How much data is needed?

[Slawomir]

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

[multiweb]

I think you're already there with Ha.

[Atmos]

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.

[Slawomir]

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.

[glend]

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.

[Slawomir]

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.

[markas]

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

[Imme]

What's the equation for that one Mark?

I'm not doubting you....just be interested to know how it is worked out!

[Atmos]

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

[Imme]

Thanks Colin....will scribe that on my obs wall for future reference!

[Slawomir]

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.