM midpoints:

That does not mean Bortle 7 imaging is impossible. It means that, for faint broadband signal, the brighter sky can require dramatically more total time and more careful gradient handling.

If the same comparison is for a dual-band emission-nebula setup, the calculator reduces the sky sensitivity. With the default 35% dual-band profile:

That is still only a planning estimate. A dark site can remain much better for broadband colour, dust, reflection nebulae, galaxies, and faint IFN.

How to use the result

• Use Bortle classes when you only have map estimates.
• Use measured SQM values when you have a Sky Quality Meter or trustworthy local readings.
• Keep the reference and target setup the same: same lens/telescope, f-ratio, camera, gain/ISO, filters, and target.
• Use broadband sensitivity for galaxies, reflection nebulae, dust, star fields, and luminance.
• Use dual-band or narrowband only for emission-line targets where the filter actually rejects most unwanted skyglow.
• Treat the sub-exposure result as a “same sky-background histogram” hint, not an instruction.
• Check real test subs for clipping, histogram placement, read noise, and star saturation.

Assumptions and limitations

• Bortle-to-SQM values are representative midpoints, not exact measurements.
• SQM usually measures a wide patch of sky and may not match your target direction.
• The model assumes background-limited imaging. Very short subs, high read noise, strong dark current, or narrow filters can break the simple scaling.
• Narrowband reduction is a heuristic, not a physical filter-transmission model.
• The model does not include focal ratio changes. If you change f-ratio, exposure time also scales roughly with f-ratio squared.
• It does not predict dynamic range, star saturation, gradients, colour cast, seeing, tracking, or processing difficulty.

For other hobby planning tools, see the Cross-Stitch Fabric and Thread Calculator and the Wood Movement Predictor.