I'm looking for a good tutorial/reference on the effects of moonlight on DSO photos. I know it affects things, but just how, and by how much?
TIA,
Mike
TIA,
Mike
Moonlight effects
Well written Engaging
HI Mike, You are really asking how long is a piece of string question. How the moon affects DSO observing will depend on the phase of the moon, the proximity of the moon to the DSO and your own night sky brightness. It will also depend on whether you are observing broad or narrow band.
Essentially is comes down to the signal-to-noise ratio (SNR). Assuming that the sky signal is much greater that the object signal or the read noise & dark current from your sensor then
SNR = Object signal/(Sky signal)^0.5
All you need to know now how much brighter the moon makes an otherwise dark sky. And the variation is distance from the moon. There is a useful table here
https://www.researchgate.net/figure/The-sky-brightness-distribution-when-the-moons-phase-angle-is-90_tbl2_236346187
Which show that a 50%+ moon illuminates the sky to 19-20mags/square arcsec depending on angle. A dark site has a sky brightness around 22.5mag/square arcsec and an urban site has a sky glow around 18mag/square arsec.
Here is another ref
https://skyandtelescope.org/astronomy-resources/astronomy-questions-answers/how-does-the-moons-phase-affect-the-skyglow-of-any-given-location-and-how-many-days-before-or-after-a-new-moon-is-a-dark-site-not-compromised/
So in an urban site the moon contributes almost as much as the terrestrial light pollution, reducing SNR by around 40%
On a dark site the moon increases the sky noise by a factors of 10-20 - reduces SNR by factors of 3 to 5.
Grossly oversimplifying, those who live in heavily polluted Bortle 8 skies (where it really pays to do narrowband all the time) will keep on doing narrow band - perhaps slightly slower. Those who live under dark skies where broad band images can go deep do to the lake of light pollution, will tend to switch to NB with the moon at 50%+ That is not to say that the effect of the moon doesn't reduce the SNR for NB by factors of 3 to 5 on a dark site - its just that you give yourself the best chance of achieve any worthwhile SNR by tuning to a specific emission wavelength, shutting most of the rest of the sky.
CS Brian
Essentially is comes down to the signal-to-noise ratio (SNR). Assuming that the sky signal is much greater that the object signal or the read noise & dark current from your sensor then
SNR = Object signal/(Sky signal)^0.5
All you need to know now how much brighter the moon makes an otherwise dark sky. And the variation is distance from the moon. There is a useful table here
https://www.researchgate.net/figure/The-sky-brightness-distribution-when-the-moons-phase-angle-is-90_tbl2_236346187
Which show that a 50%+ moon illuminates the sky to 19-20mags/square arcsec depending on angle. A dark site has a sky brightness around 22.5mag/square arcsec and an urban site has a sky glow around 18mag/square arsec.
Here is another ref
https://skyandtelescope.org/astronomy-resources/astronomy-questions-answers/how-does-the-moons-phase-affect-the-skyglow-of-any-given-location-and-how-many-days-before-or-after-a-new-moon-is-a-dark-site-not-compromised/
So in an urban site the moon contributes almost as much as the terrestrial light pollution, reducing SNR by around 40%
On a dark site the moon increases the sky noise by a factors of 10-20 - reduces SNR by factors of 3 to 5.
Grossly oversimplifying, those who live in heavily polluted Bortle 8 skies (where it really pays to do narrowband all the time) will keep on doing narrow band - perhaps slightly slower. Those who live under dark skies where broad band images can go deep do to the lake of light pollution, will tend to switch to NB with the moon at 50%+ That is not to say that the effect of the moon doesn't reduce the SNR for NB by factors of 3 to 5 on a dark site - its just that you give yourself the best chance of achieve any worthwhile SNR by tuning to a specific emission wavelength, shutting most of the rest of the sky.
CS Brian
Helpful Insightful
Thanks Brian. So how long was the string again? 
While all of what you said makes sense, I think I was curious to see a practical application of it all - a side by side shoot out (as much as that's possible). So my 3rd ever decent photo below was taken in Bortle 7-8, 90x2' subs, about another hour+ of calibration data, Canon 100-400, Canon 90D, guided, and a SkyTech CLS Clip filter. I started with no moon, but it did rise (almost full) for a good portion of the lights. Is there a way I can see how much better it would have been if there was no moon at all?
While all of what you said makes sense, I think I was curious to see a practical application of it all - a side by side shoot out (as much as that's possible). So my 3rd ever decent photo below was taken in Bortle 7-8, 90x2' subs, about another hour+ of calibration data, Canon 100-400, Canon 90D, guided, and a SkyTech CLS Clip filter. I started with no moon, but it did rise (almost full) for a good portion of the lights. Is there a way I can see how much better it would have been if there was no moon at all?
Hi Mike,
I can't give you scientific values for your question. But I guess, if you compare your subs without the moon and the subs taken with the full moon out, you will recognize the difference in contrast quite easily. I live in a bortle 4 area and I am not used to your bortle 7-8 sky. All I can say is, that you can still shoot good images with the moon out, as your example of M31 above prooves. But I can hear the screaming photographers shouting at me right now. Of course, it is not as much fun or effective as under dark skies or without the moon.
Shooting with your DSLR may not be the best option in heavy light conditions or with the moon out either. Using a CLS filter is a first step in the right direction. I had the same issues and decided to go for an astro camera and narrowband filters. They are not perfect for your kind of situation, but they work much better.
But the point is, that you may have to collect much more data than in dark and moonless nights. You also may decrease your exposure time and shoot more subs instead. As I already said, this is not as much fun. But personally, I bring the equipment out and still take images. Thats what I love and if I have to use the subs of multiple nights, I'd just do it. Image processing is a bit harder and the best images you will have, are maybe those of the nights with better conditions. I would suggest, you use the opportunities you have. Good conditions are quite rare
Don't know, if that helps you, but it is one opinion you might think about…
CS
Christian
I can't give you scientific values for your question. But I guess, if you compare your subs without the moon and the subs taken with the full moon out, you will recognize the difference in contrast quite easily. I live in a bortle 4 area and I am not used to your bortle 7-8 sky. All I can say is, that you can still shoot good images with the moon out, as your example of M31 above prooves. But I can hear the screaming photographers shouting at me right now. Of course, it is not as much fun or effective as under dark skies or without the moon.
Shooting with your DSLR may not be the best option in heavy light conditions or with the moon out either. Using a CLS filter is a first step in the right direction. I had the same issues and decided to go for an astro camera and narrowband filters. They are not perfect for your kind of situation, but they work much better.
But the point is, that you may have to collect much more data than in dark and moonless nights. You also may decrease your exposure time and shoot more subs instead. As I already said, this is not as much fun. But personally, I bring the equipment out and still take images. Thats what I love and if I have to use the subs of multiple nights, I'd just do it. Image processing is a bit harder and the best images you will have, are maybe those of the nights with better conditions. I would suggest, you use the opportunities you have. Good conditions are quite rare
Don't know, if that helps you, but it is one opinion you might think about…
CS
Christian
Helpful Engaging Supportive
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