Choosing Colors

To the question "do you choose colors randomly?", the short answer is no!

A large part of the work consists of assigning the right color to the right filter for each object to achieve a harmonious result. Thus, certain unsightly colors in astrophotography should be avoided or used with caution, such as green and magenta. This is mainly due to the fact that no star can have these colors.

Conversely, blue and cyan give an impression of a background or icy coldness, while red, orange and yellow give an impression of a foreground, fire, or warmth.

Based on this principle, Franck wanted a photo "on fire" (a combination of red and yellow), whereas I preferred a photo with a high chromatic contrast based on the classical CMY triad (combination of Cyan - Magenta - Yellow).

Note that very few areas in the resulting picture only consist in one single channel. The mix between these, and where they are located is unique to each astronomical object. CTB 1 has precisely located OIII area, a hazy Ha background with some SII linear shapes, and a very subtle mix of Ha and SII for the supernova remnant itself.

Study of different color palettes

The stars are processed separately, and their colors are calibrated using the Gaia catalog to obtain rigorously accurate star colors. This is a deliberate decision. Since the smallest observed wavelength is OIII (Blue-green at 500.7 nm), this is limiting how blue the stars are. 

In return, the nebula photographed with three filters assigned to three colors is subject to an artistic interpretation, based on the previously posted color wheel.

The results of these narrow-band, so-called starless, photos are presented below.

We recommend increasing the brightness of your screen to distinguish the sometimes very subtle differences.