One of those S. Hemisphere Nebulae
Alex Woronow
Hello friends and fellow imagers,
As I continued to explore the products available on Telescope Live, I came across this data set of a prominent nebula, IC 2948, in the constellation Centaurus in the Southern sky.
Again, I used my own "true-color" representation of the narrowband captures (Ha, SII, OIII) and processed the image after color calibration and removing the stars...subsequently re-introducing them by transferring them from the earlier color-calibrated version of this image.
Click https://astrob.in/9af5xv/0 to go to Astrobin for a larger image and more description.
For the Imagers: Re-inserting stars sound easy, but it is not. The removal of the stars in the first place distorts their colors, and the standard process of re-introducing them (e.g., by addition or screen-blending mode) does not reinstate the true colors (even when the stars are removed by "unscreening" them). So transfer seems the only alternative unless someone has an unshared secret!
The problem with such blending arises because the stars are the color. They are independent of the background... which is why we can photometrically color-calibrate them in the first place and why the color calibration can be done on either the linear or stretched images with the same result.
Another way to envision it is to take a stretched image with a blue star (not near saturation) on a red (nebula) background. Photometrically color-calibrate the image. Now, remove the stars using SXT or SN and make a stars-only image (by subtraction or unscreening). Next, boost the red saturation in the nebula. Then, replace the stars using addition or screening. Now run the photometric color calibration again. If the image colors shift, the replaced stars do not retain their previously calibrated colors, and the replacement algorithm (addition or screening) has failed to deliver the promised result. Try it if you like; I did. The color shift in the stars is a fact; it remains to be determined if the shift makes a difference to anyone.