And just as you thought you were done… Astro photo image processing consists of pre- and post-processing. Once you’ve acquired the images of your target object, certain processing steps are required to bring out the full detail and colour of your target, along with maximising the signal-to-noise ratio as mentioned in part 1.
Pre-processing: Calibration and Integration
Long exposure images contain a lot of noise artifacts: undesired information in the light frames that need to be removed through calibration files. When starting out in astrophotography these files are not a necessity, but they greatly assist in cleaning up light frames.
What is image calibration?
Calibration frames consist of:
Dark frames – Captures thermal noise and hot pixels. Dark frames are subtracted from light frames to remove these artifacts.
Flat frames – Used to correct uneven field illumination and dust particles.
Bias frames – Removal of electronic noise created by the sensor, and also aids in colour correction.
Image integration – more commonly known as “stacking”.
Once your lights are calibrated you proceed to integrate them – or rather the more commonly used term would be to “stack” them to improve the signal-to-noise ratio. With that said, you can still get great results by simply stacking light frames without calibration frames. The process can become overwhelming and might rob a beginner of the joy in simply taking a long exposure photo, so don’t get lost in the technical details just yet if you’re new. These added processes will come to you in time. For now it’s just important that you understand their benefits and why advanced imagers use them.
Various software packages are available to perform these calibration and integration tasks. They range from freeware to some bigger price tags. Deep Sky Stacker is the most widely used freeware, with programs like Nebulosity, PixInsight, CCD Stack and MaximDL at the upper end of paid-for applications.
Post-processing software is a personal choice and depends on your skill level, and the application you feel most comfortable with. The aforementioned astronomical packages do a stellar job at processing feint detail. Adobe PhotoShop is also widely used in astrophotography and many plugins and actions are available to help improve photos.