Astro-image Processing

Noise occurs randomly across multiple exposures but the signal always stays the same.

Signal can be improved by prolonging exposure (that is limited by thermal signal & tracking constraints) or shooting multiple shorter exposures and stacking them together, which is usually the better option.

For longer exposures, good to automatically reduce noise in-camera, but for shorter exposures, it is better to turn this feature off.

Stacking light frames improves signal to noise ratio . Adding increases signal, while averaging reduces noise. Both improve the S/N ratio. Averaging is generally a better option between the 2 options.

Subtracting dark frames removes thermal & bias (sensor) signals.


Using bias frames helps generate scaleable darks. Thermal signals scale with exposure but bias does not. Therefore, by removing the bias from the darks, it becomes possible for darks of any given exposure to scale to a light frame of another exposure, as long as the ISO and temperature are the same. When using bias frames, it is best to scale longer exposure darks to shorter expsoure lights, instead of the other way around. Using bias frames is not required, if the darks and the lights are of the same exposure, since by definition the dark frame subtraction will remove both thermal & bias signals.

Using flat field frames helps remove uneven illumination, vignetting & dust spots.



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http://deepskystacker.free.fr/english/index.html


For most instances, stacking multiple dark frames to create a "master" dark frame for a given temperature and then subtracting the master dark from each individual light frame will generate optimal results.

Light frames: Actual image

Dark frames: same temperature, exposure (aperture & shutter speed) & ISO as lights, but with lens cap attached. These will record thermal noise from long exposures

Bias frames: same temperature, aperture & ISO, but with the shortest shutter speed. Essentially very short dark frames, to identify intrinsic but undesirable signals with the sensor that is different from the thermal signals.

Flat frames: images with light diffusers to remove vignetting and even out the darkness

Steps 1-4: Full calibration involves creating master bias, dark and flat frames, plus a set of dark flats – at the same exposure as flat field images but with the sensor covered – to further reduce noise in flat field frames. Steps 5-6: The final process involves applying the master calibration files to each light frame and then combining the calibrated lights into a final image. Credit: Pete Lawrence

https://www.skyatnightmagazine.com/astrophotography/astrophoto-tips/deep-sky-image-processing-for-science-an-in-depth-guide/