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Hi Peter,
A perfectly legitimate question. They are both “ calibration “ frames. I’ll break it down for you into the darks and the flats, what they are, how to capture them and why we use them.
Firstly Darks;
A dark is an image that you take with your camera with the same ISO setting and the same length as your subs, ie: 180sec @ iso 800. The only thing that is different is that you have the cap on the front of your telescope so that the sensor sees no light. The idea behind this is that when your camera takes long exposures, the sensor gets warm. This inherently causes some pixels in the sensor to falsely read a signal. These are called hot pixels. Also during this time electrical signals are traveling through the sensor causing the pixels to read these signals. This is called “ read noise “. The idea behind taking these calibration images is that your processing software is able to subtract these signals from your images so that you are left with much less “ false signal “. These should also be taken at the same temperature ( as much as is practical ) as your images because the noise will increase through higher temperatures so you are trying to capture the same noise levels as your images are capturing to be able to accurately remove it.
Flats ;
These images are essentially taken to again calibrate your final image but this time it is for errors in the image arriving at your sensor. These can be dust motes, light scratches on lenses but most often vignetting. This causes by a curved image being recorded on a flat sensor or other optical anomalies that cause the center of an image to have a different level of illumination compared to the edges. Flats must be taken by evenly illuminating a white sheet or piece of paper ( some use specifically made light boxes ) across the front of your telescope and taking an image only long enough to capture a complete illumination. You can do this by simply adjusting your exposure speed down until your camera shows the correct exposure. These are not temerature dependent as such but they do require the same focus. Just bare in mind that focus of your telescope does change through temperature variance. Again, any faults in illumination, dust or imperfections can then be corrected by your processing software.
I hope this has cleared things up for you a little. There are other calibration frames and there is a more technical level to all of this but as you said you’re a beginner I thought I’d just run the the basics of what, how and why.
Cheers
Ryan
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