Choosing your filter in astrophotography: Duo-band, Narrowband, LRGB

Guide to choosing the best filter based on your sky (Bortle) and your target (Galaxies vs Nebulae).

Fighting light pollution

The choice of a filter is determined by two factors: your camera type (OSC vs Mono) and the chemical nature of your celestial target.

Broadband Filters (LRGB)

Ideal for Galaxies and Star Clusters. They let the entire spectrum through, but are very sensitive to spatial and urban pollution. Best used under Bortle 1-4 skies.

Duo-band Filters (OSC)

The salvation for color sensors in the city. By only letting through H-Alpha and OIII, they allow photographing nebulae even under Bortle 8-9 skies.

The Hubble Palette (SHO)

For monochrome cameras, using Sulphur II, H-Alpha, and Oxygen III filters allows reconstructing NASA's famous colors.

Impact on exposure time

Warning: the more selective (narrowband) a filter is, the more you will need to lengthen your individual exposures to pierce your sensor's read noise.