OSC and RGB-L question

[DougM43]

I have seen some tremendous images from both types of camera, my OSC camera just like any other as well as DSLR,s has a Bayer matrix of red green and blue pixels but if I were to buy a mono camera (and please correct me if I’m wrong) it would be normal to image with RGB and L in fact reading through older posts  It seems people often spend longer capturing (L) than RGB,So what is the (L) adding that OSC and DSLR are missing out on?

Many thanks, Doug 

 

 

 

[geordie85]

The luminance channel is where the details lie as every pixel is registering every photon. 

If you imagine the bayer matrix is like a filter, red only let's in red light, green only let's in green light and blue only let's in blue light. So if a red photon hits the green filter, it is blocked from the sensor. 

[wimvb]

The Bayer matrix in one shot colour (osc) cameras has a low transmittance, even at each filter's peak wavelength. As such, it will always be less sensitive than an equivalent mono camera.

The advantage of a osc camera is that you don't need to invest in filters and a filter wheel. But a disadvantage is that you're stuck with the sensitivity spectrum decided by the manufacturer.

A mono camera is more versatile as you can choose the filter you want to use.

The human eye is much less sensitive to slight colour variations (chrominance noise) than to intensity variations (luminance noise). By imaging colour (rgb) and luminance seperately, you can collect more L data than colour and create a high quality image in shorter time than if you collect colour alone. With a classic CCD camera you also have the advantage to bin the data from four (2x2) or nine (3x3) pixels in order to refuce noise. Cmos cameras offer less if an advantage in this respect.

There is a down side to LRGB imaging. If you image from a site with light pollution, the luminance data will contain that, as wel as its associated noise. RGB filters otoh, are generally designed to block the worst light pollution, caused by mercury and sodium lamps. These lamps emit light at wavelengths between red and green. By designing red and green filters that will transmit only light just above (red) or below (green) these wavelengths, RGB only can be less sensitive to light pollution than LRGB. Also green and blue filters are generally designed to overlap at the wavelength at which Oiii emits. Oiii signals are therefore "boosted" by RGB filters.

If you use a cooled cmos camera, and live in a light polluted area, you should at least experiment with only shooting RGB and perhaps create a synthetic Luminance by averaging the R, G, and B data.

[DougM43]

Very informative answer Wim, thank you for taking the time.

Doug

[Starwiz]

3 hours ago, wimvb said:

perhaps create a synthetic Luminance by averaging the R, G, and B data.

I hadn't thought of that.  Something to look into.

John

[ollypenrice]

7 hours ago, geordie85 said:

The luminance channel is where the details lie as every pixel is registering every photon. 

If you imagine the bayer matrix is like a filter, red only let's in red light, green only let's in green light and blue only let's in blue light. So if a red photon hits the green filter, it is blocked from the sensor. 

In truth this is not the main reason for the luminance channel containing most of the detail. It seems like a reasonable explanation but debayering algorithms are very sophisticated (some are better than others) and they use ingenious ways of working out what, say, the missing red pixel would have recorded had it existed. Imagine the curved edge of a red nebula: a red filter in a mono camera will record that curve on every pixel when an OSC will only record it on one pixel in four. However, the debayering algorithm will reconstruct that curve as a red value on all pixels. This is not quite as good but it is very good indeed. RGB from a mono camera is shot on every pixel for every colour but it still doesn't compare with luminance.

No, the real reason the Luminance channel is powerful both for finding detail and for finding faint signal is that it passes all the light of the visible spectrum at once, which is to say about three times as much as gets through a colour filter. The LRGB system was invented, pure and simple, to save time. That's what it does. When you shoot L you are capturing vastly more light than through a colour filter so you get more signal. This will drag out the faint stuff and allow you to sharpen the details in the areas of strong signal.

