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R, G and B Exposure Questions


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Hi everyone,

I have been using a mono CMOS cam + filters setup for about 6-7 months now and I'm fairly comfortable in terms of LRGB DSO imaging. Comfortable as in basic competency...still tons to learn of course! As such, I wanted to go a bit deeper into the RGB side of things.

Up to now, in terms of total exposure time, I've been basically shooting 50% Lum and 50% RGB, with the latter evenly split. However...with my filters, the R exposures need to be twice as long as the G and B exposures to achieve the same ADU (about 1400-1500 ADU per exposure for each channel at unity gain) - 60 seconds for R and 30 seconds for G and B. Results have been good, but my questions are:

(1) By shooting the same exposure time across R, G and B, I end up with half the number of R subs; am I right to say that I am capturing less R data than G and B, even if total exposure time is the same across each channels? For every 10 R subs, I get 20 G and B subs, but in ADU terms for R, I get 10 x 1400, versus 20 x 1400 for G and B.

(2) If that is the case, I suppose I can fix this by either (a) shooting twice as much R exposure as individual G and B, so an exposure ratio of 2:1:1 or (b) increasing the gain on the R filter to try and get the exposure time to reach the same ADU per sub similar to G and B. Is this the right way to think about it?

(3) That got me thinking about whether it makes sense to capture the same R, G and B total exposure (in terms of ADU) at all....for e.g. I have often read that there is not much green in space, so does it actually make sense to capture less G, for example, relative to R and B? Is there a theoretical colour channel ratio optimal for DSO imaging? My concern about varying the ratios too much would be messing up star colour...

...question (3) is the one I find most fascinating and I would love to see what others think about it!

Sorry for the long post and thanks for getting this far!

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Depends on what sort of measurement give you 1400-1500 ADU per exposure.

This topic is not easy one to answer as there is no single answer. It depends on many factors. Let's examine on example.

Let's suppose that you are imaging target that has equal distribution of wavelengths across R, G and B. When you finish your subs you measure that R is on average lower signal than G and B. This can mean couple of things. It can mean that R part of spectrum has less QE on your sensor than G and B. But it can also mean that LP is such that there is no much R in the sky.

These two give very different recommendation for duration of R channel over G and B. In case of lower QE - you need to capture more R frames than G and B to compensate. In case of LP being less in R channel - it's the opposite! You need less R frames to get to same level of SNR as with G and B!

It can also happen that you are imaging target that has different ratio of R, G and B in spectrum. Add to that LP spectrum and QE of sensor and you get very complicated calculation for optimum R, G and B exposure.

So the answer to first question is - no, that is not proper way to think about exposure length as you don't know what is the reason for lower signal.

2) Most often it is the case that B channel is suffering the most. If you are going to give priority to any channel - it should probably be B channel.

3) Although there is no much pure green out there - green part of spectrum is important for color balance and many targets do indeed have light in that part of spectrum.

Here is how you should be thinking about all of this. Level of signal does not depend on how long you expose. Read that again :D - level of signal does not depend on length of exposure. What depends on length of exposure is Signal to Noise Ratio - or SNR. If you decide to go for shorter exposure in any particular channel - make sure that you will achieve good SNR compared to other channels. Signal to noise ratio depends on two components - Signal and noise, as it's their ratio. Good SNR can be achieved by strong signal, or low noise. Strong signal depends only on your target - if target is providing enough light in that part of spectrum - signal will be strong. Noise part depends on additional factors, so it's not easy to describe.

Here is good exercise - although you have less average signal in R - stack R and B for example - same number of subs that you have captured. Process each as mono image and be careful to apply about the same level of stretch to each - see which one has less noise in it - you might get surprised - R might be higher quality data (better SNR) compared to B although "signal level" appears to be lower.

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Wow vlaiv - that is an incredible answer! Thank you - I will go over it in detail...but the salient point that SNR is the key thing to think about is well made, even though my camera is relatively low in terms of read noise (ASI1600). Also the B channel being the one to maybe think about is also food for thought. 

I have done a number of integrations with R and B data, so I will give that exercise a go to see the comparative noise.

While it makes perfect sense about LP and the signal of the object being factors, the longer exposure needed on my camera/filter setup on the Red channel has been consistent across objects. But the LP factor is probably also common across the objects from my back garden.

I am clearly in danger of overthinking this, but these sorts of insights will really help me maybe make a more informed tweak here and there. Thanks again!

 

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Regarding the need for green, I think it's particularly necessary in order to balance the blues and control the magentas. Try playing with an image with some strong blue stars: lift the blue without lifting the green and it goes badly awry. Likewise an overdose of SCNR green in Pixinsight will give the image a magenta hue.

Olly

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