LRGB

[Galen Gilmore]

Hi All,

Been reading about Luminance Channels for astrophotography, wanting to try implementing them into my images.

I have some questions about Luminance: Is there a big difference between shooting L subs and creating a synthetic L channel? Can I create L just by using the B&W setting on my DSLR? How do I affectively apply L to my RGB?

Thanks in advance.

[ollypenrice]

There is a huge difference between 'real' L and synthetic L, whether your synthetic L comes from separate RGB filters in a mono or from a one shot colour camera. It is not the fact that luminance is monochrome which matters. What matters is that the luminance channel has way more signal because, in the time available, the L filter has been passing all colours on all pixels all of the time. You might expect an L channel, therefore, to have about three times the signal per unit time as a synthetic L from RGB or OSC. However, whatever the filter bandpass graphs say, I find when I measure it that an L channel is nearer to four times stronger per unit time than a synthetic lum from an RGB shoot. An OSC camera might improve on that but it won't get anywhere near the signal realized by a mono with L filter. So the point of an L filter is to pass all of the light all of the time.

However, when it comes to processing RGB or OSC data without an L channel it is a good idea to work with an RGB layer and a synthetic L layer even if the underlying data is the same (as it is.) You can process the Syn L for sharpness and contrast and then process the colour layer for low detail, low noise, but strong colour. Combining them should improve on doing everything to the poor old RGB/OSC layer at the same time!

Olly

[steppenwolf]

There is another thread running on this which might be of some interest to the OP.

[wimvb]

22 hours ago, ollypenrice said:

What matters is that the luminance channel has way more signal because, in the time available, the L filter has been passing all colours on all pixels all of the time.

...

I find when I measure it that an L channel is nearer to four times stronger per unit time than a synthetic lum from an RGB shoot.

...

So the point of an L filter is to pass all of the light all of the time

This is probably in relation to the dark site where you do your imaging, Olly. But since an L filter passes ALL light, it also passes light pollution. As I wrote in another thread, red and green filters may be optimised to block the most common sources of light pollution, sodium and mercury. In a situation with light pollution, synthetic luminance can have its advantages.

But there's really only one way to find out ...

[Galen Gilmore]

On 2/3/2018 at 13:22, ollypenrice said:

There is a huge difference between 'real' L and synthetic L, whether your synthetic L comes from separate RGB filters in a mono or from a one shot colour camera. It is not the fact that luminance is monochrome which matters. What matters is that the luminance channel has way more signal because, in the time available, the L filter has been passing all colours on all pixels all of the time. You might expect an L channel, therefore, to have about three times the signal per unit time as a synthetic L from RGB or OSC. However, whatever the filter bandpass graphs say, I find when I measure it that an L channel is nearer to four times stronger per unit time than a synthetic lum from an RGB shoot. An OSC camera might improve on that but it won't get anywhere near the signal realized by a mono with L filter. So the point of an L filter is to pass all of the light all of the time.

However, when it comes to processing RGB or OSC data without an L channel it is a good idea to work with an RGB layer and a synthetic L layer even if the underlying data is the same (as it is.) You can process the Syn L for sharpness and contrast and then process the colour layer for low detail, low noise, but strong colour. Combining them should improve on doing everything to the poor old RGB/OSC layer at the same time!

Olly

Thanks for that, interesting info that I didn't know.

[ollypenrice]

On 2/4/2018 at 18:43, wimvb said:

This is probably in relation to the dark site where you do your imaging, Olly. But since an L filter passes ALL light, it also passes light pollution. As I wrote in another thread, red and green filters may be optimised to block the most common sources of light pollution, sodium and mercury. In a situation with light pollution, synthetic luminance can have its advantages.

But there's really only one way to find out ...

I can see the theory behind your point, Wim, but many imagers say that they find OSC more susceptible to LP than LRGB. I can't comment from experience (I'm glad to say! )

Olly

[wimvb]

29 minutes ago, ollypenrice said:

I can see the theory behind your point, Wim, but many imagers say that they find OSC more susceptible to LP than LRGB. I can't comment from experience (I'm glad to say! )

Olly

It's the L in LRGB that I have trouble grasping, unless it is captured with a light pollution filter. Anyhow, astrophotography is enough of a dark art, that a lot of it is beyond my comprehension.

[MartinB]

Yes, I've given up with osc from my light polluted site.  This wasn't based on theory as much as what I end up with from a given amount of imaging time.  I would be interested to hear from anyone who has found osc outperforming lrgb, esp with regard to signal to noise ratio

[Shibby]

Just thought I'd add that a RAW image from a DSLR is a RAW image, regardless of whether you set the mode to monochrome or not, the file will be exactly the same. So, if you are going to use this approach you would want to use all the subs for each 'layer'.