Lesson Description
You learn two ways to select by “chromatic information”: (1) masks per channel, based on the luminosity of a channel (R, G, B...); and (2) masks by colour, based on colour similarity in the already composed image. You will see clear cases (aircraft red, aurora green) and a typical narrowband astro example, where it usually works better to select by channel and not by colour.
What you will learn
- Differentiate when a mask is appropriate by colour (well-defined colour) vs. by channel (separate signal per channel).
- Select and adjust only one red/green area without affecting the rest using colour masks.
- In astrophoto, hide the star layer before creating the mask so as not to “contaminate” the selection.
- Enhance the blue of OIII with Hue/Saturation using a blue channel mask and then return the stars intact.
Advice
In the “starless + stars” flow, generates the mask only from the starless layer (hiding stars before) and apply the adjustment; at the end, show stars again so that they are outside the adjustment.
Practical Checklist
- I choose colour mask (red/green) for a very defined colour and check that it selects just that area.
- In narrowband, I hide stars and try a channel mask to isolate the signal.
- I apply Hue/Saturation with blue channel mask for OIII and restore the star layer at the end.
Technical Glossary
Starless: Starless layer used to apply adjustments to the object/background without touching stars.
RGB channel: black and white image representing the intensity of a colour in each pixel (red, green, blue).
Narrow band: flow where each channel can represent different signals (e.g. Hα, OIII, SII) assigned to the final colour.