I recently came across a data set that a user had made available for anyone to download and process to see what could be achieved by individuals who have more experience in processing. This data set was loaded with issues from frames with terrible gradients, incomplete data, severe vignetting and much more. In short, the images proved to be very challenging to work with.
Because none of the data came with any calibration frames, I was pretty much left with what the author had shot and stacked on the light frames. Not a good place to begin.
It reminded me of the importance of calibration frames in the imaging process. Not only a matter of taking them, but of ensuring that yiou have good high quality calibration frames to work with because these are the antidote to many problems that crop up in your imaging data. Let's review the basics.
Lights - These are your data frames. You open your camera and expose tjhe target you are shooting, collecting photons and saving the individual exposures which you will later stack. These frames have all of your imaging data. They also contain all the thermal noise from your camera, all of the uneven illumination from your optical configuration, and even some inherent electrical noise from just operating the camera. What we want to do is use these frames for the data, and remove the other information from the final signal.
Darks - These are a type of calibration frame that is made by convering the optical opening fo the camera and exposing a number of frames for the exact same duration as your light frames. In doing this, you will have a resultant image comprised of only the thermal noise from the camera.
Bias - These are yet another type of calibration frame that is made by rapid exposure of the camera. These frames will collect any fixed pattern noise in the camera and thus serve yet another unique purposes in allowing us to remove this from the final image.
Flats - These calibration frames are done by exposing a perfectly evenly illuminated field across the entire optical train and will show the resultant uneven illumination across the frame allowing us yet another calibration frame to complete our final processing. I usually create flats using a white t shirt and placing it tightly over the telescope to provide an evenly illuminated white field to expose.
Putting it all together - The process is very basic. All the lights get stacked together to make a master light. All the darks get stacked togehter to make a master dark, All the bias frames get stacked together to make a master bias and all the flats get stacked together to make a master flat. The master dark is subtracthed from the master light, removing the thermal noise from the camera making a new master, the master bias is subtracted from that new master light to remove the fixed pattern noise, and yet a new light is made. Finally the flat is subtracted from the new light to make the final image.
The final image will still need some work It may exhibit the effects of gradients, improper color calibration, optical abberations, lens flares, even a greenish tinge. All of these flaws are part of the process of collecting photons and processing them into final images. Processing is a whole new technique that all of us must learn to be good at this.
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