Light pollution and stacking

Hi everyone,

I managed to take a stack of 27 photos of 15 second exposure at f5.6 and 800 iso. The following shows what the light pollution is like:

HTTP response: NOT FOUND | MIME changed to 'image/gif' | Redirected to error image by FLICKR

Where abouts in wales are you?

North Wales. I took the photograph from the bedroom window as I was not prepared to go out after midnight with my camera on a Friday night! That is why there is quite a bit of light pollution as I was in the town.

Ahh ok.... ive taken a few night exposures in deepest darkest wales... and theres very little light pollution! Definitely worth a trip out!

I may do it in the winter as you know sunset will soon be just before 10pm and here it is currently dark after 11:30pm.

You're overexposing the image way way WAY too much if you get that much light pollution. All the dimmer stars in your image are covered up in the sky glow and you'd actually have more stars if you did a shorter exposure.

Try Noel Carboni's Photoshop tools, there is a light pollution removal action that can work wonders.

The image where you did remove the LP has a really bad gradient in it and needs to be flattened out so it's more even across the image.

zeldaboy101 wrote in post #10194677
You're overexposing the image way way WAY too much if you get that much light pollution. All the dimmer stars in your image are covered up in the sky glow and you'd actually have more stars if you did a shorter exposure.

Not true, zeldaboy. The sky glow is always the darkest part of the image, no matter how bad the light pollution is, because the sky glow is always the darkest thing you can see in the sky. With lots of sky glow, bringing out the faint stars in the image becomes more challenging, but shorter exposures make it harder, not easier. There is no substitute for exposure time if you're trying to capture faint objects.

Those of us who deal with severe light pollution learn the techniques needed to get the most out what we have, or travel to darker sites, or simply don't succeed. One of the methods of dealing with it is using a lower ISO setting, which enables you to capture the same amount of photons without overwhelming the sensor. Another is to capture good calibration frames. It's easier to stretch an image to bring out details that are close to the sky glow if the field illumination is flat.

Don

DonR wrote in post #10195133
Not true, zeldaboy. The sky glow is always the darkest part of the image, no matter how bad the light pollution is, because the sky glow is always the darkest thing you can see in the sky. With lots of sky glow, bringing out the faint stars in the image becomes more challenging, but shorter exposures make it harder, not easier. There is no substitute for exposure time if you're trying to capture faint objects.

Those of us who deal with severe light pollution learn the techniques needed to get the most out what we have, or travel to darker sites, or simply don't succeed. One of the methods of dealing with it is using a lower ISO setting, which enables you to capture the same amount of photons without overwhelming the sensor. Another is to capture good calibration frames. It's easier to stretch an image to bring out details that are close to the sky glow if the field illumination is flat.

Don

Overexpose was a poor word choice, while the light pollution is the darkest point, there is a limit as to how much you want your black point to be raised by the light pollution. When shooting in LP you're definitely going to expose enough to have it show up and brighten the background, i'm not saying make the image so dark you have a black sky to start, but the image posted above was bright orange and in the tons of astrophotography i've done in similar skies i've never had better results by exposing so long that everything is a bright orange glow. There is definitely a point where the light pollution will overcome the faint stuff in the image, finding that point where it starts to become too much is key in your skies and everyone's skies are different.

I'd recommend shooting images at something like 5, 10, 15, 20 sec and so on and do a stack of like 5 of each exposure length, to see which images stack up to give you the best combination of some light pollution without overwhelming the stars. Getting to around 30 seconds is ideal, but sometimes the skies are so bad you can't do that.

Thank you for the information on Noel Carboni Photoshop Tools, I will have a look into it. I think I will need to spend more time on post processing manually if I am unable to get the plugins. I have decided that in order to take decent photos, I will have to go away from the town and out into the countryside. I have been taking photos from the house to practise astrophotography because when I tried it once, I had limited time and made quite a mess on some of the shots with many being out of focus. I have managed to learn many things from reading this forum and other places on the Internet.

I will experiment with reducing the ISO values down to about 400 and 200, but still use stacking. I did another stacking set a fortnight ago and was told to increase the time as it would show more stars. I guess that even though I managed to find some really faint stars, the light pollution increased. The problem at the moment is that the nights are disappearing now and the number of clear nights in this part of Wales are rare.

Thank you once again for the advice.

zeldaboy101 wrote in post #10195385
There is definitely a point where the light pollution will overcome the faint stuff in the image...

I disagree with this statement, but perhaps it's the choice of words again. The light pollution never overcomes anything as it always remains the darkest part of the image. This was not true with film astrophotography due to reciprocity failure - the sky glow intensity would actually catch up to the subject eventually, a concept known as the "sky fog limit". But that concept doesn't apply to digital photography since digital sensors aren't subject to reciprocity failure. What happens is that the dynamic range available for the subject decreases when the range taken up by the sky glow increases. This is sometimes refereed to as the digital equivalent of "sky fog limit", but it's really a totally different phenomenon. The stacked image naddieuk posted is nowhere near a reasonable limit for sky glow when working under light polluted skies. I suspect he still had 80% of his camera's dynamic range remaining. With longer exposures, the brightest objects will eventually saturate the detector resulting in loss of image data, but the faintest objects will never be lost or merge into the sky glow until the entire image is solid white.

zeldaboy101 wrote in post #10195385
I'd recommend shooting images at something like 5, 10, 15, 20 sec and so on and do a stack of like 5 of each exposure length, to see which images stack up to give you the best combination of some light pollution without overwhelming the stars. Getting to around 30 seconds is ideal, but sometimes the skies are so bad you can't do that.

Longer is always better, up to the point were your tracking or lack of tracking causes unacceptable star trailing. You are better off reducing the ISO and shooting for the longest exposure time you can manage when light pollution is an issue. You will acquire four times as much signal at 20 seconds and ISO 200 as you acquire at 5 seconds and ISO 800, and the sky glow intensity will be the same. After stacking several exposures, the SNR will be significantly lower with the longer exposures, and post-processing to kill the sky glow and enhance the subject will be easier.

Don