Moonlight imaging - what did I do wrong?

[ram]

Hi all, I was experimenting with some broadband imaging with the PowerMate at 2x and since I didn't want to waste nights, this was during a full moon. I was able to find M81 my target for the night and frame it for the night. Each five minute frame looked pretty weak as usual (especially at F15 - 1500mm FL with a 100mm aperture) but when I stacked all the images (and I shot several hours), I didn't get much better than a single frame. I was wondering why this occured. Could it have been the moonlight? Basically I wasn't able to get beyond the sky limited exposure? I don't get it...

Once again I appear to have a clear moonlight tomorrow and I could try to narrowband focus but I really want to get the PowerMate stuff working too. I could try doubling a smaller scope so going from F6 to F12 (840mm with a 70mm aperture) so this way I free up 100mm for NB imaging which is what I want to do now but if the moon was the issue the last time then obviously there's no point. Thanks!

--Ram

[yobbo89]

f15 is pretty slow for deep sky , yes there is a background cut off for your sky where you'll just keep soaking in lp with your longer exposures, this can make processing later a bit harder ie trying to seperated it from your signal, it's best to minimise sky background in your images prior , steve did a tutorial with a program that determines your exposure settings to your sky background , i think my sky and scope gave me about 340 seconds exposure for optimal signal . i can guarantee you'll get much better results once the moon is gone , the moon is enough to cut down signal even on the brightest objects when imaging wide ban , if you run your frames through deep sky stacker it can tell you the frames sky background percentage .

deep sky imaging with a barlow is typicaly not advisable but there is an exception ie.., imaging extreamly bright deep sky objects like eta carina homunculus cloud which is mag -1.0 which is typicaly normal for astronomers imaging this target , in reference m81 is mag 6.9

[ram]

Thanks. I'm just trying to get these galaxies to come up larger in my frames (rather than having to digitally zoom them up). Looks like there's no easy way out of it short of lifting my C925 which I won't do until I get a permanent set up...

--Ram

[ram]

Also, does RGB vs. OSC imaging make a difference in moonlight? Probably not (since it is just reflected starlight) but in terms of light pollution, RGB should be a bit bit better right? Given the narrow bandpasses of the three colour filters rather than something more broad band that lets unnecessary light through?

--Ram

[KathyNS]

When dealing with light pollution, folks are often tempted to double or triple the exposure time to catch more light from their DSO target. Unfortunately, sensors do not discriminate, and doubling or tripling the exposure time will double or triple the light pollution photons as well. The ratio of target photons to LP photons remains the same. Exposure time is not going to help.

Moonlight may be natural, but it is still LP. It is white light, so broadband colour filters will not make any difference to the proportion of LP recorded in your image. After all, an OSC camera is using the same broadband colour filters: it just uses them at pixel size rather than macroscopic size.

Broadband colour filters are called broadband for a reason: their pass-bands are very broad and actually overlap with each other. They won't help even with artificial light pollution. You could perhaps get a slight reduction in LP with a red filter if your primary source of LP is from mercury vapour lighting, which is blue, or with a blue filter and sodium vapour lights, which are orange. But that means travelling to different locations for the different filters, and finding a truly dark site for your green filter. And, with the trend towards LED lighting, which is broadband white, you are back to being unable to filter it.

LP filters will work to a point. They are broadband filters with narrow exclusion bands for the more common narrowband light sources, like sodium or mercury. But they, too, are ineffective against LED lighting or moonlight.

The bottom line is that there is no substitute for dark skies, other than to shoot narrowband. I got some nice Ha data under the full moon the other night.

[yobbo89]

all i can say, there is a slight difference in quality of rgb filters over one another and they do cut of slightly a small ammount of sky background , the moon is just going to destroy anything that isn't narrowband really unfortuantly and even in tight 3nm filters to an extent , i typicly stay away from a moon thats at 80+ % narrowband and only image rgb wideband when there is no moon present , bortle 6 is already killer , the moon adds so much more.

[STEVE333]

Just to jump into the conversation.

The noise in the stacked image should be less than the noise in a single image by the factor [1/sqrt(N)] where N is the number of images stacked. This is true whether the noise from LP is large or small. So if you stack 25 images the noise in the stacked image will be lower than the noise of a single image by the factor 1/sqrt(25) = 1/5. If that isn't the case with your images there may be something else going on.

As for the Exposure Time I agree with most of what Kathy said. The only point I might disagree with is that I would expect that longer exposures might help some. Because you are at f/15 you will collect 9 times less signal than if you were using a f/5 telescope. So, your 5 min exposure would be equivalent to 5/9 min = 33 sec which is pretty short. Your noise might still be dominated by the read noise rather than the noise from the LP (which gets larger with longer exposures). Just a thought.

Steve

[ram]

Thanks all, all that squares with my experience as well. I find imaging in moonlight to be a losing proposition even with narrowband (O3 specially). Anything with over half moon starts to get in the way especially for very very faint targets (like the Squid which I am doing now, and the squid itself is all O3 signal). But I was wondering if anyone had any tricks to get around it since it's wasted clear nights otherwise (this is why I experiment on clear skies with moonlight but that results in a confounder and thus my question which it seems like unless I try without moonlight I'm screwed).

I'm in dark skies otherwise (rural area).

I suppose one thing to do is go for bright objects like planets but I'm not ready for that yet.

--Ram

[yobbo89]

3nm narrowband is your best bett against lp during moon activity.