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Old 13-07-2013, 08:24 PM
Garbz (Chris)
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Combining good data with average data

I'm attempting to do a multinight shot of the same target and one thing I've noticed is my first night I was getting far better results than my second, however I only have 1 hour of data from the first night and 3 hours from the second.

The data from the second night is significantly more washed out (the moon is out now). Certainly when looking at single frames the first night was great, and the second night is very average.

This got me thinking that there's a point where it no longer makes sense to keep capturing data as the stacking process would result in your good frames getting dulled by the bad. Is my thinking right? Is it worth augmenting good data from a perfect night with data from an average night?

Guess I'll stack two different attempts and process them separately tomorrow and see what happens. Bit of an experiment.
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Old 13-07-2013, 10:56 PM
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dolbs (John)
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Once the moon starts becoming an issue I would personally think it better to give imaging a miss. Just the same it will be interesting to hear how the two stacks turn out. You may be able to get away with it.
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Old 14-07-2013, 11:22 AM
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What's wrong with your "average" data? If the seeing was good and the only issue is a brighter sky background then you may get some benefit from stacking all the data and then trying to remove the gradients afterwards. You'll get some shot noise from the sky glow but the extra data may make up for that. As you suggest, best to suck it and see!
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Old 15-07-2013, 12:00 PM
jase (Jason)
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Originally I thought the context of this discussion was exclusively focused on FWHM where it easy to discriminate the difference between good and average data. However when it comes to gradients from the moon or other sources it does add complexity.

From a data rejection perspective, its imperative for the algorithm to determine what is outlier data i.e. not normal. Combining several average subs with a gradient will see the information you want to reject be viewed as normal. Care should be taken to exclude or reduce the quantity of average subs but having a couple in place should do no harm. Equally, depending on the gradient it is likely not too difficult to clean up post processing.
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Old 15-07-2013, 09:59 PM
Garbz (Chris)
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The issue is not so much gradient but rather lack of contrast. A moon free night with very good atmospheric transparency compared to a 25% growing moon with average transparency. Don't get me wrong there's just lower signal-to-noise data in there. So we stack to increase SNR, does that mean an hour of perfect conditions is the same as 2 hours in slightly non-perfect conditions?

The results worked well. Here's the final: http://www.iceinspace.com.au/forum/s...d.php?t=109766

Attached is a screenshot of two pictures. No DBE applied, just a simple and identical histogram curve and some debeyering. Now obviously the data on the left 5 min exposure is better than the data on the right one. But the right one is still fine.

In this case I had about 1 hour worth of excellent data, and 3 hours worth of ok data. The stack of only the excellent data did not have the same quality as the stack of all 4 hours.

But obviously there is a point where this no longer applies. If I take data in a full moon during a dust storm I imagine the results would be worse when combined with good data then just having the good data alone.

Attached Thumbnails
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Old 15-07-2013, 10:21 PM
jase (Jason)
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That's indeed a challenge. If the well depth is small then you'll reach the ceiling faster i.e. the pixel charge will fill with undesirable noise from the moon glow over desirable extended object signal. Have you tried altering your exposure times to see if that makes a difference. You are right that there will come a point of diminishing returns.

Have to say given the circumstances, the resulting image is a solid outcome. DBE is fantastic for such challenges.
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Old 16-07-2013, 12:30 AM
Garbz (Chris)
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How does well depth make a difference? The "noise" that is the light polluted sky would cover the signal in the same way regardless of exposure or well depth would it not?
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Old 16-07-2013, 09:17 AM
jase (Jason)
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Well depth doesn't make a huge difference on its own but there are other factors at play here that influence the result. Perhaps a more influential component is the focal ratio of the system. Faster systems, say F/5 and lower will have a higher sky background flux when imaging from the burbs than a slower system. There is considerable information online related to optimum exposure times and sky conditions.
http://starizona.com/acb/ccd/advtheoryexp.aspx
http://www.hiddenloft.com/notes/SubExposures.pdf

A general principle still applies, go as long as you can before you hit the sky flux limit.
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Old 16-07-2013, 11:11 AM
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I think there is a common misapprehension that capturing subs longer than the sky limited exposure is bad. I thought that myself until I did some more reading and thinking on the topic.

What happens when you go longer than the sky limit is that you no longer get a significant benefit from reducing the impact of read noise. That doesn't mean that you don't get an improvement in S/N. The longer exposure still gives you better S/N for both the wanted signal from the object and the unwanted signal from the sky glow.

The disadvantage of longer subs is that you'll lose more exposure time if you have to throw one of them away (due to a problem like a guiding issue) and that rejection may be less effective with fewer subs.

In the case where the sky limited exposure is short compared to the time needed to set up and download a sub (e.g. a bright sky glow with a slow camera like my STL11K) it does make sense to expose longer than the sky limit. The longer subs still improve S/N and the dead time waiting for the camera to download is reduced.

Cheers,
Rick.
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