Would it be possible to have an additional frame subtraction to account for light pollution?

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DrBobAZ
Posts: 71
Joined: Thu Feb 25, 2021 11:41 pm

Would it be possible to have an additional frame subtraction to account for light pollution?

#1

Post by DrBobAZ »

Greetings

I am a new user of sharpcap 3.2. I have been reading the forum posts and have noticed that this subject has been brought up before, but in light (no pun) of more and more people observing in high Bortle number skies, having such a feature might have increasing relevance.

As a bit of background, I am using a low end r2 imager, so I use AV to USB under sharpcap. I have used the dark frame feature in sharpcap to crudely remove the light pollution in the following way. After setting exposure and gain on the camera, and locate the target, I slew off to a direction in the sky that has few stars. I defocus the telescope and take a dark frame, but with the aperture open, so it picks up the sky background (and hot pixels and amp glow per standard dark frame). I use that as the dark frame, re-engage the object and live stack. Attached it the result of this pretty tedious procedure. This image of M42 was taken last night with a nearly full moon quite close to Orion, and Bortle 7.8 skies (Scottsdale AZ). (Not sure how the small thumbnail will look, the full image was .gt. 1MB) If you are familiar with the r2 imager, it is more of a EEA device, not a full up astro-photography camera, so my concern is not the star roundness and sharpness, but what was done to the background. I think the color rendition is largely correct, and the star field is true.

Note that I do not want the entire image to be darker, just the background.

Anyway, what I was thinking was it would be cool to have an additional preprocessor frame (dark,flat, and now LP). You could select a region of the current object frame that did not have many stars, possibly eliminating the need to slew away and defocus. From that sub-region, create a full frame image that would be subtracted in exactly the same way the dark and flat is subtracted.

Except for the selection coding (which I think might be there from other sub-frame selection option) should the coding to do this already be available in sharpcap?

I did a web search, and the guys using DSLRs with telephoto lenses and tracking mounts have a similar procedure.

I would love to hear what you all think of this, especially Robin

Thanks for listening, and clear skies

Bob
Orion_full_moon_close_Bortle_7.8.jpg
Orion_full_moon_close_Bortle_7.8.jpg (13.09 KiB) Viewed 570 times
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Re: Would it be possible to have an additional frame subtraction to account for light pollution?

#2

Post by admin »

Hi Bob,

yep, background subtraction is already on my to-do list for a future version of SharpCap. I think it will appear in live stacking first as a simple option to remove a constant background. Later there's a possibility of trying to deal with gradients in the background too.

Not sure about adding it outside live stacking- normally if you process in another application you would deal with the background as part of the processing.

cheers,

Robin
DrBobAZ
Posts: 71
Joined: Thu Feb 25, 2021 11:41 pm

Re: Would it be possible to have an additional frame subtraction to account for light pollution?

#3

Post by DrBobAZ »

Thanks much for your consideration and reply!
Bob
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