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Scheduler questions ...
#11
Hi,

You said:

"if I'm imaging in skies where the limiting magnitude is 10 due to sky brightness and I'm shooting an object that is magnitude 12, I can be taking lights for the rest of my life and I'm not going to get anything ... but ST4 already takes this into account, by increasing the exposure time needed to catch something when SQM goes down, and decreasing it when SQM goes up ..."

Sure, from one location to the next, SkyTools takes the sky brightness into account. It is modeled from the base sky brightness + weather +sun + moon + altitude above the horizon. But I hear people talking about changes in SQM from night to night, and this is something altogether different.

From the rest of what I say I gather that you are taking SQM at face value. It is the brightness of the sky background on that night overall, which relates directly to the background sky level on your images. But to say it has to do with extinction... that's on slippery ground. Because the sky can be darker for a lot of different reasons (smoke, dust, aerosols, moisture, less skyglow, etc.) and how much light is getting through isn't connected in a straight forward way.

I'm curious, are you altering the sky brightness for your location to match your SQM readings?
Clear skies,
Greg

SkyTools Developer
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#12
(2018-09-28, 03:32 AM)theskyhound Wrote: Hi,

You said:

"if I'm imaging in skies where the limiting magnitude is 10 due to sky brightness and I'm shooting an object that is magnitude 12, I can be taking lights for the rest of my life and I'm not going to get anything ... but ST4 already takes this into account, by increasing the exposure time needed to catch something when SQM goes down, and decreasing it when SQM goes up ..."

Sure, from one location to the next, SkyTools takes the sky brightness into account. It is modeled from the base sky brightness + weather +sun + moon  + altitude above the horizon. But I hear people talking about changes in SQM from night to night, and this is something altogether different.

From the rest of what I say I gather that you are taking SQM at face value. It is the brightness of the sky background on that night overall, which relates directly to the background sky level on your images. But to say it has to do with extinction... that's on slippery ground. Because the sky can be darker for a lot of different reasons (smoke, dust, aerosols, moisture, less skyglow, etc.) and how much light is getting through isn't connected in a straight forward way.

As a practical matter for imaging the extinction caused by higher SQM isn't a factor, because extinction (limiting magnitude) is a function of aperture and FL and that helps quite a bit ... unless I'm going after a mag 14 or less object in my 11's it's not an issue.  Where it *does* become an issue is for guide star candidates, where the scope doing the guiding is smaller than the imaging scope and the guide camera has to be able to get a good exposure in a second or less ... in that case for my C6's using SSAG's at SQM 16.5 guide stars become useless less than mag 9, at SQM 20.5 guide stars are useless below 13 mag.  People using ONAG's can use the full aperture/ FL of the main scope, but they still have to be able to get an exposure off the pick prism in a second plus have to deal with the FOV of that ONAG ...

And you're quite correct, there are many factors that can cause SQM to vary, which is why I see enough seasonal variation that it's worth the hassle to separate image projects by SQM ...

I'm curious, are you altering the sky brightness for your location to match your SQM readings?

Not exactly; recall that I create "Locations" that are identical except for the sky brightness value, then match Imaging Projects to that location/ SQM value ... so I have 3 locations "Site A SQM 19.0", "Site A SQM 19.2", "Site A SQM 19.4" which have the sky brightness set at 19.0, 19.2 and 19.4 respectively. Say I've got an object I want to image, M 100 which is a galaxy, so it's only useful to image with seeing at 1.0" or less ("Fine").  I will create three Imaging Projects for that object, "M100 SQM 19.0 F", "M100 SQM 19.2 F" and "M100 SQM 19.4 F".  On the day I do my planning, say Monday, I look to see what the previous night Sunday SQM had been, say it was at 19.3 ... it's unlikely to vary much this evening, so imaging the "M100 SQM 19.2" for Monday using the "Site A SQM 19.2" location is what I will choose to image.  Let's say the SQM for Monday has been really good at 19.5 (so my 19.2 rocked!), for Tuesday I would schedule the "M100 SQM 19.4" project for imaging at the "Site A SQM 19.4" location.  If on Tuesday the SQM was actually 19.1 I would feel sad and schedule the "M100 SQM 19.0" project for the "Site A SQM 19.0" location on Wednesday ... if the SQM dropped below 19.0 I would just stay inside ...

So here the sky brightness picks the projects and the locations, not that I dynamically modify the project location to fit the sky brightness (although once I get used to how ST4 manages consistent signal SNR numbers across varying sky brightness values, I might well start doing that!) ...

The seeing also picks the projects for the evening as well; in the daytime I check the MeteoBlue and ClearDarkSkies numbers to get an idea of the seeing for the evening and pick projects for that, as I mentioned earlier ... "Fine" for seeing better than 1.0, "Coarse" for seeing 1.0 to 2.0, "Binned" for 2.0 to 3.0, "HyperStar" for 3.0 to 4.0 and don't bother imaging if it's worse than that ... 
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#13
Ok, now I feel really stupid. There were two different threads and for some stupid reason I thought I was talking to two different people! If you were wondering why I asked you about things you already told me, well... I'm sorry. But thank you for taking the time to explain things to me!

I'll stop pestering you now.
Clear skies,
Greg

SkyTools Developer
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#14
(2018-09-28, 04:13 AM)theskyhound Wrote: Ok, now I feel really stupid. There were two different threads and for some stupid reason I thought I was talking to two different people! If you were wondering why I asked you about things you already told me, well... I'm sorry. But thank you for taking the time to explain things to me!

I'll stop pestering you now.

No worries, it's me who keeps pestering you! :Smile Keep up the good work!
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