3 hours ago, wimvb said:

 

There is a down side to LRGB imaging. If you image from a site with light pollution, the luminance data will contain that, as wel as its associated noise. RGB filters otoh, are generally designed to block the worst light pollution, caused by mercury and sodium lamps. These lamps emit light at wavelengths between red and green. By designing red and green filters that will transmit only light just above (red) or below (green) these wavelengths, RGB only can be less sensitive to light pollution than LRGB. Also green and blue filters are generally designed to overlap at the wavelength at which Oiii emits. Oiii signals are therefore "boosted" by RGB filters.

 

Wim, I follow your reasoning but have read many times that OSC cameras are more susceptible to light pollution than mono. This from people who've tried it. I've only ever imaged at a dark site so I have no experience but this is the first time I've heard OSC praised as a light pollution buster.

Processing an OSC image using a synthetic lum in the way one would use a real lum is sound advice but I know from experience that there is no comparison in signal strength between OSC or RGB and genuine luminance. Theory might suggest that real lum might be 3x stronger than synthetic but I think it's more like 4x stronger in my data.

In a nutshell, Lum catches all the signal and RGB is used to distinguish between the colours. There is no need to waste photons by shooting only colour.

Olly

[geordie85]

17 minutes ago, ollypenrice said:

In truth this is not the main reason for the luminance channel containing most of the detail. It seems like a reasonable explanation but debayering algorithms are very sophisticated (some are better than others) and they use ingenious ways of working out what, say, the missing red pixel would have recorded had it existed. Imagine the curved edge of a red nebula: a red filter in a mono camera will record that curve on every pixel when an OSC will only record it on one pixel in four. However, the debayering algorithm will reconstruct that curve as a red value on all pixels. This is not quite as good but it is very good indeed. RGB from a mono camera is shot on every pixel for every colour but it still doesn't compare with luminance.

No, the real reason the Luminance channel is powerful both for finding detail and for finding faint signal is that it passes all the light of the visible spectrum at once, which is to say about three times as much as gets through a colour filter. The LRGB system was invented, pure and simple, to save time. That's what it does. When you shoot L you are capturing vastly more light than through a colour filter so you get more signal. This will drag out the faint stuff and allow you to sharpen the details in the areas of strong signal.

Wim, I follow your reasoning but have read many times that OSC cameras are more susceptible to light pollution than mono. This from people who've tried it. I've only ever imaged at a dark site so I have no experience but this is the first time I've heard OSC praised as a light pollution buster.

Processing an OSC image using a synthetic lum in the way one would use a real lum is sound advice but I know from experience that there is no comparison in signal strength between OSC or RGB and genuine luminance. Theory might suggest that real lum might be 3x stronger than synthetic but I think it's more like 4x stronger in my data.

In a nutshell, Lum catches all the signal and RGB is used to distinguish between the colours. There is no need to waste photons by shooting only colour.

Olly

I learned something new today. 

[rickwayne]

Of course, the other option for imaging from light-polluted areas is to pick a narrowband wavelength which predominates in the light from your target, and use that as a luminance channel. It will emphasize whatever color its wavelength falls within (e.g. H-alpha will preferentially brighten the red areas of an emission nebula), but sometimes that's perfectly fine. If I'm shooting the North American Nebula, it's all red anyway.

That won't do very well on, say, a reflection nebula. Using H-alpha for luminance on the Trifid is going to leave you with some pretty dim blue areas.

I don't think the wavelengths of the three commonest narrowband filters map very well to creating a synthetic L that includes R, G, and B. Hydrogen-beta for blue, maybe, and O-III for green, but it's hardly ideal.

Edited July 14, 2019 by rickwayne

[wimvb]

1 hour ago, ollypenrice said:

Wim, I follow your reasoning but have read many times that OSC cameras are more susceptible to light pollution than mono. This from people who've tried it. I've only ever imaged at a dark site so I have no experience but this is the first time I've heard OSC praised as a light pollution buster.

I probably wasn't clear enough. What I meant was r, g, and b filters as used with a mono camera, NOT the bayer filter array of an osc.

Eg Astrodon's

Or Baader's

Notice the gap between red and green, and the overlap between blue and green.