LRGB v RGB
Posted: Thu Apr 16, 2020 2:42 pm
I was going to post this on Cloudy nights but wanted to run it passed here first.
Question is it better to take more Luminance images than just RGB? i.e. take a set of LRGB or just RGB.
I would like to revisit this debate because I think the equipment and software used to capture and process astro images has moved on and would like to have my thought process tested.
I am one of those who has moved from taking OSC DSLR (mirrorless DSLR) astro images to dedicated astro mono camera with a CMOS sensor (ZWOASI1600MMPro). I was lead to believe from the Astro community wisdom that was the way to go. In my ignorance I thought it was simply a matter of taking R + G + B images through wide band filters. I was not fully aware of the issue of taking images using a Luminance filter or indeed its significance. Now after reading the very informative debate on Cloudynights I can see that there are good arguments for both methods. But to me it comes down to what equipment you are using and your software method of processing.
I have to say that I am more persuaded by the RGB arguments than the LRGB. I have no doubt that the maths show LRGB to be better but ignores what some would call the true signal (i.e. the useful bit of signal). If I am to look at the ZWO filters set that came bundled with my ASI1600MMPro I see that the Blue filter has a response between 400-500nm, The Green overlaps this a bit between 470-590nm and the red between 620-700nm. There is a significant gap (590 -620nm) between the red and the green. I am assuming that there are not many DSO’s out there that shine light at this wavelength and it is more likely than not to be from light pollution (unwanted signal = noise). Now the Clear filter used for luminance has a much wider band between 400-710nm including the gap between the red and green. It seems to me therefore that the clear filter will show more “noise” because of capturing this bit of the spectrum, which I don’t want.
Now some of the claims on the LRGB side is that for a set imaging time you get better results, but requires you to be binning your R+ G + B frames 2 x 2 and hence having reduced time exposures. The reasons for doing this appear to be because of read noise in CCD sensors being significantly reduce by binning. But wait I am using a CMOS sensors I image at a gain of 129 = a read noise of about 1.8 (e-rms). I have read that because the read noise is so low there is no benefit of binning at the sensor for a CMOS camera, so I do not bin. That means if I go with the LRGB method I now collect less RGB for the same imaging session because I have to take Luminance frames.
This becomes very easy to prove now as all I need to do is take a set of LRGB images and process one as a LRGB and one as a RGB set. Now I use Startools and that has a very useful feature of being able to process luminance and colour in parallel. So I have taken some images of the Hickson 44 Galaxy group with my iOptron RC6 no reducer or field flatterner. Clear filter 60 x 60sec, Red filter 55 x120sec, Green 55x 120sec and Blue 55 x 120sec. The first image was processed in Startools using the Luminace, colour L, RGB total integration time 6.5 hrs. The second image was processed using the L + Synthetic L from RGB, RGB total integration time 5.5hrs. The third set was taken by creating a synthetic luminance from the LRGB and using that new luminance file what I think they call LLRGB. All were processed in exactly the same way following the loading and blending of the files. I have not changed anything in the colour module just accepting the default values. Now the only difference I can see is that the LRGB image shows more noise and a very slight colour loss and that is despite it having longer integration time. If I am missing something then the difference is so small that at the moment to me it would come down to other factors.
So my take on this for my setup is that the difference is so small as makes no difference I can get longer integration taking just RGB and the real benefit is I can ditch the Clear filter and put in an OIII narrowband filter in my 5 position Electronic filter wheel to go along with the Ha already in there.
Question is it better to take more Luminance images than just RGB? i.e. take a set of LRGB or just RGB.
I would like to revisit this debate because I think the equipment and software used to capture and process astro images has moved on and would like to have my thought process tested.
I am one of those who has moved from taking OSC DSLR (mirrorless DSLR) astro images to dedicated astro mono camera with a CMOS sensor (ZWOASI1600MMPro). I was lead to believe from the Astro community wisdom that was the way to go. In my ignorance I thought it was simply a matter of taking R + G + B images through wide band filters. I was not fully aware of the issue of taking images using a Luminance filter or indeed its significance. Now after reading the very informative debate on Cloudynights I can see that there are good arguments for both methods. But to me it comes down to what equipment you are using and your software method of processing.
I have to say that I am more persuaded by the RGB arguments than the LRGB. I have no doubt that the maths show LRGB to be better but ignores what some would call the true signal (i.e. the useful bit of signal). If I am to look at the ZWO filters set that came bundled with my ASI1600MMPro I see that the Blue filter has a response between 400-500nm, The Green overlaps this a bit between 470-590nm and the red between 620-700nm. There is a significant gap (590 -620nm) between the red and the green. I am assuming that there are not many DSO’s out there that shine light at this wavelength and it is more likely than not to be from light pollution (unwanted signal = noise). Now the Clear filter used for luminance has a much wider band between 400-710nm including the gap between the red and green. It seems to me therefore that the clear filter will show more “noise” because of capturing this bit of the spectrum, which I don’t want.
Now some of the claims on the LRGB side is that for a set imaging time you get better results, but requires you to be binning your R+ G + B frames 2 x 2 and hence having reduced time exposures. The reasons for doing this appear to be because of read noise in CCD sensors being significantly reduce by binning. But wait I am using a CMOS sensors I image at a gain of 129 = a read noise of about 1.8 (e-rms). I have read that because the read noise is so low there is no benefit of binning at the sensor for a CMOS camera, so I do not bin. That means if I go with the LRGB method I now collect less RGB for the same imaging session because I have to take Luminance frames.
This becomes very easy to prove now as all I need to do is take a set of LRGB images and process one as a LRGB and one as a RGB set. Now I use Startools and that has a very useful feature of being able to process luminance and colour in parallel. So I have taken some images of the Hickson 44 Galaxy group with my iOptron RC6 no reducer or field flatterner. Clear filter 60 x 60sec, Red filter 55 x120sec, Green 55x 120sec and Blue 55 x 120sec. The first image was processed in Startools using the Luminace, colour L, RGB total integration time 6.5 hrs. The second image was processed using the L + Synthetic L from RGB, RGB total integration time 5.5hrs. The third set was taken by creating a synthetic luminance from the LRGB and using that new luminance file what I think they call LLRGB. All were processed in exactly the same way following the loading and blending of the files. I have not changed anything in the colour module just accepting the default values. Now the only difference I can see is that the LRGB image shows more noise and a very slight colour loss and that is despite it having longer integration time. If I am missing something then the difference is so small that at the moment to me it would come down to other factors.
So my take on this for my setup is that the difference is so small as makes no difference I can get longer integration taking just RGB and the real benefit is I can ditch the Clear filter and put in an OIII narrowband filter in my 5 position Electronic filter wheel to go along with the Ha already in there.