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Old 25-05-2019, 05:27 PM
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Ryderscope (Rodney)
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Effect of Moon Glow

I guess that this is the sort of thing that you do when idle and waiting for the end of twilight and for an imaging run to kick off.

I was just reviewing the luminance subs that I captured of M64 from an image run on Thursday night just gone (23 May). Whilst doing so I noticed a significant brightening in the background of the subs towards the end of the imaging run. I then realised that the end of the run had overlapped with the rising of the moon which was 78% illuminated. Interesting I thought so I decided to get a bit more scientific and use a couple of methods to determine exactly how much brighter the subs were. I ran the subs through the sub frame selector tool in Pixinsight as well as taking a sample of the background ADU in each image. I then plotted the background ADU over time as well as producing a graph output from the sub frame selector tool showing the change in SNR weighting.

As you can see from the attached images, the effect of the moon glow becomes apparent approximately 30 minutes before the rising of the moon and significantly increases once the moon pops its head over the horizon. The moon was at an angular separation of about 110 degrees at the time as M64 was just past the meridian.

Needless to say, the last few subs will be going in the bit bucket.

Cheers.
Rodney.
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Old 26-05-2019, 06:38 PM
Startrek (Martin)
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I use a DSLR to image in my 6” and 8” newts and I don’t even have to look at my planetarium or gaze up at the night sky as my histogram tells me sub after sub that mr moon is on his way up in the east
It’s quite noticeable and annoying to say the least especially if your halfway through your imaging run
I guess we have to make the most out of clear nights regardless of obstacles like moon glow
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Old 26-05-2019, 08:01 PM
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Atmos (Colin)
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It’s fascinating how much earlier before moon rise that our subs start to become affected. I know I use moon rise as my cutoff at times but maybe I should be looking a bit earlier than that in the future.
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Old 26-05-2019, 09:52 PM
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strongmanmike (Michael)
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Very interesting Rod, thanks for that. Depending on the specific phase (usually don't worry about it, if it is any sort of crescent Moon) I have generally stopped my Lum subs just before moon rise for this exact reason but to see it so graphically illustrated like this is very cool

Mike
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Old 27-05-2019, 09:16 AM
markas (Mark)
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Interesting! How dark is your sky without the Moon?


Mark
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Old 27-05-2019, 09:34 AM
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Thanks for your responses everyone. Of course, all this is doing is confirming what we know all ready but sometimes it is interesting to be a little more precise.

@ Mark - The last time that I measured the sky (with no moon present) with my SQM meter it was between 21.1 and 21.3. That is pointing straight up at the zenith. I am 10km from the eastern edge of Bathurst so my sky is quite reasonable with the exception of the light dome from Bathurst which extends about 35 degrees up into the western sky.
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Old 27-05-2019, 12:43 PM
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I have not been all that scientific about it but I was messing around with mosaic planning in SGP last week and shot a basic, 2 subs per panel sequence on 15 panels (Glutton for punishment, 15 panels, my first mosaic!)

You can clearly see what order the panels were shot in by the progressively brighter sky background as it got closer to moonrise.
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Old 27-05-2019, 01:07 PM
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multiweb (Marc)
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Very interesting Rodney. I had no idea the moon would brighten the sky that much prior to rising but when you think about it depending on the phase it makes a lot of sense. Having said that from my backyard the glow that affects your pristine dark skies is swamped into the local LP and probably would account for 0.1% of the total sky illumination here.
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Old 27-05-2019, 04:07 PM
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A pictorial version of Rodneys data. A bit of a gradient in the first couple of panels (Bottom right) from neaby Kilmore but the approach of moonrise is all the rest. This was a simple 2 X 2 minute sub per panel test of framing for an image I am thinking of doing.
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Old 27-05-2019, 04:26 PM
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Quote:
Originally Posted by multiweb View Post
Very interesting Rodney. I had no idea the moon would brighten the sky that much prior to rising but when you think about it depending on the phase it makes a lot of sense. Having said that from my backyard the glow that affects your pristine dark skies is swamped into the local LP and probably would account for 0.1% of the total sky illumination here.
Ah, yes. At least I am fortunate to have reasonable skies so that I can see the effect of the rising moon. Can't complain really .
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Old 27-05-2019, 04:29 PM
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Quote:
Originally Posted by The_bluester View Post
A pictorial version of Rodneys data. A bit of a gradient in the first couple of panels (Bottom right) from neaby Kilmore but the approach of moonrise is all the rest. This was a simple 2 X 2 minute sub per panel test of framing for an image I am thinking of doing.
That's a good example Paul and shows how it is critical with mosaics (and especially large, multi panel ones) to try and avoid gradients wherever possible. Its hard enough to make them go away at the best of times.
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Old 27-05-2019, 06:19 PM
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I was pondering the other day whether it was worth using some data I grabbed when the moon was up to add to data I had from a moonless sky, and was just doing a quick back of the envelope calculation when I saw this thread.

You can use essentially the same calculation to decide which of your subs to throw away Rodney, ie how bright does the background have to be in an image before using that image in your stack would make the stack’s final SNR worse?

With some (perhaps oversimplifying!) assumptions, it seems that the answer is that if the background brightness of the new image is less than twice the background brightness of the majority of the images, then it’s beneficial to use it. (This is if you have a lot of images and are only looking at using a few extra bright ones – the more general case is the one I was originally looking at, where you can be adding quite a few images, and there you can get away with using even brighter images).

Note: by brightness I mean average ADU above the dark frame –not sure what that would be in your case Rodney.
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Old 27-05-2019, 08:04 PM
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Quote:
Originally Posted by DiscoDuck View Post
I was pondering the other day whether it was worth using some data I grabbed when the moon was up to add to data I had from a moonless sky, and was just doing a quick back of the envelope calculation when I saw this thread.

You can use essentially the same calculation to decide which of your subs to throw away Rodney, ie how bright does the background have to be in an image before using that image in your stack would make the stack’s final SNR worse?

With some (perhaps oversimplifying!) assumptions, it seems that the answer is that if the background brightness of the new image is less than twice the background brightness of the majority of the images, then it’s beneficial to use it. (This is if you have a lot of images and are only looking at using a few extra bright ones – the more general case is the one I was originally looking at, where you can be adding quite a few images, and there you can get away with using even brighter images).

Note: by brightness I mean average ADU above the dark frame –not sure what that would be in your case Rodney.
I guess the issue Paul is that it comes down to the weighting that one chooses to allocate against SNR and where you might set that threshold between the keep or throw in the bin. Any increase in noise from what ever source is going to decrease SNR assuming the signal level remains constant. I am in the fortunate position with my own observatory that I can be less precious about keeping subs that are less than optimal. Alternatively if one has to travel to a dark site and set up just for the weekend (which I have done previously) then one tries to hang on to as many photons as possible.
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Old 27-05-2019, 09:29 PM
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Quote:
Originally Posted by Ryderscope View Post
Any increase in noise from what ever source is going to decrease SNR assuming the signal level remains constant.

In that sub, sure. But the SNR will typically increase from averaging subs, even including such poorer ones. That's why we stack, after all. As long as the noise is not too extreme in any of the subs, as I said. I was just trying to work out how extreme the noise need be before it doesn't help the final SNR of the stack and actually lowers it.



Quote:
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Alternatively if one has to travel to a dark site and set up just for the weekend (which I have done previously) then one tries to hang on to as many photons as possible.

Absolutely
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