Calibration frames are images taken by astrophotographers to help calibrate and correct for various types of noise and other artifacts in their astrophotography images. These frames include bias frames, dark frames, flat frames, and sometimes also offset frames.

• Bias frames: Bias frames are images taken with the same camera settings as the astrophotography images but with a very short exposure time (usually less than one second). Bias frames capture the electronic noise present in the camera’s sensor and readout electronics. These frames are used to remove the bias noise from the astrophotography images.

• Dark frames: Dark frames are images taken with the same camera settings as the astrophotography images but with the lens or telescope cap on, so no light reaches the camera sensor. The exposure time is typically the same as the astrophotography images. Dark frames capture the thermal noise generated by the camera’s sensor and electronics during an exposure. These frames are used to remove the thermal noise from the astrophotography images.

• Flat frames: Flat frames are images taken with the same camera settings as the astrophotography images but with a uniform light source (such as a lightbox) illuminating the camera’s sensor. Flat frames capture the unevenness in the camera’s optics and dust particles on the camera’s sensor. These frames are used to correct for vignetting, dust motes, and other optical defects in the astrophotography images.

• Offset frames: Offset frames are similar to bias frames but with a longer exposure time (usually a few seconds). Offset frames capture the low-frequency variations in the electronic signal, such as the signal from the camera’s power supply. These frames are less commonly used in calibration but can be helpful in some cases.

Calibration frames are typically taken before or after the astrophotography session and are used to process the raw data in specialized software such as DeepSkyStacker or PixInsight. By subtracting the noise captured in the calibration frames from the astrophotography images, astrophotographers can produce cleaner, more detailed images of celestial objects.