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Unexpected year bar calculation
#11
FYI: weather can make a big difference for this calculation, which is why I keep asking about it. But assuming that all the weather data is the same, then I suppose we can rule this out. Just make sure all the settings are the same, including the monthly settings. That is really important.

BTW changing the sky brightness can have counter-intuitive results. This isn't an absolute calculation that can be compared between two different locations or for the same location with different levels of light pollution. It's not meant to answer those types of questions. It is a relative calculation based on a comparison between the best SNR that can be expected at a given location (and light pollution) under optimum conditions and the SNR that can be achieved under the specific conditions of the night in question (or in this case for each night of the year). Changing the amount of light pollution does not simply make things better or worse. It changes the basis for the relative comparison.

I suspect something is broken beyond just the Yearbar. I think that's just where the problem is showing up. So I need to figure out what's wrong. I am asking questions in an attempt to rule out some of the more obvious possibilities. Please send me the following:

1. Screen captures of both locations showing all the settings (I already have the one, thanks)

2. Open the Imaging Systems, select your system, and click the Save button to save it as a .stx file. Send me that file so I can do some testing.
Clear skies,
Greg

SkyTools Developer
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#12
Sent to your skyhound email.

Thank you!
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#13
Hi Greg,

Something doesn't look right with the Year Bar calculation. What is the observation time that's applied to the Year Bar calculation. ST3 Pro allowed the observation time to be set for the Year Bar, but ST4i v4.0e has no user controlled observation time that I could see. The object in question (NGC 6781 in Aquila) should be observable in early May according to the Night Bar and the Exposure Calculator around 3:30 to 4AM near Columbus, Ohio (40 deg North latitude). I have the weather set to 'Auto'. My Year Bar doesn't show that (See attached files). Neither the month nor moon separation seem to affect the results of the Year Bar calculation.

Also the Database Power Search still doesn't allow a search by constellation. It's not a big deal because you can sort the search results by constellation and choose the one that you want.

Hope this sheds some light on the issue,

Phil S.


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#14
Hello,

Thanks for sending the info. I was able to figure out what was going on. But first we need to start with how this actually works, and how to interpret the display. This is not adequately explained in the help (I will work on that in the future).

If you look at the Target Selection tool it has three columns that display the start, duration, and quality of the observation window on that specific night. What the Yearbar does is plot this same duration for every night of the year. So if the duration on the Target Selection tool is calculated to be exactly 60 minutes on July 12th, then there will be a little green line that extends vertically by 60 minutes on the Yearbar that corresponds to July 12. The Target Selection tool tries to select the part of the night that achieves as "A" quality rating, but if it can't do that, it will settle for a "B" period of time (or in some rare cases even a "C").

This calculation is based on a comparison between the SNR for the best conditions that can be expected at this location and the conditions on this night. What makes this so useful is also what can sometimes make it very hard to understand. The calculation is the result of a complex interplay between the spectral-energy distribution of the target object, your optics, camera, light pollution and weather/seeing conditions. The weather/seeing is going to affect the dependence on SNR with the altitude in the sky, twilight, and moonlight. The image scale of some systems makes them very sensitive to the effective resolution, which makes them even more sensitive to these things than other systems.

What I discovered in this case was that your monthly weather settings--particularly the humidity--were making it difficult to achieve an "A" period for the night. For a single night, as on the Target Selection tool, it simply defaults to the "B" period and displays that. But the Yearbar considers the entire year. There are nights where an "A" rating is achievable, so it chose to display only these nights. That limis the number of nights and the duration of the time to image on those nights, resulting in a pretty sparse graph.

So, as usual when you use a very smart model such as this, the first lesson is that it will always outsmart us. The second lesson is that it only knows what we tell it, so the age-old saying "garbage in = garbage out" is in full force. It may still outsmart us, but at the same time we may be inadvertently making it dumber than it should be.

The second lesson tells us that we need to be meticulous in setting everything up before we start questioning the results. In this case, I suggest entering your monthly weather as accurately as you can. This should be the typical temperature and humidity on a typical night that you typically observe on during these months. Give some careful thought to the seeing selection as well. If you tend to only observe on nights of good seeing, set it to that.

Now, there is another half to this problem. The Yearbar isn't telling you enough about what it is doing and that led to frustration. It decided to only show you nights that it could achieve an "A" quality period, which are more rare than we might expect, and it did not communicate this fact to you. Moreover, it occurs to me that the quality should be a user selection. So I am going to add a selection for the minimum IQ to the Yearbar dialog. In your case, selecting a minimum IQ of "B" would produce a lot more nights to image on the Yearbar and hopefully not be as confusing.

A side note: I was surprised to see that you had restricted your exposures times to being so short. I know this is likely the result of what people suggest for this camera (and probably due to your light pollution conditions) but unless you have some practical reason (such as guiding) to use such short exposures, I recommend allowing longer exposures and letting SkyTools determine the proper exposure time based on the conditions at the time and the properties of the target object. In other words, if possible, let it do its thing rather than tell it what to do.

Phil: note that the Yearbar is for the whole year, so it's not going to matter which month you start it in, as long as its still 2019.
Clear skies,
Greg

SkyTools Developer
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#15
Hi Greg,

Thanks for the detailed reply, that explains a lot. Indeed, adding the quality filter help very much, particularly that we cannot always restrict ourselves to A nights. Could you also add a time filter, meaning the time of day? I need to go to a dark site for any observing that's not narrowband so anything after 2am is not feasible. Or perhaps you have a better idea how to handle this restriction, but I wouldn't be surprised if many users have this restriction. 

On the short exposures: I mainly do EAA so short exposures are the norm. I've recently got an equatorial mount so I'll probably do some light imaging but most of the observing will be EAA. As you guessed, high light pollution is one reason. Another one is airplanes (Toronto has a busy airport and even the dark site is under the path of planes) and given that the cooled CMOS camera has a low read noise, short exposures are preferable. Now with the eq mount I'll pay a closer attention at the recommended exposure time.

Clear skies,
Razvan
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#16
Hi,

Yeah, I could add time filters, but I have a some concerns about that. It may be putting too fine of a point on it, meaning that the Yearbar is more of a starting point, and other tools are more suited for specific nights. That, and in practice, time constraints don't always work very well because the winter is often so different from the summer. A AM may only make sense for part of the year. Then you run into questions like, should it keep your selection, which could lead to confusion, or make you enter the time filter every time you use it? But I will certainly consider it.
Clear skies,
Greg

SkyTools Developer
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#17
Fair enough. With a quality filter it should be useful to get a sense of when the object can be considered for observation; the Target Selection window can do the rest when it comes to a specific day.

Thanks and looking forward to the future releases!
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