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  #1  
Old 09-09-2018, 05:32 PM
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cometcatcher (Kevin)
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Luminance ratio to RGB

Is there an optimum luminance ratio to RGB? I want the biggest bang for my time so to speak. Don't we all I suppose.

So if I take an hour of Red, an hour of Green and an hour of Blue, what's the optimum luminance time? 1 hour, 2, 10?
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  #2  
Old 09-09-2018, 06:23 PM
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A great question Kevin - I would also like to know people's experiences with LRGB. My guess is at least 5-10 hours of Lum would be desirable, depending of course on how dark the site is.

I think that RGB data, as long as it has decent SNR, might also be included in the process of making of the Luminance (combining Lum master and RGB masters to make an improved Lum in terms of SNR).
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  #3  
Old 09-09-2018, 06:49 PM
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I did a bit of work on this about a year ago and came up with some interesting results. There are two trains of thought you can do.

1)
Get just enough RGB for good statistical rejection; 5-8 subs and then pump everything into luminance. This allows you to do HEAVY noise reduction on the RGB and then be gentle on the deep luminance.

2)
Forget luminance altogether... for shorter integrations anyway. By bang for time I’m guessing you’re thinking of a few hours per target? When you go with pure RGB you have much better colour rendition and noise control when doing the deeper stretches.

#1 will go deeper with less accurate colour and a terrible background while #2 will be almost as deep but better controlled and actually less noisy overall.
What I found in my testing is that pure RGB and equal amounts of LRGB had less noise than lots of Lum and Little RGB.

Depending on how much time you want to put into a single target, I’d suggest that the best way of imaging is to collect RGB until better SNR requires a lot more data. This is when you turn to Lum.
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  #4  
Old 09-09-2018, 08:00 PM
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Interesting suggestions and findings Colin - thank you for sharing.

Just going purely from theory, or at least from what I think makes sense, is that Luminance collects more photons from the target per hour of exposure (given the skies are sufficiently dark) than RGB filters that block a significant portion of the signal from a given dso, therefore I would go with as much Lum as practical to maximise SNR at the expense of less time dedicated to RGB filters.
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  #5  
Old 09-09-2018, 08:36 PM
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Thanks Colin and Suavi. So it looks like I can't really go that wrong, just depends on colour accuracy or depth priority?
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  #6  
Old 09-09-2018, 09:16 PM
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I usually go at least as long wrt time for L as for R+G+B, although still getting a feel for it.

I find that 40+ subs per channel hits the range when the channel output is cleaning up nicely. With galaxies, hundreds of subs to really target the SNR. This is with my 1600 of course. Prefer a few more with the 178 and its tiny pixels.

YMMV though as your scope is much bigger and faster (thus higher SNR!) than my little refractor
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  #7  
Old 10-09-2018, 05:54 AM
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RGB imaging works well for bright targets but is slow to build SNR on dimmer ones.

I once read a claim, based on empirical evidence, that you should do equal amounts of L, R, G and B. I didn't find the argument completely convincing, but it's probably a reasonable place to start.

I reckon the optimal strategy varies depending on the target and what you're trying to emphasise in the object. If you're trying to bring out dim dusty stuff then you will want to go heavy on L. If you have a colourful galaxy then you'll need a decent swag of RGB.

Cheers,
Rick.
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  #8  
Old 10-09-2018, 07:51 AM
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Sounds reasonable. Depends on target. Otherwise just 1:1:1:1 is easy to remember for now, for a starting point.

See this is what swayed me to a mono cam over an OSC, the flexibility to fiddle with the channels to get the result I desire.
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  #9  
Old 10-09-2018, 05:05 PM
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And then slide in the Ha too
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  #10  
Old 10-09-2018, 05:11 PM
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Quote:
Originally Posted by Camelopardalis View Post
And then slide in the Ha too
Based on Kevin's M27 image I reckon he'll use an Oiii filter tilted so that the bandwidth is shifted to H beta
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  #11  
Old 10-09-2018, 06:36 PM
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How are you adding the L to the RGB image ? Added to any particular channel eg just emphasizes that channel. Does anyone use the Lab Colour option ?
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  #12  
Old 10-09-2018, 07:11 PM
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Quote:
Originally Posted by Slawomir View Post
Just going purely from theory, or at least from what I think makes sense, is that Luminance collects more photons from the target per hour of exposure (given the skies are sufficiently dark) than RGB filters that block a significant portion of the signal from a given dso, therefore I would go with as much Lum as practical to maximise SNR at the expense of less time dedicated to RGB filters.
What youíre saying here is true, luminance does capture a much stronger SNR in the same amount of time as RGB; but this isnít the whole story.
LAB mode allows for a luminance and two chrominance channels. In my tests I replaced the LAB luminance with a true luminance, recombined the LAB into RGB and then checked the noise levels of RGB (2 hour each), LRGB (1.5 hour each), 3LRGB (3 hour L 1 hour RGB) and found that, at least in shorter integrations (6 hours) a pure RGB had less noise.

The caveat of this is that 3LRGB was deeper than LRGB which was deeper than RGB. The increase in depth wasnít hugely significant in the field I was working with which had no IFN but also had very little above Mag 17 and had galaxies showing around the Mag 21-22 Mark.

Without doing some more testing I would suggest that RGB is better with shorter integrations (<10). LRGB for the medium (10-30) and 3LRGB for the long integrations (thinking Rolf here).
Iíd say that after 10 hours per channel of RGB youíve hit the SNR wall and luminance becomes the best way forward from there.
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  #13  
Old 12-09-2018, 01:13 PM
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Quote:
Originally Posted by RickS View Post
Based on Kevin's M27 image I reckon he'll use an Oiii filter tilted so that the bandwidth is shifted to H beta
You know me well lol.
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  #14  
Old 13-09-2018, 02:32 AM
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Hmmmm, , Very interesting Thread Kevin
I was just reading on Ha filters, whether and/or which I could need, to combat heavy LP.
Being totally new to mono have realised to consider which objects to capture or like to capture is an important step to choosing
All the best with your imaging endeavours btw, going forward in leaps & bounds
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  #15  
Old 13-09-2018, 08:18 AM
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Thanks Bob. Narrow band definitely cuts through light pollution, which would be great for imaging nebula in LP.
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