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.