Capturing nebulae with a light pollution filter

Before reading this article, I recommend to read about method of deep shooting using a tracker and the method of removing stars in StarNet++.

The ability to remove stars from an image and isolate them into a separate image opens up enormous possibilities in processing astrophotography. The simplest example is separate processing of stars and background (for example, with a nebula). While sorting through the archives of images testing the Optolong L-eNhance EOS-C light pollution filter, I discovered images taken with a short shutter speed to show by the background brightness how much the shutter speed can be increased when using a filter. I’ve long had the idea in my head to separately photograph the stars without a filter and separately the nebula with a light pollution filter, and then superimpose it on top of the other. The fact is that L-eNhance transmits only a few narrow bands of the spectrum (H-alpha, OIII, H-beta), and the stars are painted in a peculiar green color. Oddly enough, with the L-eNhance filter I was able to do photometric calibration in the SIRIL program, although this is most likely still not entirely correct. I would like to warn you that the L-eNhance filter is only suitable for shooting diffuse (self-luminous) nebulae. To photograph galaxies, star clusters, and dusty nebulae in bright conditions, you can use the Optolong L-Pro filter, but it is still better to get away from the city to the dark sky.

Approximate shooting and processing algorithm

– First photograph the object without a filter. The ISO (gain) can be set lower, the shutter speed longer, but so that the bulk of the stars are not burned out.

– Capture the nebula through a light pollution filter. Set the shooting parameters so that the nebula can be clearly captured. If the mount allows, we increase the shutter speed and reduce the ISO (gain). Don’t pay attention to burnt out stars.

– Process pictures with stars without a filter. Apply photometric white balance in SIRIL (if necessary). Add saturation.

– Remove the stars from this image, then select them into a separate file.

– Calibrate and stack images with a nebula, taken through a filter, into DeepSkyStacker. Stretch the histogram using Graxpert, flatten the background using Graxpert or Fitswork.

– Removing stars from the image with the nebula. Adjust levels, curves, apply noise reduction.

– In Adobe Photoshop, overlay the image of the nebula with stars with a separately saved image of stars taken without a filter. If necessary, shift, rotate and scale the layer with stars relative to the layer with the nebula so that some stars overlap exactly with others. Merge the layers and save them as a separate image (for example, “rotated stars.tif”).

– In Adobe Photoshop, overlay this image on a photograph of the nebula WITHOUT STARS (which already has levels and noise reduction), in the blending settings we specify “Linear Dodge (add)”.

– Adjust the saturation of stars (top layer) and their number (in levels).

– Merge the layers, crop the frame, and save it as the final image.

Sample final picture

Туманности М16 и М17, 22 июля 2020 года
M16 and M17 nebulae, July 22, 2020

– The color of the stars is preserved
– The nebula has been processed

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