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I must admit that I am thoroughly confused on how ST4 works. I might be overthinking it. For example, I was looking for something that would help me determine something like optimal sub exposure time for various targets. I put this into practice recently and I'm just not seeing something correctly. I am using a QHY163M camera along with a Skywatcher 80mm evostar at 480 mm (.8FR). I shot M42 binned 1x1 at a gain of 50 and an offset of 35. (I included my settings etc, under a post about the Crab nebula where I was equally confused). The median ADU value in a 30 sec. exposure of M42 yielded ~1250 ADU whereas a 5 sec. exposure gave a median of ~683. Under the exposure calculator for the same night using a single exposure time of 30 secs., ST4 is showing : Signal: 359 ADU -Sky: 101 ADU/pix

What am I missing? Am I just using the application incorrectly? I had all of the settings for the same night I actually shot the images (1-5-20). I have experimented with this for other objects and I never seem to be getting ADU values from ST4 that jive with what I see in reality. Sorry for being so dense but I could use some mentoring....

I suppose what I thought ST4 would do is tell me what sub-exposure times and how many images I should take for a particular filter under the conditions I give it. Maybe my misunderstanding is in the SNR concept. I do know what SNR is but perhaps I'm a bit weak on how much I need in total and for individual pictures. I also use a spreadsheet I made from information I gathered online which supposedly tells me the min and mix ADU values I should shoot for regarding sub-exposure times in order to swamp read noise. That calculation seems to work but doesn't give me anything like I am getting from ST4. The spreadsheet calculates a min of 662 DN and an ideal of 899 DN. Thanks for any help. Again, you can see/obtain my settings a few posts down about the crab nebula.

Hello Jon,

It sounds to me like SkyTools can do what you expect it to do, but there is something set up incorrectly such that it is not giving you the correct results. Your spreadsheet is just a simple rule of thumb that fails to take into account all factors, so SkyTools should be able to do much better. But for the most part, the two should more or less agree.

Here are some thoughts:

At first blush your measured numbers don't appear to be consistent. If all else is the same, an exposure that is 6 times longer should approximately have 6 times the signal (more or less). 6 x 683 is 4098, so I would start by trying to understand why you would get something so much smaller (1250) for the longer exposure. This would seem to imply that something about your experiment wasn't consistent.

One thing to careful about is comparing apples to apples. When you select your target in ST4 you also select what part of the target you are measuring. For example, if you choose the brightest portion then that predicted signal must be compared to the brightest parts.

M42 may be the most imaged object in the sky but it makes a terrible test subject. This is because it is so complex, and somewhat ironically, ST4 does not have accurate emission line data for it yet. Unlike other objects, the pros have had no need to do this work for us. I am working on a project to better characterize M42 and many other emission line objects. But even then, the result is going to be highly dependent on where and how you take your measurement. For testing, I suggest a less complex object that has better data available. Stars make the best test targets, and Landolt standard stars in particular. But bright galaxies and planetary nebulae work well too, as long as you make comparisons to the same part of the object that you are measuring, and it is critical that ST4 know all of the specific details, such as the night, time, weather, etc.

Remember to look carefully at all of your settings. I have seen everything, including people who had the latitude and longitude switched for their location, or the wrong units for the aperture of their telescope, which are both good ways to get meaningless results. Check everything, especially: location, weather, telescope OTA, and camera.

Once you have some confidence that ST4 is prediction the signals correctly, then we can talk about interpreting SNR.

Lastly, I have been neglecting ST4 Imaging because I am working very hard to finish ST4 Visual. But that project is almost finished, and when it is finished I am going to release an update for St4 Imaging with some new features and many fixes and improvements. Come summer I expect to release more improvements, including a stand alone calibration app that will compare the ST4 model to actual images, computing efficiency factors that will ensure accurate calculations.
Clear skies,

SkyTools Developer
Thanks Greg for the reply. I think I am maybe beginning to see what I am doing wrong. If I check the regions as selected in the "Expose for" section of the calculator, my images do seem to come much closer to the ADU values expected by ST4. Rather than focus on the median or mean ADU values, I should be looking at the object's ADU values instead. And that should be based on my selections in the "expose for" section in the calculator. I don't have an explanation of why the readings in my images aren't growing linearly other than perhaps the mean or median values aren't growing linearly due to the brighter sections of the image becoming saturated and fewer pixels are actually increasing? The sky ADU readings in ST4 match perfectly with sky ADU samples from my different exposures.
Thanks again...I'll make a few more tests when I get some clear skies.

Hi Joe,

I think you are right about the median values. I wasn't really sure what you meant by that. I am happy to hear that the sky values are in the ballpark. That's a good sign. If you have questions about how to use the software or SNR or whatever, please feel free to ask.
Clear skies,

SkyTools Developer

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