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  #1  
Old 19-06-2018, 09:56 AM
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Slawomir (Suavi)
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How much data is needed?

Hi all,

I have started collecting data for the Omega Nebula - I imaged it once only when I just started my adventure with astro and ever since wanted to redo the image.

It will be a narrowband image. I tend to use only Ha for Lum (unless OIII or SII have a good SNR). I already have about 11 hours of Ha and I have been wondering whether I should add another 5 or so hours before I move on to OIII and SII.

Histogram Only: https://www.astrobin.com/full/351798/B/?nc=user

Damaged stars with Local Histogram Equalisation: https://www.astrobin.com/full/351798/0/?nc=user

Thank you in advance for advices/criticism.

Suavi

Last edited by Slawomir; 19-06-2018 at 02:39 PM.
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  #2  
Old 19-06-2018, 10:06 AM
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multiweb (Marc)
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I think you're already there with Ha.
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  #3  
Old 19-06-2018, 10:36 AM
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The Ha looks smooth enough as is. The Ha is also going to be suppressed in favour of boosting the OIII and SII aid you go for a SHO image as well.
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Old 19-06-2018, 10:48 AM
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Thank you Marc and Colin.

I may then start collecting OIII.

I feel this is a challenging object to process if one wants to show the bright and darker areas simultaneously while not making the image look flat. That's why I thought more signal might be useful.
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Old 19-06-2018, 11:10 AM
glend (Glen)
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Depend on your camera imho. If your using a very low noise camera you can get away with less signal build.
I don't mean to be unkind but it looking at the stars, they seem fuzzy, maybe abit out of focus to me, Ha is usually tack sharp on stars.
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Old 19-06-2018, 11:22 AM
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Slawomir (Suavi)
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Quote:
Originally Posted by glend View Post
Depend on your camera imho. If your using a very low noise camera you can get away with less signal build.
I don't mean to be unkind but it looking at the stars, they seem fuzzy, maybe abit out of focus to me, Ha is usually tack sharp on stars.
Thanks Glen. I was asking whether the data I already have is sufficient. You are not unkind at all, Andy was also suggesting soft data in my previous image of the Lobster, so I could look at autofocus routine just in case. Image scale is 1.18 arcseconds pp and it is a 4" refractor, so stars are perhaps at the limit of resolution for this aperture, but at least are not under-sampled blocky-shaped. Coastal tropical location might also have some influence here.
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Old 19-06-2018, 11:29 AM
markas (Mark)
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Adding an extra 5 hours to the 11 you have will improve SNR by a factor of 1.2 - all other conditions being equal. Whether or not that is worthwhile is debatable!


Mark
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Old 19-06-2018, 11:56 AM
Imme (Jon)
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Quote:
Originally Posted by markas View Post
Adding an extra 5 hours to the 11 you have will improve SNR by a factor of 1.2 - all other conditions being equal. Whether or not that is worthwhile is debatable!


Mark
What's the equation for that one Mark?

I'm not doubting you....just be interested to know how it is worked out!
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Old 19-06-2018, 12:06 PM
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If you double your exposure you only get a ~41% increase in SNR.

(22/11)^0.5 = 1.41

(16/11)^0.5 = 1.21
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Old 19-06-2018, 12:45 PM
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Originally Posted by Atmos View Post
If you double your exposure you only get a ~41% increase in SNR.

(22/11)^0.5 = 1.41

(16/11)^0.5 = 1.21
Thanks Colin....will scribe that on my obs wall for future reference!
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  #11  
Old 19-06-2018, 02:42 PM
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Slawomir (Suavi)
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Thank you all for your input.

I agree with Glen - stars look soft so I checked what might have caused that - it was a speedy processing with Local Histogram Equalisation with no star mask.

So here is unadulterated version with Historgam stretch only.

https://www.astrobin.com/full/351798/B/?nc=user

I can sleep peacefully again. Thank you Glen.

Either way - I shall collect flats for Ha and move on to OIII.
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