How to obtain flat frames

Flat frames) are obtained by pointing the telescope at a uniformly illuminated field with the same optical image calibration, ie without moving the camera or the focus so that the light path is exactly the same.

This can be achieved in three ways:

• Twilight sky - at dawn or dusk when the telescope is pointed to a semi dark area of the sky with no visible stars and with telescope tracking turned off.
• Light box - a box is constructed with the background uniformly artificially lit.
• T-shirt - the scope is covered with a tight and wrinkle-free white T-shirt and pointed to some source of homogeneous light.

I used the technique of the T-shirt until I built my light box. The twilight sky technique can only be used if camera angle and focus were not changed and you must wait until dawn.

The exposure time and camera temperature may be and are often different from the image to be calibrated. The exposure should be adjusted so that the image obtained is between one third and two thirds of the camera dynamic range.
Flat frames should be calibrated with their own drarks because they usually have different exposure times object frames so don't forget to take them after taking the flat frames.