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Old 01-07-2012, 12:57 AM
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Tandum (Robin)
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M8 Ha Wierdness.

Tweeked the RC8 this arvo and tested exposure times tonight with a guide scope on top instead of an OAG.

I got 9 usable 20minute subs (3hours) and 12 x 5minute subs (1hour).

I don't see a lot of difference between these. Anyone know why?
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Last edited by Tandum; 01-07-2012 at 09:42 AM.
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Old 01-07-2012, 07:41 AM
stevous67 (Steve M)
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I'd be buggered too Robin. Normally one of my Ha subs at 10mins looks much dimmer than those at 20mins. Smoothness of my 20min subs I suppose is a major difference too. But surely they must have looked different to each other prior processing? In these lower res images, I cannot see detail differences, is there any? What's nice about the top image, it shows more contrast, and M8 has lots of contrast people normally miss.

Steve
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Old 01-07-2012, 09:34 AM
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Tandum (Robin)
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I might have another fiddle with the data steve. The hystograms on single subs seem to think the 20minute subs hold more data.
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Old 01-07-2012, 12:31 PM
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RickS (Rick)
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Robin,

As Steve said, it's difficult to tell too much from the < 200KB images stored as attachments, but a few comments:
- you will only get about a 1.7 times reduction in noise from 3 times the exposure - sqrt(3)/sqrt(1)
- M8 is a pretty bright target, so the S/N is fairly high to start with. Reducing it by 1.7 times may not be that noticeable.
- Have that fiddle you mentioned. Processing will make a difference so maybe you can squeeze a bit more out of the longer subs
- Have a close look at the background. Any difference in noise will be more visible there
- Try stacking them all together. You might get a small improvement from that

Cheers,
Rick.
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Old 01-07-2012, 08:24 PM
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Tandum (Robin)
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The 20meg tiffs look the same as the jpegs just as the 200K jpg looks like the 70K jpeg, the upload process did that by the way.

I'm not worried about noise or sqrt calculations, what seems odd to me is 20minute subs appear to show the same content as 5minute subs. I'll admit this is the first time I've captured Ha through the RC8 with this camera so I'm probably just adjusting to the combo.
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Old 01-07-2012, 08:54 PM
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The histogram for the subs looks like you got some benefit from the longer exposure. Now you just need to figure out why you can't see it, Robin
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Old 02-07-2012, 09:37 AM
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DavidTrap (David)
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Your 5 min subs definitely look sharper - my hunch is less guiding error. It should require less stretching of the image, so theoretically the tonal range should be better.

I did a similar experiment on the weekend with 10 vs 20min Ha exposures. I'll have to process them properly and have a look. I did it mainly to see if stars were filling the pixel wells on longer NB exposures.

DT
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