How to make a synth Lum from DSLR

[torsinadoc]

Is there a way to make synthetic lum then process it separately then combined with RGB that was processed? Any reason to do this in ST?

[Amaranthus]

I'd be interested in this reply, and also a follow up. As a rule, do you recommend we make a synthetic Luminance in pre-processing, rather than just use the clear-filter channel data, for LRGB work in ST? (i.e. full stack of all LRGB greyscale subs as a Lum channel)? Or is it best to create a synthetic Luminance channel within the LRGB (or color?) modules of ST? I always collect my RGB data at binning x1 (life's too long to bother with binning), so they are compatible with the L subs.

[admin]

Hey guys,

Is there a way to make synthetic lum then process it separately then combined with RGB that was processed?

There is, but it's a little tedious. http://www.startools.org/forum/viewtopi ... 262&p=1493

Any reason to do this in ST?

If the reason for creating a synthetic luminance frame, is wanting to process color and luminance separately and your data is from a DSLR or OSC, then there is no need! StarTools already processes luminance and color separately as long as Tracking is on, simply because it yields better (less noisy) results and is more flexible in terms of color renditions.

However, if you use a mono CCD with color filters, and you have been acquiring narrowband data, or L, R, G and B separately, and/or your R, G and B have different exposure times, then you would want to use the procedure outlined in http://www.startools.org/forum/viewtopi ... 262&p=1493 to create a synthetic luminance frame.

Indeed, adding your R, G and B information to L is a great way to improve your signal!

As a rule, do you recommend we make a synthetic Luminance in pre-processing, rather than just use the clear-filter channel data, for LRGB work in ST? (i.e. full stack of all LRGB greyscale subs as a Lum channel)?

Yes - if you other software to create synthetic luminance frame with, feel free to use that, instead of the procedure outlined in http://www.startools.org/forum/viewtopi ... 262&p=1493.

[Amaranthus]

Based on this (and the fact that I'm using a mono CCD), I'll definitely be following this procedure and going down the synthetic luminance route as part of my standard workflow! It sounds a little complex at first, but I like the way ST does this - it's logical, and powerful in the way it lets one control/specify the relative weightings.

Just a question about weightings - in theory, the L component of each R, G and B channel sum up to the total luminance of the L channel. So in terms of signal, R+B+G = L, right? So let's say, for example, I collected 6 hours of L data and 3 hours each of R, G and B. Would the correct weightings for total luminance be 6:3:3:3 (i.e. 2:1:1:1), or would it be 6:1:1:1 (given that only 1/3 of the total luminance signal is in each of the R, G and B channels)? Or something else, given that the filters differ in the amount of L transmitted?

[admin]

Based on this (and the fact that I'm using a mono CCD), I'll definitely be following this procedure and going down the synthetic luminance route as part of my standard workflow! It sounds a little complex at first, but I like the way ST does this - it's logical, and powerful in the way it lets one control/specify the relative weightings.

Just a question about weightings - in theory, the L component of each R, G and B channel sum up to the total luminance of the L channel. So in terms of signal, R+B+G = L, right? So let's say, for example, I collected 6 hours of L data and 3 hours each of R, G and B. Would the correct weightings for total luminance be 6:3:3:3 (i.e. 2:1:1:1), or would it be 6:1:1:1 (given that only 1/3 of the total luminance signal is in each of the R, G and B channels)? Or something else, given that the filters differ in the amount of L transmitted?

That's a good question! I'm inclined to say 6:1:1:1 without knowing your filters' transmission characteristics (and spectrum response / overlap, etc.), indeed the reasoning being that, in an ideal world, all photons collected in the red spectrum + all photons collected in the green spectrum + all photons collected in the blue spectrum would constitute all possible photons collectable (L).

[Amaranthus]

Yep, thank Ivo, the 6:1:1:1 confirms my thinking too, in the absence of further information on the relative transmissibility of the RGBs. It's a decent working hypothesis, anyway

These are my current LRGB filters - you can see from the transmission diagram that the assumption approximately holds:
http://zwoptical.com/Eng/Cameras/filters/index.asp