Starguide - A Practical Guide to Astro-Photography

Astro Photography 

 Electronically Assisted Astronomy (EAA)

updated: 2025-07-12

Image Artifact

Compensation Measures During Image Capture

Compensation and Calibration during Image Post Processing

Thermal noise: 

Each photodiode on the sensor has its own characteristics and collects thermal photons at its own rate. Some photodiodes are so active that they are called hot pixels and are visible in the image, meaning they are always 100% active. In addition, the sensor's electronics can excite photodiodes in such a way that this can be clearly seen in the images

can be reduced by cooling the sensor. Modern astro cameras offer integrated cooling down to -20°C, which significantly reduces thermal noise.

To correct for thermal noise, a series of dark frames is recorded. During this process, the sensor must be completely darkened and operated at the same temperature as during the subsequent recording. Additionally, the same recording duration and signal amplification (gain or ISO) must be used. The resulting master dark image is later subtracted from the actual image during the calibration process to eliminate these errors

Bias signal: 

Even with the shortest possible exposure times, not all pixels contain the expected zero values, but rather slightly positive values distributed across the entire image.

Set the camera to the shortest possible exposure time and take a series of bias frames with the sensor completely darkened. The resulting master bias frame is later subtracted from the actual image during the calibration process to eliminate these errors. 

Uniformity error: 

like dirt on the sensor, dark spots in the image, and vignetting

can be minimized by thoroughly cleaning the sensor, filters, and other optical components before use. 

Point the camera toward a uniform white or gray background and take a series of white or flat frames to reveal any image errors. The resulting master flat frame is later subtracted from the actual image during the calibration process to eliminate these errors.

Blooming errors:

due to overexposure of individual photo cells

can be prevented by keeping the exposure time as short as possible. However, this results in other image components being significantly underexposed (too few photons are collected). This can be compensated for by taking many images (up to hundreds or thousands) of the same section of the sky and later superimposing them using the stacking process (see Alignment and Integration with PixInsight). 

Some Blooming effects, such as thick star blotches, can be corrected during post-processing. For more information, see:

• Star Reduction with the EZ Processing Suite
• RC-Astro BlurXTerminator
• PixInsight StarMask