What are calibration frames in astrophotography?

Calibration frames are additional exposures used to improve the quality of astrophotography images and reduce noise from outside sources. There are three common types of calibration frames used, Dark Frames, Bias Frames, and Flat Frames. From thermal noise and hot pixels to sensor readout noise and uneven field illumination, each type of calibration frame tries to reduce a specific type of noise from the initial images. These calibration frames are normally taken alongside your images while the camera is in the same configuration and temperature and are then subtracted from the initial images reducing the noise and improving the quality.