The use of bias, dark and flat frames to calibrate light frames is fundamental for astrophotography. While bias and dark frames can be created with relatively little effort, a more complicated setup is required to create flat frames. Flat frames require a mask that is as evenly illuminated as possible in order to be able to correct differences in brightness in the light frames. These flatfield masks can be purchased from specialist astronomy dealers for several hundred euros. For larger aperture diameters, the price can easily reach into the mid to high three-digit range. In my case, I would need a flatfield mask with a diameter of around 240 mm. These would cost around 500 € from specialist dealers. If you want to spend less money on a mask, you can also build one yourself. In this blog post, I would like to show you how I built the flatfield mask for my Bresser NT203/1000.
