Saturday, 20 March 2010

Looking through the clouds

During the eighteen months or so of this blog's existence I've occasionally described the conflict I have about clear skies versus cloudy ones. Clouds make for beautiful pictures at sunset and sunrise (see above) but also have the potential for ruining our astronomical observations.

In the comment section of "Less wind, more cloud", my good friend Keera asked if we could observe through clouds and wondered what the point of building telescopes on a high mountain is in the first place. (I apologise for the paraphrase!). My unhelpful answer was yes and no.

The quick answer is that optical/infrared telescopes can only "look through clouds" if they are thin enough to allow light from whatever they're looking at to pass through them. Clouds attenuate light from a star and if there's enough attenuation, the telescope won't see the star. This is where things get complicated however. If the cloud is thick and no starlight gets through, then we give up and go home. If some starlight gets through, then there are still often things that we can do, but trying to measure how bright a star is can get tricky.

In our case, we use a camera that has a very large field of view (FOV) which means that we often see enough stars in our images that at least one or two of them already have very accurate photometry (brightness measurements) from 2MASS. That means we can see what the cloud does to the brightness of the known stars and can use that information to deduce the brightness of the other stars in the field, i.e., use them as calibrators. This can get difficult though because having such a large FOV means that the thickness of the cloud can vary from one part of the image to the other. If the cloud is so thick we can't see the calibrators, or even any other star, then it's time for bed!

Being rather familiar with infrared astronomy, I know of further problems. Although our current setup doesn't involve this, observing in the thermal infrared (beyond a wavelength of around 2.3 microns) becomes almost impossible if there's cloud. Mauna Kea is one of the best sites on the planet for thermal infrared astronomy but if there's cloud above the summit then it's another early night. High altitude clouds emit infrared radiation at exactly these wavelengths due to their cold temperature, so instead of detecting stars, even the thinnest of clouds will shine so brightly that it overwhelms the light from a star. Some years ago I spent much of my time commissioning a mid-infrared instrument on UKIRT and Gemini called Michelle and learned quickly that it was a superb cloud detector!

On the other hand, some telescopes on Mauna Kea benefit from high cirrus clouds. As I'm sure my readers know, clouds are made from water. Most clouds, those at low and mid-level, consist of water vapour and tend to be below Mauna Kea's summit. High clouds, i.e., cirrus, tend to occur at 20-30,000 feet where it's so cold that water turns to ice and that's what these clouds are made of. That's bad news for infrared astronomy because they shine very brightly but our submillimeter colleagues down in the valley (e.g., JCMT and CSO) thrive in these conditions!

Submillimiter telescopes require dry conditions because they are excellent water detectors. That's why they're built in high places like Mauna Kea because they're above most of the water in the atmosphere. So you might ask why they thrive when there are clouds made of water above them? Simple. Water molecules vibrate and rotate and these give off wavelengths we can detect (it's a quantum mechanics thing). Their vibrations can be detected in the infrared and the rotational transitions in the submillimeter. The thing is, if they're frozen then they can no longer rotate. That means the sky is almost transparent to the submillimeter guys. They can, unfortunately, still vibrate, which is what us unfortunate infrared astronomers see instead...

Maybe you can see why my answer to Keera was yes and no!

Oh well, those bad nights for infrared astronomy means I get to take some nice pictures.

Hualalai in the clouds and vog.

As ever, you can click on the images for larger versions.


Keera Ann Fox said...

Keera is quite satisfied with Tom's yes and no answer. Now. :-)

The things I learn here! It hadn't occurred to me that the clouds would be frozen, for example. And that ice is a great lens (if I've understood you correctly) for some types of viewing.

Tom said...

I'm sure you've seen a large halo around the moon on several occasions - the famous 22 degree one. That's caused by ice crystals in high altitude cirrus clouds!


PS. There are so many other reasons to build telescopes on Mauna Kea-like mountains, none of which I touched on. I wrote about them some time ago but haven't the energy to find the post/s just now! One problem is that there are few if any other Mauna Kea-like mountains on the planet...

Tom said...

PS - not a lens, the cirrus clouds (if consisting of ice) simply aren't detectable in the submillimeter - they're just like clear glass while to us working in the infrared they're like stained glass!

Keera Ann Fox said...

I know I should know this about clouds (great, just gave myself a Joni Mitchell earworm) but since it's not part of my everyday experience, it just doesn't occur to me that logically, the water vapor is frozen at such high altitudes.

Clear glass, not lens, gotcha.

And if Iceland's current burp doesn't quit, maybe there will be another Mauna Lea-like mountain on the planet. :-)

Qfenestrate said...

Interesting description of the problems with viewing through cloud cover. Ran across a similar situation recently where I could see through upper atmosphere clouds and see stars through the clouds, but could not see stars where the clouds weren't as I was too close to Chicago city limits. I wonder if the cloud does not act transparently but similar to one of those view boxes you use for seeing comments, cutting down background light.

Tom said...

Qfenestrate - I understood everything you wrote until the last sentence. Could you clarify what you mean?

Thanks for popping by my blog!


Qfenestrate said...

I don't know if someone who uses a telescope regularly ever had use for one but back in the 80's as Halley passed overhead they were giving out pieces of cardboard that folded into what appeared to be a cereal box with no ends. When you were close to the city and looked through one end of the box you could see the comet better because the box allowed your eyes to ignore the background light from the city. If the cloud was acting as both transparent and a blanket in various portions of the sky it would reduce the light from stars so that you can focus even better on single stars. If that makes sense

Tom said...

Hi Qfenestrate,

Sounds interesting and I think I understand your original comment now - yes, it makes sense.

This isn't the sort of thing done on pro telescopes since we don't use our "eyes" the way you would have done - everything goes through computers and we can sort out things afterwards, plus we tend to look at very specific parts of the sky anyway.

One of our biggest problems is dealing with the moon. Although we may not be looking anywhere near it, it's so bright it can reflect off various surfaces (dome, telescope, something else) and appear as some very odd artifacts in our images. We've mostly solved this but every so often we get surprised by some odd reflection we never thought could happen!