Astronomical Observing - Dealing With Weather

An (old) picture of the 2.1 Meter Telescope with lightning.

Ah, yes.  Can't write a series of posts about observing without mentioning the weather.  It is particularly appropriate right now - since I have time to write this post because weather has shut down the telescopes.

We'd hoped for clear skies tonight.  The forecast was for no clouds, and reasonable seeing conditions.  But sometimes it just isn't that easy.  When the weather is obviously bad, say you have lightning like in the picture, you know you won't be getting any data.  It is a very black and white situation.  When you have heavy clouds, or it is raining, that is also straightforward.  You can easily see that the conditions are simply not conducive to observing.  But there are less obvious factors that are just as important.

One of these factors is wind speed.  Most telescopes cannot be open in relatively high winds (say 45mph in some places) as this can damage the telescope and is a safety issue.  Wind is something you can feel, of course.  You may not know exactly how fast the wind is blowing, but you can look at a wind gauge and assuming it is working properly, it will tell you if it is safe to open or not.  Mostly.  The wind is not always steady, of course, and it can change value or direction, or there may be sudden wind gusts.  It can be much harder to determine what is or isn't safe under those conditions.  So it is possible to be sitting inside the dome on a clear night, unable to open, because the wind is occasionally gusting a bit too fast for comfort.  That can be a little frustrating.

Sky Chart predicting clear skies ... and high humidity.  At least for a while.

Another factor is humidity.  This factor is particularly frustrating, and is the reason we are closed at the present moment.  The issue in this case is not that we can't see the sky, but as with wind, that there is a possible issue with the telescope.  It isn't a safety issue, but rather that under the right conditions, water can condense right onto the telescope mirror.  This is very bad for the telescope, and could damage the mirror.  One way to avoid this is not to open the dome if the humidity is high outside, but that is not always a perfect indicator of dew formation.  Condensation (dew) is also a function of the air temperature, the temperature of the telescope, wind speed, and more.  To add to the frustration, humidity is highly variable across short distances.  It is possible that one telescope on the mountain is just fine to open, while another is not.

Bright Jupiter in a haze of humidity.

It isn't particularly obvious how humid it is outside until you look up and see the fuzz of light around the planet Jupiter.  As in this sky shot, the sky is pretty clear, but pretty much straight up (in the center) is a bright spot with haze around it.  That is Jupiter, and the haze is water in the atmosphere.

So here we are sitting under a clear sky, unable to open.  Adding to the frustration is the constant checking.  I am the sort of person who is uncomfortable with gray area.  I want it to be cloudy or not.  Raining or not.  Constantly checking the humidity, (and getting four numbers for four different instruments) is maddening.  Hopefully things will improve later, the sky charts says they might.  But our little corner of the mountain, well, who knows.

Image Credit:  2.1 Meter in lightning from http://www.noao.edu/image_gallery/html/im0297.html  NOAO/AURA/NSF.  Clear Sky Chart from http://cleardarksky.com/c/KittAZkey.html?1 Kitt Peak Clear Sky Chart.  Sky Shot from http://www-kpno.kpno.noao.edu/Info/Mtn_Weather/  Kitt Peak National Observatory.