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Old 01-12-2019, 11:38 AM
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NGC 1097 from the 'burbs

Tried a bit of an experiment to see if, with enough data, a decent image could be obtained under light pollution. A quick calculation suggested that, for my location, about 7x more data was needed than in a dark sky site - so about 40 hours or so should be the same as one night at a dark site.

The experiment is not conclusive without doing the same object at a dark site! But even after 40+ hours of data, there is still a lot of noise in this image and not great detail - though admittedly some nights the seeing was appalling, so may be a bit of an ambiguous outcome to the experiment.

Though I think it's cool seeing the detail brought out by stacking (see the one sub vs luminance stack comparison image).

Oh, and the 2000+ luminance frames made PixInsight (and my poor CPU) sweat a bit! :-)
Full res available on Astrobin here, though please don't go pixel peeping as, as I said, it's a bit rubbish I think.
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Old 01-12-2019, 01:50 PM
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Hi Disco. There seems to be some unusual streaking in the background, very reminiscent of DSLR images. Other than that, nice galaxy!
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Old 01-12-2019, 02:59 PM
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Man, that's a huge effort Paul, good on ya for giving it a shot.

Even if you put in enough hours to get the SNR to be theoretically the same as a dark site, I think the data is still going to be very challenging to work with. IMO galaxies really need dark skies.

I second the above comment re the "walking noise" in the image. Did you dither these images? It looks like you didn't dither and if that's the case I think you'll find you could make quite a big improvement by doing that. Maybe something to think about for next time.
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Old 01-12-2019, 03:07 PM
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Thanks guys. Yes, they were dithered. Perhaps I need to dither more! Don't know if there's anything unique about this situation that makes the fixed pattern noise more obvious.
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Old 01-12-2019, 03:15 PM
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A dedicated effort for sure and a decent result Paul . I agree with Lee (not Bruce), dark skies also help because you have to do less "work" on the image too, less gradient removal (or none) etc. Dithering is very important I recon, I don't even use darks with my H694, just a 3 pixel dither between light frames, then median combine them in pre processing, it's great

Mike
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Old 01-12-2019, 03:43 PM
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Quote:
Originally Posted by strongmanmike View Post
A dedicated effort for sure and a decent result Paul . I agree with Lee (not Bruce), dark skies also help because you have to do less "work" on the image too, less gradient removal (or none) etc. Dithering is very important I recon, I don't even use darks with my H694, just a 3 pixel dither between light frames, then median combine them in pre processing, it's great

Mike
Agreed that dark skies help - but in their absence was wondering what could be done! I think your comment re less "work" is on the money - took a lot of processing to get something that'd be trivial with decent data!
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Old 02-12-2019, 09:20 AM
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It’s a very good test Paul, I did something similar not long ago with NGC 253, testing galaxy imaging under Bortle 7-8 skies.

My theory was that brighter objects aren’t quite as important for dark skies, whether it be bright nebula or bright galaxies. After that it largely comes down to seeing for the amount of detail that you can pick up. Your stars look great but there may be a bit too much noise reduction showing in the galaxy itself; it might be blurring some of the details but then again, not so great seeing destroys that too!

All in all, a great shot Paul
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Old 02-12-2019, 09:54 AM
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It’s a very good test Paul, I did something similar not long ago with NGC 253, testing galaxy imaging under Bortle 7-8 skies.

My theory was that brighter objects aren’t quite as important for dark skies, whether it be bright nebula or bright galaxies. After that it largely comes down to seeing for the amount of detail that you can pick up. Your stars look great but there may be a bit too much noise reduction showing in the galaxy itself; it might be blurring some of the details but then again, not so great seeing destroys that too!

All in all, a great shot Paul
Thanks Colin. There wasn't too much noise reduction applied to the galaxy core - I think the limited detail was just the bad seeing. Some nights were up to about 4.5" FWHM and yet I still used that data (I know!). (One night the average FWHM of some frames was well under 2", with the stars in the centre about 1.5". I've never seen seeing that good before. Shame it was only for a bit of a night!).
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Old 02-12-2019, 10:59 AM
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Something you could try is having your standard luminance stack with its current FWHM and only stacking luminance subs with FWHM less than 2”.

