In deep sky astrophotography, the photographed objects (galaxies, nebulae, star clusters) have a weak signal. For this reason, it is necessary to use a long exposure time per photo to pick up the weak signal from these objects. With these long exposure times, there are different noises that are added to the image of the object. Here are the different noises included in the raw image of the deep sky object. At the end of the talk, I present the noise reduction techniques with a link to access the details. Also, by consulting these links, there are detailed explanations for each of the noises presented below.
Bias
Bias represents the readout noise of the camera. This is a constant spurious electronic signal that adds to the image of the deep sky object.
The Black (Dark)
Dark represents the thermal noise of the camera.
Light pollution signal
In a sky of light pollution, it is necessary to take into account the signal of the background of the sky which is added to the image of the deep sky. The challenge is to determine the maximum exposure time per photo according to the light pollution of the observation site so as not to overexpose the background of the sky.
photon noise
This noise appears during the acquisition of the deep sky image. As the signal from the deep sky object is weak, photons of light from the object always accumulate in the same place in the acquired image, resulting in a noisy image. To make this noise disappear, we take several images of the object. As the light photons of each image appear in different places for each acquired image, by assembling the images (compositing technique), this photon noise is greatly reduced. This phenomenon is explained scientifically by quantum physics (for each acquired image, photons of light appear in different places).
Here is a graphical representation of all these noises
Noise reduction techniques
Deep sky image preprocessing
To remove the signal from read noise (bias) and thermal noise (dark).
Light pollution signal
You have to determine a maximum exposure time per photo so as not to overexpose the background of the sky.
Deep sky object signal (photon noise)
To greatly attenuate the photon noise during the acquisition of deep sky images, several images of the deep sky object are composited. Click on this link and go to section The compositing of several images. It is also recommended to use the composite method Sigma clip and use the Dithering during image acquisition.
Richard Beauregard
Sky Astro - CCD
Created on 2022/09/06