Saturday, February 22, 2014

Making stars white in narrowband images

Stars always have this intense magenta color in narrowband images. One way is to tone down magenta in the image, but that also affects the other objects and might destroy faint nebulosity.

A few months ago, somebody talked on a mailing list of Straton - a tool to remove stars from an image. Or reversely extract the stars from an image.

Let's try this out on a recent image that took (color combined, basic stretches and level adjustments and sharpened):

For processing this image, I already have a synthetic luminance image (combination of the Ha, OIII and SII image):

I loaded this in Straton and removed the stars from it and then subtracted this from the luminance image:

But looking into the lower part of the image shows that there is some nebulosity left:

After loading the file into photoshop, I adjusted the levels to carefully remove these fainter parts (careful, so that I don't remove faint stars). Here is the same part as above:

And finally, I layered this on top of the original image:

Comparing this with the original image above shows that the stars are white, but the nebula itself looks exactly the same. But there are still pretty strong magenta halos left. In SII (red), the stars are larger then in OIII (blue) and those are larger then in Ha (green).

From Carboni's Astronomy tools, I used the "Make Stars Smaller" action before overlaying the white star mask:

Although, now the nebula lost some contrast ...