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Tri-colour exposure time

09 Jan, 2020 20:28
Hi, I've recently purchased a set of tri-colour filters to enable me (try too!) to take colour images on my ZWO mono camera.  Can any one offer advice on the ratio of exposures I need for each of the filters, or should I use the same exposure for all 3 filters.  I'm planning to image M42.  Thanks for any help.
24 Jan, 2020 10:15
I haven't done any mono imaging, but from what I read it is similar to choosing the right integration time for OSC cameras: Try to put the main peak of the histogram to 1/3rd from the left. Most people image the same time for R, G & B. For luminance channel L you need to have a different exposure time, usually shorter.

One thing mono allows is to image with 2x2 binning on the colors and only do the luminance 1x1. I think you need more integration time with L than with RGB - something like: time-for-L = sum(time-for-each-color).

I hope this helps.
24 Jan, 2020 20:01
Thanks for the help.
25 Jan, 2020 06:34
The advice to bin 2x2 for color is more appropriate for CCD cameras and not CMOS. CMOS sensors are read pixel by pixel in hardware regardless of binning instruction, so there is no noise advantage to 2x2 binning of color when reading a file (unlike in CCD cameras where there is a noise reduction by binning). Yes, the resulting "superpixel" has 2x better SNR, but no different than binning the final image after the fact in software without irretrievably  throwing away the resolution in the raw data which you will do if you bin while reading. The only reason to bin while reading a CMOS camera is if you are file space constrained. The ZWO is a CMOS camera.
Edited 25 Jan, 2020 06:41
26 Jan, 2020 13:04
Thanks Arun.
27 Jan, 2020 06:25
Absolutely suggested to take the same exposure time on RGB. It will get easier color calibration. Another advice is to shoot in a roulette mode R-G-B-R-… if you have an automatic filterwheel. This will grant homogeneous gradients, far easy to remove directly in the RGB combined image.
With CMOS, I always suggest also to keep Gain=0 to preserve dynamic range to the maximum scale, then go for the longest exposure you can. Instead looking at the histogram peak, I use to look at its left foot: it should not shift much from the graph’s 0 (this would mean your background isn’t black). I also look at histogram’s peak because it shouldn’t be cut on its top (saturated parts).  M42 is quite a particular object because it hasn’t any background in its surroundings, so just be careful to not saturate nebula. Even in this case, M42 is particular: the core needs 10s exposures, the surroundings will be better with 1800s. Take both if you can, much 1800s shots and 3x10s, 3x60s, 3x120s, 3x300s, 3x600s, 3x1200s to achieve a perfect HDR composition.
If you want to gather also luminance, it is recommended to get longer times on it, both for single exposure and total integration.

I took M42 HDR last year. Try to take a look in my gallery to get more info smile
27 Jan, 2020 06:41
Here it is:
my error here was not to take HDR shoots even on RGB and going on shooting with high humidity
27 Jan, 2020 11:35
I find with RGB only you don't reap the benefits of mono imaging so much unless you also use a luminance filter for the detail.  I find RGB (for me) simply adds the colour and for that reason I bin all my colour data but DON'T bin the luminance where all the detail is.  I find I can get away with much less RGB data so long as I get a decent length of time on the luminance.  Yes try to get the same amount on each of RG&B.

I don't use a CMOS camera though, I use mono CCD, so if any of my comments are irrelevant to CMOS I am sure some-one will correct me.

I have just imaged M42 in a LP location, you can take a look at the specs of my image and see what I did for my result.

Edited 27 Jan, 2020 11:37
27 Jan, 2020 12:59
The big advantage of luminance filters is very simply that they collect more light in any given time interval - about 2.5 to 3x - as any given color filter. You build up Signal to Noise ratio much more rapidly than with a color filter. As Carole alluded to, the human eye is much more sensitive to detail than to color. LRGB imaging sacrifices color information for detail. Generally it is a good trade off for broadband objects like galaxies and reflection nebulae. M42 certainly falls in that category since it is a broadband object and the reflection nebula from the dust surrounding it benefits hugely from luminance frames. Luminance imaging is less beneficial for narrowband objects (eg. the California nebula).
27 Jan, 2020 12:59
Edited 27 Jan, 2020 13:11
27 Jan, 2020 16:12
Arun, Carastro and Alessio, thanks for all your suggestion.  I have a flywheel holder and a luminance filter so will try out your suggestions.  Just need some clear sky now.  Thanks again. smile
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