There must be a reason why Lum is required

[DJScotty]

Hi all...

I just came across an image on Astrobin, where the imager has stacked all of the LRGB subs into one (as he calls it) Super Luminance.

This led me to thinking, why can't the RGB subs be all stacked to create the luminance rather than a new set of data?

I know some people bin their RGB data for speed, and then there is the need for different sets of flats, but if one was not inclined to bin their RGB data, what is the overall benefit of shooting a luminance channel rather than stacking the RGB subs together?

Should you not be getting essentially the same required data?

The post is here http://www.astrobin.com/full/285119/0/

Just a thought.

Cheers,

Scott

[Slawomir]

Hi Scott,

A good question and the answer is yes, you can and many do it. Easily done in PI with ImageIntegration tool :-)

You can/should also combine narrowband channels to create a synthetic Luminance that will show more info as opposed to using say Ha alone as a Lum.

Here is a link to a bit more detailed discussion: http://pixinsight.com/forum/index.php?topic=6042.15

[codemonkey]

The difference is that when you're shooting lum, you're getting light from red, green and blue all at the same time, so you're getting better SNR, thus showing up fainter stuff more quickly and cleaning up the noise more quickly.

I combine my RGB with my L, since they're all shot using 1x1 binning; basically throwing away data if you don't.

[RickS]

RGB only imaging can work well, but is best suited to bright targets (or automated scopes with access to frequent clear skies). As Lee noted, LRGB will get you the same SNR in less time.

Cheers,
Rick.

[Shiraz]

Some methods seem to be significantly more efficient than others (time wise). The attached spreadsheet shows the results of a very simplistic analysis based on scaling to get to the same SNR from some different strategies. Time in hours has been presented for my system for a bright target, but the main point is the relative times of the different approaches, not the absolute time.

The four possibilities summarised in the attached spreadsheet are:
1. Lum + RGB full res where the RGB subs are not long enough to be sky limited – ie they cannot be added to the luminance stack, but may be adjusted for the desired dynamic range.
2. As above but with RGB subs that are also sky limited and that are added to the luminance stack. This approach does not allow the RGB subs to be deliberately shorter to increase the colour dynamic range.
3. Lum + 2x2binned RGB where the RGB data only requires ¼ as long, but has lower resolution, so cannot be added to the luminance – question is, does this lower res introduce any complications?
4. RGB with subs long enough to be sky limited, but with no luminance. This cannot be equivalent to 1. above, so have chosen the time required to get to the same luminance SNR, but with higher quality colour results. It can take a long time to get results, but maybe they are worth the wait?
5. Rick's method - RGB 1x1 subs long enough to add to luminance, then software bin RGB to generate the colour data.

Based on this very simple comparison, there may be significant gains to be made in time required for an image, simply by choosing an appropriate strategy. I have been using strategy 3. above with reasonable results and it is definitely quicker to get to a given result than strategy 1. The worst possible efficiency comes with RGB at full res - you never sample more than about 1/3 of the available photons.

[DJScotty]

Are you trying to convert me AGAIN?!!! :D
So what I am thinking of doing is stacking all of the RGB data in DSS to "create" the synth lum. I only ever use one set of flats for all filters, so that shouldn't be an issue...

Hmmmmm, maybe that's why me images look crap! :lol:

[DJScotty]

Rick, Lee, Ray and Suavi,

Thanks for your responses.

So what I am taking away from all of this is:

Binning 2x2 saves time
It is possible to do, and is done
If you decide to actually shoot Lum and then add unbinned RGB data, there is better SNR and thus overall benefit to the image in the end


Definitely something to think about. Thanks gents!

[Shiraz]

the one possible disadvantage of adding RGB to luminance is that, if you have short RGB exposures, you could possibly end up with slightly lower SNR because the read noise you introduce from the RGB subs overwhelms the slight reduction in shot noise from the extra signal in those subs - if you are going to add in RGB subs, the exposures should be roughly 2-3x as long as the lum subs for best effect. Probably always worth a try though if using PI:D - it does a good job of getting the best possible stack results, even if the exposures are not optimum.