So, what are calibtation frames I hear you ask?
When taking pictures with any camera through an optical system be it a telescope or camera lens there are a lot of imperfections that are caused by various factors. These include - thermal electronic noise, vignetting and dust blobs.
As astrophotography involves taking long exposures, these imperfections will become more prevelant in your final image, especially during stacking and subsequent post-processing.
We take calibration frames to either totally eliminate or at least drastically limit the effect of these issues in our final processed image.
There are three main calibration frames, but you should usually only need to use two of the three listed as follows:
• Dark frames - these remove thermal electronic sensor noise and include- hot, cold & dead pixels and amp glow.
• Flat frames - probably the most important calibration frames you will take. They remove vignetting and dust blobs.
• Bias frames - these remove dark fixed-pattern noise that is a result of the manufacturing process.
Of the above three calibration frames, you will only really need to use darks and flats. Bias pattern noise can sometimes be removed with just dark frames, but you may find that you need bias frames in place of dark frames if the darks alone do not work removing pattern noise.
Some people are happy to try to remove noise, vignetting and dust blobs during the post processing part of their workflow but this can be very time consuming and hard to achieve. Taking calibration frames at the end of an imaging session makes life a lot easier when post-processing.
Calibration frames are taken at the end of each imaging session apart from flat frames which should be taken after each target imaged. They are then added to your choice of stacking software and they will remove any thermal noise, vignetting and dust blobs during the stacking process if they are present.
The below link will open up a presentation that visually describes how calibration frames work and how to take them.
As it is a presentation, on a PC or laptop just use the left and right directional buttons to navigate to the next slide. If you are on mobile or tablet then just swipe from right to left to navigate the slides. If the presentation does not resize to fit the screen just refresh the page and it will resize.
