When NOT to image with LRGB, SII & OIII filters

[souls33k3r]

I've always wondered, i know when the moon is near full it's not a good idea to image using SII and OIII filters but what exactly is the limit, when should you say to yourself that moon is (percent) lit up tonight so i shouldn't image using these filters and stick to Ha only? Also same for LRGB filters?

Thanks

[vlaiv]

Well SII is so close to H alpha that I don't really see moonlight making too much difference between 656nm and 672nm.

Maybe only difference being that targets are much brighter on average in H alpha then SII.

Anyways, imaging when the Moon is up is not impossible, it is the matter of having to spend more imaging time to get same SNR than when Moon is not there. I've read somewhere that zenith sky brightness on night of the full moon on otherwise very dark skies is somewhere around mag 18. That is the same sky brightness I get on moonless nights in my LP skies (red zone, ~18.5 mag), and I happily image in such conditions. Granted I use LP filters which help a lot with LP but won't help at all with moonlight - being just reflected sunlight - so full spectrum.

I would say that general rule that I would apply: don't image if moon is less than 10 degrees away from your intended target and brighter than a quarter, leave narrow band (any kind) for nights when there is moon out, more moonlight - go for narrow band where target is brighter. Don't combine shots from moonlit night with those taken on moonless night - meaning same filter / channel shots, unless you have a way of combining low and high SNR images with appropriate weights (special algorithm needed to judge SNRs of individual frames and assign weight to each).

I guess one should use the time under stars to image even if moon is out, with understanding that doing so will result in lesser quality frames and less SNR for same amount of imaging time.

[souls33k3r]

Thanks @vlaiv, that's very helpful advice. Much appreciated.

[alan4908]

43 minutes ago, souls33k3r said:

I've always wondered, i know when the moon is near full it's not a good idea to image using SII and OIII filters but what exactly is the limit, when should you say to yourself that moon is (percent) lit up tonight so i shouldn't image using these filters and stick to Ha only? Also same for LRGB filters?

Thanks

Given that we have limited clear nights in the UK, I'm also interested in what I could image when the moon was up without a significant degradation in image quality.  I currently follow a method that is based on working out the minimum angular distance you need to be from the moon given a particular filter type and the moon phase. The method, called the Lorentizian approach, was developed by the University of Berkeley Astronomical Department.  When the moon is full, I generally don't image objects that are closer than 60 degrees with my Ha or SII filters. My OIII and LRGB filters are set to 120 degrees. When the moon is not full then you can get closer. 

Since I'm into automated imaging, I wanted a capture program that would taken this factor into account, since to me, it would seem key to maximizing imaging efficiency - this is one of the reasons that I went down the ACP route which implements the Lorentzian algorithm.

Alan

[graemlourens]

Hi.

I've done extensive testing on this in the past 2 years and have come to following conclusion:

Most filters except HA are very much affected by moonlight. HA is pretty resistant, but it depends on what target you're going for. If its a strong HA target you can go pretty brutal on your scheduling.

I have absolutely stopped imaging SII, OIII or RGB if the moon is more 5% as it damages the result a lot. But even HA will suffer a lot if moon is >50% and even if its a little further away from your target.
I'd suggest you to also do a few tests yourself. Pick a target and then regularly on purpose just make one frame in moonlight, and then one without moon, and compare the background bias levels.

I have also seen that moonlight in summer is much less drastic than in winter as in summer the moon rises much lower in the sky than in the winter where it gets up pretty high.

It all depends a little though how much imaging time you have. If you have very little, then you can't afford to just 'waste' a moonlit night, so just try and get something that makes a little sense. If you have 'plenty' imaging time or have a remote observatory then its easier to skip a night with high & large moon.

For example yesterday was a pretty large moon during the night: My observatory spent the night making 'testshots' of various targets (for mosaic purposes). Well used time, even if its not data for a specific image, but for planning purposes or just to see whats there...

Just my 2 cents.

Kind regards, Graem

[wimvb]

Me, I just wing it. Too few clear nights to let the moon bother me. I just stay away from faint targets from my light polluted, moonlit lawn. I used to not go out when the moon was more than about 2/3 full, but that gave me too few imaging nights.