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I have noticed that when I change the weather conditions (temperature and humidity) it can dramatically affect the IQ line and Start/Duration/IQ column data in the Target Selection window.  However, when I make these same changes in the Project window for my observations, it makes no difference in the SNR of the project.

If I put in the correct data in the weather conditions for last night (19C and 95% humidity), the Target Selection window thinks the IQ is so bad it won't even give it a D rating, and there is no exposure window.  The columns are all "---".  And yet, the images I took look great, SNR seems ok, and when I add last night's imaging session into my project as an observation, it happily increases the SNR.  If I change the weather data for the observation to much better conditions, there is no change.

What's going on?
Hi,

Unfortunately, the whole point of these calculations is that there is no generic answer. Each target is unique, and they are affected differently by seeing, T and RH. The altitude of the target also makes a big difference.

So I can't answer your question without a lot more specifics. I need to know your target object. Also, you say weather conditions, but are we talking about seeing? Or temperature and humidity? If so, what are your specifics?

I am also confused when you say, "I make these same changes in the Project window for my observations, it makes no difference in the SNR of the project." Where does it make no difference? The Scheduler? The project itself has no date or time attached to it, so it is neutral regarding weather...

Please try to be more specific and maybe add some screen caps.
Hi Greg: 

>> I need to know your target object.

The target is the Cave Nebula.

>> Also, you say weather conditions, but are we talking about seeing? Or temperature and humidity? If so, what are your specifics?

As mentioned, it is temperature and humidity.  In the Target Selection window for Sept 21/22, if I enter "Fairly Good Seeing" with 60°F and 70%RH, The IQ column shows A, and the exposure window is about 5.5 hours.  If I change only the humidity to 95%, the blue line on the graph drops way down, the IQ column shows "---" and there is no exposure window at all.

>>Where does it make no difference? The Scheduler? The project itself has no date or time attached to it, so it is neutral regarding weather...

As mentioned, I was changing the weather conditions for the Observations, which are in the project window.  As I enter each observation, I enter the time and date, and the temperature and humidity.  I would assume that if the temp/humidity has such a huge effect on the exposure window in the Target Selection window, it should also have a large effect on the resulting SNR calculated for the project when I change the data there.  However, changing the humidity in the Observation, the same amount as I changed it in the Target Selection window, I see no change at all in the SNR that SkyTools calculates in the project window.

It seems that the temperature and humidity in a project Observation have zero effect on the calculated SNR in the Project.  Is that true or am I misunderstanding something?

Here's the screen captures you asked for:

1) Target selection window at 70%RH
[attachment=389]

2) Target selection window at 95%RH
[attachment=390]

3) Project window with 70%RH Observation (highlighted):
[attachment=391]

4) Project window with 90%RH Observation (highlighted):
[attachment=392]

-Dan Kuchta
Hi Dan,

The IQ on the target selection tool is a relative number meant to tell you if you would be better off waiting for a different night. It doesn't mean you can't get any SNR at all. So there is a bit of an apples and oranges going on when comparing to actual SNR values. This calculation also assumes your broadest filter and it may well be that filter is affected much more by humidity than the IDAS LPS-V4, which is what you are comparing to in your specific observations.

I see the difference in RH not having an effect on the SNR for your observations. For the IDAS LPS-V4 filter and the relatively small SNRs, there is an outside chance that the humidity isn't affecting it that much even at 95%. Again, this is why these calculations are useful. If you really are observing in 95% humidity, which would be unusual, open the Exposure Calculator for your objects and select the LPS filter to look at results for that filter explicitly.

That said, the code that handles this is more complex than it might first appear... While it is possible that somehow the weather conditions for the image (for your observations) are being ignored, I think it may be that it isn't properly accepting the change after the original entry. In other words, it may be stuck on the original weather that was entered, regardless of what is displayed.

Regardless, I am looking into it and will get back to you when I determine what is happening and whether there is really a bug or not.
Hi Greg:

Thanks!

>> If you really are observing in 95% humidity, which would be unusual ...

Sadly, it's all too common around here. For the last week, the humidity for much of each night has been well over 95%. When I get up in the morning to close up the observatory, everything is covered with water from being at or below the dew point. I'm actually surprised that it doesn't seem to interfere with imaging too much. Lots of dew heaters! :-)

In any case, thanks for looking into it!

-Dan