Hello,

my boundary conditions due to the houses and trees only allow objects close to the zenith. The maximum observation time is about four hours. The city sky is quite bright and at new moon I measure SQM values ​​for sky brightness from 18.8 to 19.4. For three years I have been using the Optlong-L-Enhance dual narrow-band filter together with the ASI1600MCC OSC camera. This combination has enabled me to photograph a number of objects that I previously thought would be unfeasible. However, I lack the patience to photograph an object with a longer exposure time over several nights.

Now, over the years, the possibilities in image processing have also expanded significantly. So I was curious to see what can be obtained from exposures with a broadband filter (Hutech LPS-P2) compared to the dual narrowband filter. In the past, the bright sky background and weak object contrast often only led to mediocre results in processing.

I chose the Cocoon Nebula as the object and photographed it with the two filters at G=150 and T=-20°C with about the same total exposure time:

L-Enhance: 30 shots of 240 s each
LPS-P2: 58 shots of 120 s each

Due to the brightness of the sky, the histogram with the broadband filter is already shifted relatively far to the right, so I used a shorter exposure times.

The image processing uses DSS for stacking, AstroFlatPro for background removal, StarNet for star removal and Affinity Photo for further processing including TopazDenoise for noise reduction. By subtracting the original image from the starless version, a layer is generated that only contains the stars. This enables separate processing of the starless background (stretching, sharpening, color saturation and de-noising) and the stars (color saturation). Finally, the two levels are merged again. TopazDenoise has the problem that the stars are often changed ugly when de-noising. This approach circumvents this problem.

I'm amazed at what can be extracted from the original low-contrast broadband image. The small reflection nebula around the star TYC 3608-755-1 to the right of the Cocoon Nebula is more evident here than in the narrowband image. Of course, this brings out the H-alpha region more clearly and is characterized by smaller stars. For nebulae with weak H-alpha areas, however, you have to use a narrow-band filter under my boundary conditions. 

Greetings and CS 

Christian