Integration Time

[Rossco72]

Question for the brain trust:

If i take 3 hours each of LRGB and combine them, do i have 12 hour’s integration time or 3 in my image?

[Tomatobro]

If you stack, say, 10 images, each 1 minute long, your integration time is 1’ x 10 = 10 minutes. 

[vlaiv]

10 hours ago, Rossco72 said:

Question for the brain trust:

If i take 3 hours each of LRGB and combine them, do i have 12 hour’s integration time or 3 in my image?

You have 3 hours of LRGB each.

Does not make sense to add up integration time of components - unless you just want to compare your previous attempts on same target or something else, and then you say, I have total of 12h of integration time.

You might just want to record total imaging time - in that case, it makes sense to note down that it took total of 12h of exposure to make that image.

[ollypenrice]

I would call that 12 hours of exposure. I'm sure that's pretty much a universal convention.

Olly

[Rossco72]

25 minutes ago, ollypenrice said:

I would call that 12 hours of exposure. I'm sure that's pretty much a universal convention.

Olly

I was just thinking about it and although it took 12 hours, with it being 3 hours per channel surely the SNR is based on 3 hours?

I stumbled across a SNR prediction tool that could estimate the exposure time needed to achieve a given SNR and for example I got say 84 hours was needed based on my 5 minute sub. Then I was thinking, is that actually 84 hours per channel, or does something clever happen to the individual noise when you combine channels?

I know I can get "decent" images without 84 hours, but I am curious to understand the maths of it.

 

[Rossco72]

1 hour ago, vlaiv said:

You have 3 hours of LRGB each.

Does not make sense to add up integration time of components - unless you just want to compare your previous attempts on same target or something else, and then you say, I have total of 12h of integration time.

You might just want to record total imaging time - in that case, it makes sense to note down that it took total of 12h of exposure to make that image.

That is what got me started on my thought process. So if I need say 84 hours to achieve a given SNR with my gear/skies etc. do I actually need 84 hours per channel? I just didn't know if there was some form of averaging going on when channels are combined that improves SNR

 

[ollypenrice]

Is there just one SNR ratio? Or two? Luminance noise and chrominance noise?

And what is the consequence of shooting more luminance than colour, as many do? The luminance, of course, captures more photons.

Do you make a synthetic luminance from your RGB and add it to the L filter luminance?

Olly

[The Lazy Astronomer]

I would also say 12 hours total, and agree that this pretty much universal. I'd approach the split slightly differently though: instead of 4 x 3hrs for all channels, I'd do 6hrs L, and 2hrs each RGB.

[tomato]

I also follow the conventional method of counting the time I have photons hitting a sensor, irrespective of what filter is in front of it so 1 hour with a L filter, and an hour each with the R, G and B filters is 4 hours integration. I’m fortunate to have dual rig, so I have one scope on L all of the time and the other doing RGB, so I usually capture 3 x more L than R, G and B.

[vlaiv]

1 hour ago, Rossco72 said:

That is what got me started on my thought process. So if I need say 84 hours to achieve a given SNR with my gear/skies etc. do I actually need 84 hours per channel? I just didn't know if there was some form of averaging going on when channels are combined that improves SNR

 

If you calculated that you need 84h to achieve given SNR on faint part of target, given your conditions - one parameter of your calculation was filter to be used.

SNR makes sense per channel or per filter - not overall.

When combining LRGB data - about 90% of noise in final image is in form of luminance noise. I've already done this on several occasions, but I'm going to repeat it - here is an experiment.

This is our baseline image (random internet search for a bird):

This is our baseline with noise added to luminance data (L channel of Lab)

This is our image with same amount of noise added to two channels (each gets that amount of noise) that represent chrominance data (a and b components of Lab model):

Noise in luminance data is much more noticeable and obtrusive even if there is more noise in second image (two channels polluted versus only one).

However - one must pay attention that LRGB is not Luminance + chrominance model unless it is treated so in processing - where only chrominance is extracted from RGB and L is used as luminance of final image. Then above makes sense and it makes sense to spend much more time on luminance channel.

[Rossco72]

How does that apply to narrowband SHO filters?

[vlaiv]

21 minutes ago, Rossco72 said:

How does that apply to narrowband SHO filters?

Same / similar. For SNR - actual filter used will participate in calculations (sky flux passed through filter and brightness of target) - so each one will have its own SNR.

It really depends on how you combine them SHO palette is created so that H takes place of green color.

Out of RGB colors - green is most responsible for luminance, or rather greatest part of luminance comes from green color. It is hard to explain, but here is something that can help to clarify this:

This is sRGB to XYZ conversion matrix. These are still linear values and Y component of XYZ is effectively luminance (or linear luminance).

If you look at XYZ to Lab conversion - you will see that L in lab is just a function of Y and not other two components (unlike a and b), so L is some nonlinear version of Y - but both represent luminance.

In any case, from above matrix we can see that

Y = ~0.213 * R + 0.715 * G + 0.07 * B

Almost 70% of luminance is made up from G (in linear light and sRGB red green and blue).

People kill off green in their SHO images for some reason - but assigning Ha to green makes perfect sense as most SHO images benefit from having Ha related to luminance and green will be large part of that.

Only difference to RGB is that with narrowband Ha naturally has high SNR and other two components have lower SNR. But if you wish, you can do SHO image composing for color and then use Ha as luminance in similar way as above (Lab or XYZ composition).

 

[dan_adi]

On 05/05/2023 at 14:50, Rossco72 said:

That is what got me started on my thought process. So if I need say 84 hours to achieve a given SNR with my gear/skies etc. do I actually need 84 hours per channel? I just didn't know if there was some form of averaging going on when channels are combined that improves SNR

In the CCD equation, the math is done for a specific filter. 

So if the calculator said you need 84 hours for a SNR=100 using the L filter, then this is what you need. For a different filter (R or G or B or I or etc) the same SNR=100 will produce different exposure time, not 84 hours, because there is a different bandwidth , different QE, different filter extinction etc.

So, my approach is to compute the exposure time needed for a SNR using the L filter. RGB data is added to make the picture prettier.

Here is the calculator I made to better understand my hobby  https://clearskies.go.ro

In the process of converting it to a windows app... I am one of those guys that has Linux as a main OS so bare with me :)) it will take some time

Edited May 7, 2023 by dan_adi