Let’s call the full stack FS and the sharper stack SS.
After you have stretched your two luminance images to a similar level. Check the average background level (in ADU) and that’ll be your ABL.

iif(FS>(ABL/65535), FS*(1-FS)+SS*FS, FS)

What this this do is add the sharper data in on brighter areas (which intrinsically have better SNR) without affecting the dimmer areas and won’t affect the background at all.

Typing this on my phone at work but I think that code will work in PixelMath.
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Old 02-12-2019, 11:03 AM
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Quote:
Originally Posted by Atmos View Post
Something you could try is having your standard luminance stack with its current FWHM and only stacking luminance subs with FWHM less than 2Ē.

Letís call the full stack FS and the sharper stack SS.
After you have stretched your two luminance images to a similar level. Check the average background level (in ADU) and thatíll be your ABL.

iif(FS>(ABL/65535), FS*(1-FS)+SS*FS, FS)

What this this do is add the sharper data in on brighter areas (which intrinsically have better SNR) without affecting the dimmer areas and wonít affect the background at all.

Typing this on my phone at work but I think that code will work in PixelMath.
That's a great idea Colin.Will give it a go. Thanks
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Old 02-12-2019, 11:13 AM
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Originally Posted by DiscoDuck View Post
That's a great idea Colin.Will give it a go. Thanks
I donít often use it for this application but I have for increasing dynamic range in objects like M42.
120s exposures completely blow out the core so Iíll do 10s exposures just for the core. You pre-stretch the 10s stack so that itís quite bright and then do something similar and itíll add the non saturated data in and not touch the dimmer areas.
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Old 03-12-2019, 04:24 AM
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Hi Paul,
that's actually quite a good image.
The center of the galaxy is overexposed but could be improved by
only using RGB data & no Luminance.
The blotchy background can be fixed easily as per here:
http://bf-astro.com/backgndrepair.htm
I get the prize for the worst NGC1097 when
I tried 8 years ago with a DSLR -
13 frames at 5 minutes each at ISO 800
from a light polluted location.

I won't even link the pic it was so bad but it's in my Flickr photos.



cheers
Allan
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Old 03-12-2019, 06:53 AM
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Lol. Thanks Allan.
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Old 03-12-2019, 03:26 PM
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Very pretty galaxy. Love that sharp dust lane and the vivid colors.
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Old 03-12-2019, 05:41 PM
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Very pretty galaxy. Love that sharp dust lane and the vivid colors.
Thank you Marc.
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Old 04-12-2019, 06:09 AM
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Lol. Thanks Allan.
Hi Paul,
if you are willing to link all the FITS files including darks & flats
I would love to have a go at processing your picture.
I don't have the time right now to do imaging but I do have some time for processing.
Your picture is a challenge but I think I'm up to the task.

cheers
Allan
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Old 04-12-2019, 07:53 AM
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Quote:
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Hi Paul,
if you are willing to link all the FITS files including darks & flats
I would love to have a go at processing your picture.
I don't have the time right now to do imaging but I do have some time for processing.
Your picture is a challenge but I think I'm up to the task.

cheers
Allan
Unfortunately Allan it is way too much data to upload ... of order 150gb I recall.
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Old 04-12-2019, 10:27 AM
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Unfortunately Allan it is way too much data to upload ... of order 150gb I recall.
You mean 1.5 Giga Bytes?
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Old 04-12-2019, 11:17 AM
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You mean 1.5 Giga Bytes?
With more than 2000 Lum frames it would be close to 90gb of files in total
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Old 04-12-2019, 12:11 PM
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With more than 2000 Lum frames it would be close to 90gb of files in total

Of course - I forgot that it was nearly 47 hours of data.
OK then - maybe just the stacked FITS files for LRGB
already stacked and aligned &
the LRGB flats stacked results &
the dark frame stacked results for 60s & 180s.

That would be 10 FITS files if Paul was kind enough?

cheers
Allan.
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