The Astrophotography Manual by Chris Woodhouse

Author:Chris Woodhouse
Language: eng
Format: epub
ISBN: 9781351658928
Publisher: Routledge

Dark Frames

The purpose of a dark frame is to generate the same amount of dark current as is present in the light frame. For this to be accurate, the exposure has to be taken on the same sensor and have:

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no light

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same sensor temperature as the light frame

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same exposure duration as the light frame

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same binning level as the light frame

Each dark frame exposure captures the dark current, bias and associated random noise. The random noise is reduced by combining many dark frame exposures to form an average dark file. (This process is called integration by some applications. In its simplest form it is a computed average value but can also be a median or a more advanced statistical evaluation of the corresponding pixel values.) The dark current is isolated by subtracting the master bias pixel values from the average dark pixel values. The result is saved as the master dark file (fig.4) Take note that some applications, like PixInsight, do not subtract the master bias at this stage.

In practice several sets of dark frames are taken, for different combinations of exposure time and temperature and the software calculates intermediate values by scaling for temperature and exposure duration.

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