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Very low SNR contribution from Observations
#1
Hi Greg,

I'm still interested in understanding why some observations are contributing between 0 and 1 to SNR.  I trust your calculations - I'm sure I have a configuration issue, but so far I haven't been able to find it.  E.g., I now have between 1 and 3 hours of data per filter for NGC 2903 and the SNR per filter is showing as 1 or less.  That means to get to my goal of SNR=60 I'd need over 300 hours of data (five filters), which doesn't quite feel right.  

I'll attach two screen shots, Exposure Goals and Filters, and Observations.  I'm not seeing these very low SNR results for all my projects, just some of them.  

       

Thanks for any help.  

Hope you're enjoying your holidays!
Rowland
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#2
Hi Rowland,

I noticed that in your Exposure Goals section you have selected 'Expose for' as 'Target Object' 'Faint Halo'. In the 'Info & Suggested Settings' section the MagA value is 23.68. I looked at NGC 2905 in the Exposure Calculator and the surface brightness difference between 'Main extent, arms' and 'Faint Halo' is ~2 magnitudes. That might account for the reduced SNR that you're getting.

I don't know the characteristics of your imaging system so I can't be sure. Maybe you're after the faint halo. In that case, ignore my observation.

Have a Merry Christmas & Happy New Year!

Phil S.
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#3
Hi Phil - and Merry Christmas and Happy New Year to you too!

That sure sounds like a good theory and thanks for taking the time to think about it. I went back to the project and changed the goal to "Main Extent and Arms," clicked Done, then re-opened the project. The SNR results didn't change for any of the existing observations.

I happened to capture 32 more frames of this last night so I added that observation, and it showed up with an SNR of 1.

Maybe that's the reality of this object - it's just so dim that even though it looks to like I'll get an "OK" image with maybe 20 hours of data, the SNR is just very low. In that case, I should probably switch my goal to hours of exposure, or re-adjust my thinking about what an acceptable total SNR should be.

Cheers,
Rowland
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#4
Hi Rowland,

Have you checked with the Exposure Calculator for your Imaging System & observing location to see what kind of SNRs it predicts? I don't know if you can revise your Exposure Goals for an observation after it's been collected and change the SNR. It looks like the Halpha filter is going to take forever. The Rstars value is 58.6. NGC 2905 is a galaxy. Is it a strong Halpha emitter or are you looking to image HII regions?

Hope this helps,

Phil S.
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