When photographing distant objects in space, faint starlight travels through the telescope and camera and is converted into electronic signals. However, not all of these signals come from the stars themselves. Some are caused by noise produced by the equipment. To create images that are closer to reality, astrophotography uses a process called image calibration, which mainly includes three types: Bias, Dark, and Flat.

Bias:

Even when no light enters the camera, the image sensor produces small random electronic signals. To avoid negative values, the sensor adds a fixed signal so that all pixel values stay above zero. To record this effect, a bias image is taken with the lens cap on and the shortest possible exposure time.

Dark:

During long exposures, heat inside the sensor creates extra electronic signals. To measure this thermal noise, a dark image is taken in complete darkness using the same exposure time, ISO, and temperature as the actual photo. This image is later subtracted to reduce unwanted noise.

Flat:

Uneven light can be caused by the optical system, such as darker edges in the image or dust on the lens or sensor. To correct these brightness differences, a flat image is taken using a smooth, even light source, such as the sky at dawn or dusk or a light panel, with the same equipment setup. This image is used to balance brightness across the frame.

By applying these three calibration steps, astronomical images can more closely match the true light from space, allowing each photograph to better show both the scientific value and the beauty of the starry sky.