What is narrowband astrophotography?

When shooting deep sky objects in light-polluted skies, you need to filter out the light frequencies of the light-pollution. To do this, you use narrowband filters. Narrowband filters allow only narrow bands of selected light bands to go thru the filter eliminating the light-pollution but picking up the nebula light. To create a color RGB image different filter bands are used, one for H-alpha (H-α), one for Sulphur II, and one for Oxygen III emissions. These are the selected frequencies of light captured by the camera.

Narrow band light bands
Example of filter bands used in narrowband imaging

Using a monochrome camera, one created for astrophotography, several long-exposure photographs of the same deep sky object are taken in each light band. All the monochrome narrowband image data is combined into a single RGB false-color image using a program such as Adobe Photoshop. Because the narrowband data is not in the same frequencies as a regular RGB image we map the captured data to RGB channels in the following manner. The Sulfur image data is mapped to the Red channel, the H-α image data is mapped to the Green channel, and the Oxygen data is mapped the the Blue channel of the RGB image to create the famous Hubble Palette of false-color astronomical images. To see more detailed information on the subject, see this Wikipedia page.

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