Darks and skyglow experiment

[riklaunim]

Light pollution is quite big in the city, so with 2sec exposures with DMK21 I get gray background from the LP (when no filter is used etc.) So I've got an idea to use that sky-glow as dark in image processing.

I've chosen Alnitak which also has a smaller star/stars close to it. 2 sec exposure with 35 frames stacked in avistack. Then unfocused and moved bit away from Alnitak to get rid of his unfocused view (until I got "clean" gray background) - and made avi with that sky-glow. Covered the scope and saved true dark (150/750 SkyWatcher Newtonian).

Clean image: Alnitak 2sec no-dark on Flickr - Photo Sharing!

With Dark: Alnitak 2sec dark on Flickr - Photo Sharing!

With Dark-Glow: Alnitak 2sec dark+glow on Flickr - Photo Sharing!

As you can see, the true dark frame did only minor improvement (as the camera isn't noisy), but using the glow as dark did a huge improvement, even darkening Alnitak (so dark-glow with shorter exposure could be more useful)

Here each "background" side by side:

[Sam]

Very interesting and thanks for posting this. The process seems very similar to what Pixinsight does with the dynamic background extraction tool.

[dph1nm]

I always use offset sky frames as 'darks' to remove light pollution (median combined to remove the stars). But it is a compromise, in that unless you take many more offset frames than object frames you will lower your signal-to-noise in the final image.

NIgelM