Scheduler questions ...

[theskyhound]

Without a screen capture I am having trouble following the rest, but regarding the second part, SNR does not stack linearly. You are not going to add 17% each night.  You are going to add less and less SNR each night.

Hmm ... OK, then what's the value of having 'multi-night" exposures and tracking at all?  Obviously it's to take advantage of stacking adding signal at a higher rate than noise is added (and if we can keep the noise as low as possible, by controlling things like the SQM we expose at and the seeing of that night we can optimize that process). 

I'll provide additional screen captures but my question about why the Imaging Project didn't let me make forward progress on a particular session remains ...

I never said that there was no value in stacking images. In your question you made a calculation. Your calculation does not reflect the reality of how stacking images and SNR works. You can't expect twice the SNR every time you double the number of images in the stack. It just doesn't work that way. This is why we need complex software.

I need the screen capture in order to answer your question.

"Doing more research into what makes the filter "register" or not (to prevent my "setting myself up for failure"), it appears that Exposure Calculator is single-session oriented ... the numbers on the left let me answer the question "what is the most SNR I can get for the time I've given it", and the numbers on the right answer the question "what is the most SNR I can get for the entire session available" ... so, if my IQ A time is 30min and I'm interested in getting a single SNR 20 picture for that night it will tell me "X exp at Y sec" which gives me 5min total (leaving 25min left in the IQ A session).  If I want to see what I'd get for the whole 30 min IQ A period it will tell me "Z exp at A sec will give an SNR of 45" (using the entire 30min IQ A session ..."

Yes, that is mostly correct. The SNR computation works for a single night. But you can set the Exposure time calculation to "Ideal Conditions" and it ignores the night and time. It just assumes ideal conditions into forever, just like the numbers in the table on the Exposure tab of the Imaging Project. If your interest is in knowing the *minimum* exposure time that will be required to reach the target SNR, then the table in the Exposure tab of the Imaging Project is what you want to look at. It is a minimum because it assumes ideal conditions.

"It's also unclear what an "SNR 4" light will look like; there are about as many ways to calculate signal SNR as there are stars in the sky, all I can do is take some current objects I've got and compare them.  One of my experiments I'm planning once the skies darken will be to take a series of SNR 4 lights of a galaxy and of a planetary nebula, stack them up and compare them to images I've already made of those in the past (with, of course, the same SQM).  I'll then use PixInsight's ImageIntegration tool to get an SNR value for various regions of those objects ... that will answer (for me) if a bunch of SNR 4 lights really can give me a usable picture with a sufficiently large number of stacks as well as tell me what "SNR 4" for ST4 equates to with PixInsigt's tool."

When it comes to extended objects there is only one way to calculate SNR. Stars are calculated differently, and there are two ways to calculate SNR for them. The tool you mention above uses the method for non-stars, so it should be similar to what SkyTools computes for extended objects. But you need to be consistent. Take a galaxy. You will measure a higher SNR in the bright nuclear region where there is more signal, and a much lower SNR way out on the extended halo. So when comparing SNR you need to compare apples to apples.

Also keep in mind that SkyTools is assuming that you did proper calibrations on all of your images and the SNR is computed for the final image stack. The calculated SNR ignores non-random noise sources such as cosmic rays and satellites.

Remember, untimely the point is to help you choose suitable targets and to obtain the highest quality images possible, not to reproduce the SNR exactly. What would that really mean for an extended object anyhow? The brightness varies across the image, so the SNR will do so as well. 

As far as what an SNR of 4 means to you, SkyTools is not meant to replace you or your experience. You will need to develop a feel for what an SNR of 4 means.