Creating Starless Images

Astro imaging using narrow band filters (Hydrogen alpha, Ha; Oxygen-iii, O3 and Sulphur-ii, S2) is a popular part of imaging. Not only does it enable amateur astro-photographers like me to take images comparable to the Hubble Space Telescope and other land based observatories but they are a great weapon against the ever increasing light pollution we are all experiencing. They are very effective against light pollution because they only allow a very small range of light to pass (hence, narrowband). The NB images are false color as you assign the three colors of the RGB palette to each NB filter. The Hubble palette (SHO) assigns red to the S2 filter, green to the Ha filter and blue to the O3 filter. Another palette I use is the HOO - a bicolor palette as it assigns red to the Ha and both green and blue to the O3 (no S2 data). 

An obvious down-side to NB imaging is that the stars appear highly tinted in largely purple hues because the full range of color is not processed in NB treatment. So, not only is the nebula rendered in false color so are the stars, which makes them somewhat unappealing.

To correct for this I usually image the main subject using the three NB filters and then run a set of short exposure subs using the standard broadband filters to capture the true color of the stars. Then, in post-processing I create two images, one that has all the stars removed, and another that contains only the RGB stars. Then the two images are combined resulting in a NB nebula but with stars of their proper color and brightness. 

Extracting the stars is a fairly easy process that works well. Removing them to create a starless image is much more difficult. Some of the tools I use to remove the stars leave behind artifacts that must be manually corrected, and sometimes not all the stars get removed - especially bright ones that appear bigger on the image.

Recently, a fellow forum member, Steve, over at The Sky Searchers (TSS) forum posted about a multi-step process to remove stars from an image leaving no unsightly artifacts. Although I haven't fully tested it out on multiple images the results are thus far impressive.

Here are the pre- and post- star removal images of the Rosette nebula.

Unfortunately I don't have a set of RGB stars for this image so I couldn't add them back for the final photo (some folks like star-less images standalone). 

I will certainly add in the RGB session on my next NB project.