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  #61  
Old 15-10-2016, 10:11 PM
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What kind of increase in pixel values are you seeing in the nebulosity?
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  #62  
Old 16-10-2016, 05:29 AM
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This is where I need to learn much more Dunk! I don't yet have a grasp of how to measure ADU or pixel values. We are away right now, will post comparison pics on return and try and get some stats using pi. Normally I cannot see much difference in comparison shots but in this case I could definitely see an advantage with only minor sensor glow. Will try 15 next time
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  #63  
Old 16-10-2016, 07:00 AM
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open the fits file up in fits liberator and just hover the pointer around an area and you'll see values in the upper right pane...besides x and y you've got the pixel values, or ADU.

You can do the same in PI, but I think by default it uses normalised values just to confuse the issue.

If you register the images on top of each other you should be able to make some pretty accurate comparisons. Pick a favourite region of nebulosity, and some stars, and see what sort of values you get. That's it!
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  #64  
Old 16-10-2016, 07:14 AM
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provided the sky didn't change much, the ADU values above bias will have doubled if you doubled the exposure time .

in PI, you can also put a small preview block in a smooth region (keep away from hot pixels and stars) and then select it the "statistics" process - mean will give the average signal and MAD is normally a good estimate of the noise.

Last edited by Shiraz; 16-10-2016 at 07:30 AM.
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  #65  
Old 16-10-2016, 09:59 AM
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Thanks chaps, will give it a go. Not sure what the record length exposure is for the 1600, will try to break it's record soon!
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  #66  
Old 17-10-2016, 07:18 PM
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Right, finally got onto the PC and did some visual comparisons, yowzers! Definitely more detail and contrast with the 10 min subs versus 5, both at 75 gain (default gain is meant to be 139 but I wanted to try minimal, which apparently have more noise than 139, but just wondered what the results would be).

Links to original 300 sec and 600 files.
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  #67  
Old 18-10-2016, 10:46 AM
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Originally Posted by SimmoW View Post
Right, finally got onto the PC and did some visual comparisons, yowzers! Definitely more detail and contrast with the 10 min subs versus 5, both at 75 gain (default gain is meant to be 139 but I wanted to try minimal, which apparently have more noise than 139, but just wondered what the results would be).

Links to original 300 sec and 600 files.
Maybe want to be a bit careful just looking at subs Simon. The subs will continue to get better as you increase the sub exposure - without limit. But what you are actually after is the best result in a stack and there is a point where the stack quality does not improve with longer subs.

if you want to compare 300 with 600 seconds, you should stack two of the 300sec subs to get the same overall exposure.

Last edited by Shiraz; 18-10-2016 at 11:03 AM.
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  #68  
Old 18-10-2016, 10:56 AM
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Ha measurements for the 1600 at gain200:

with a 6nm Ha filter and f4 optics, sampling at about 0.75arcsec.
measured dark sky in 10 minute sub = 11.7 photoelectrons
measured noise = 3.87 photelectrons rms
expected shot noise (from 11.7 photoelectrons) = 3.41 photoelectrons rms
read noise <1e rms

ie, even with subs as short as 10 minutes the shot noise of 3.41e is completely overwhelming the read noise of <1e under what was fairly dark sky - very much shorter subs could have been used without penalty. It would have been close to sky-noise limited even at gain 100, where the dynamic range is much better. For comparison, my H694 would not have been anywhere near sky-noise limited at 10 minutes, since it has read noise that is significantly higher than the measured shot noise

Also, the measured noise is a fraction of an electron higher than the combined shot noise (due to the signal) and the read noise - ie there is a smidgin of fixed pattern noise in there, but it is vanishingly small. This camera seems to work predictably right down to the theoretical limits.

The maximum useful sub length needed for Ha imaging with my system is around 10 minutes at gains of 100+ and shorter subs will do the job if the sky is bright.

Last edited by Shiraz; 18-10-2016 at 02:24 PM.
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  #69  
Old 18-10-2016, 12:09 PM
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That's very useful, thank you Ray. I will try your method to determine optimal sub length for my gear, as I relied on SGP or PI for doing that for me, with varied success. Also, I can't wait to actually take my telescope out - it has been ages...
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  #70  
Old 18-10-2016, 01:06 PM
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Great stuff thanks Ray, I was thinking of experimenting with some narrowband from the city so that gives me something to grapple with. I expect I'll be sky limited pretty quickly by the looks of it
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  #71  
Old 18-10-2016, 01:15 PM
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Originally Posted by Slawomir View Post
That's very useful, thank you Ray. I will try your method to determine optimal sub length for my gear, as I relied on SGP or PI for doing that for me, with varied success. Also, I can't wait to actually take my telescope out - it has been ages...
Quote:
Originally Posted by Camelopardalis View Post
Great stuff thanks Ray, I was thinking of experimenting with some narrowband from the city so that gives me something to grapple with. I expect I'll be sky limited pretty quickly by the looks of it
it isn't actually my idea - M&T did the same sort of thing earlier in this thread and it seems to go back as far as these papers at least: http://www.hiddenloft.com/notes/SubExposures.pdf
http://www.stanmooreastro.com/eXtreme.htm

it is a good way to work out how your system is performing and theory behind it is the basis for the various optimum sub-length calculators and also the "Really Easy" method.

yeah Dunk, if you have read noise of say 2e, you only need about 6e shot noise to overwhelm it - and you get that from just 36 photoelectrons, which is really miniscule - it won't take long under bright sky. In fact there may be a case for going for longer than optimum just to keep the processing load down - the only thing you will lose is dynamic range.

Last edited by Shiraz; 18-10-2016 at 02:30 PM.
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