What are Calibration frames & how do you take them?
There are three main Calibration frames which should be used with your light frames to create one image. This page will explain what they are and when/how you should take them.
1. Bias Frame
Bias Frames are very important as it subtracts the natural noise that your camera sensor produces when taking an image. The Bias image example above shows you the noise and banding from a single short exposure image from your camera. Each camera looks different. (The image above has been stretched so you can see the noise.) ​
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When taking this frame, make sure you...
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Use the same Gain as your light frames
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Cover your telescope
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Use the shortest possible shutter speed.
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Take at least 100 bias frames
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Bias frames can be stored for up to 3 months. You will then need to take a new set. As your camera ages, the noise pattern can change. ​
The focus or temperature is not important when taking bias frames.
Bias frames are not magic. They wont remove all the noise from your image. (That would be nice though)
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2. Dark Frame
Dark frames is just as important as the bias frame because it helps to remove thermal noise, hot pixels and amp glow that your camera sensor produces when taking an image at longer exposures. The Dark frame example above shows you Amp glow from your camera and hot/cold pixels which could be mistaken for stars during stacking, if you did not use dark frames. (The image above has been stretched so you can see the noise.)
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When taking Dark frames, make sure you...
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Use the same Gain as your light frames
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Cover your telescope
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Use the same exposure as your light frame
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Camera is the same temperature as your light frame
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Take at least 60 Dark frames
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Dark frames can be stored for up to 3 months if you have a cooled camera. If you have a DSLR or any other uncooled camera, you will need to take dark frames after every imaging session.
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3. Flat Frame
Flat frames will remove any dust spots and vignetting from your light frames. Adding flat frames will really improve your images. The image above you can see dust spots around the frame. Without these flat frames, your final image will have these dust spots. (The image above has been stretched so you can see the dust motes.) The most popular way to take flat frames is the t-shirt method. Place a white t shirt over the lens and hold it down with a rubber band. Then point the telescope at an even light source. Some people use the daytime sky but you risk your scope changing focus. I would suggest using a tablet or something with a white illuminated screen. You can also purchase a dedicated illuminated flat panel.​
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When taking flat frames, make sure...
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You Ideally use the same Gain as your light frames but is not important
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to use a t-shirt to cover the lens and point the telescope at an illuminated light source.
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The focus is the same as your lights
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You take at least 30 flat frames
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Flat frames will be different every time. You should take this after every session.
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There you have it. You should now understand what Calibration frames are. Start adding them to your images!