Digital processing or data reduction as it was called in the olden days.

 

Re-processed Messier 35 widefield Credit: Pip Stakkert

"Pip Stakkert decided to rework the data for this image we previously published. The original showed too many stars with pixelated shapes. 

Stars represent quite a challenge for astro-photographers. Nebulae and extended objects are much easier to image and represent. Stars are big, very big or enormous but all, other than the Sun, are far far away. As a consequence of their extreme distance, they should present as point sources of light of varying degrees of brightness and colour. Unfortunately, when the light from a star passes through a lens, be it on a telescope a camera or the ones in your eyes, the point source becomes a small disc. This is a physical and unavoidable feature of light and lenses! The colour, which is determined by the stars temperature, migrates to the edge of the disc leaving the brighter centre pure white and often saturated. To capture faint objects the astrophotographer primarily increases the length of exposures and consequently brighter stars are over exposed and colour is lost. Your eye and brain combination, may not be able to detect the light from faint objects that are easy for the cameras sensor to register, but working together  can compose a more coherent and dynamic view of the night sky. Getting the stars to look 'natural' in a widefield image requires the data processor to decide how many stars to show, how to ensure their disc shape is circular, how to differentiate between bright and dimmer stars and how much colour to display.

The new neural network based software StarFixer is an interesting development and is likely to improve over time. At the moment we use it in combination with other software in regulating the shape of stars. The new astrophotography V15 macros developed by James Ritson for Affinity Photo 2.0.0 are astoundingly good at rendering stars in a realistic and natural way.

The open cluster Messier35 is much clearer in this image version as is the fainter open cluster, NGC2158, to its immediate south west. Overall the image, fewer stars are on display and there is a bigger dynamic range between the dimmest and brightest stars. In a nutshell, this image of stars looks less busy, is a better representation of what the eye brain combination might see if it was more sensitive at low light levels and more naturalistic all round (excuse the pun)". - Kurt Thrust current Director of the Jodrell Plank Observatory.