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I must be honest, I don't always do this! But I should. Flat fields are an important part of the way we calibrate our hard-earned images to remove various artefacts in our optical sytem.

My preferred way of capturing flat fields is to use the twilight sky, which is one reason I don't always do it as I may not be there at twilight! Anyway, enough excuses! At twilight, just before the stars have appeared, I direct the scope to an altitude of about 60 degrees, opposite the setting sun - this is where the sky will be most evenly illuminated, a key aspect of capturing good flat fields.

I then take a test exposure to see how bright the image is - I'm looking to get exposures which generate ADU values of around 20,000 to 30,000 (65,000 is the saturated value on my CCD). Sometimes I have to wait until the sky darkens a little more. When the levels are about right I capture about 10-20 flats. If stars are starting to appear on the flat images I slew the scope about - this ensures that when I 'median' combine the flats later on, the 'median' algorithm will remove the stars as they have moved between successive exposures.

Now we wait until dark....

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