Most of the columns I write for Telescope Topics concern the design, building, and testing of telescopes, or telescope optics. But the glass and its mechanical supports are only a part of the system that enables us to explore the universe. The remaining part is the atmosphere.
When you think about it, it seems pretty amazing that we can see through a mixture of different gasses, vapors and dusts. I mean, there are days when I wouldn’t be surprised if I looked out the window and saw a thick green fog, slowly dissolving the creatures too slow to get out of it. (Although that may be just nostalgia for my lost youth, which was spent in an industrial town in Ohio).
I realize that we have evolved to be able to see through the air, so from a standpoint far from the exhaust of the smokestacks of H-w Chemical, perhaps a transparent atmosphere is not so remarkable after all.
Our atmosphere is not really perfectly transparent, though, even to our highly evolved eyes. Dust makes it translucent, the scattering of light from molecules adds background noise in the form of the blue color in the sky, and temperature and pressure variations can cause one of the atmosphere’s components to become opaque and condense, which is good for the grass, but bad for concrete, especially if it happens to be near H-w Chemical.
All of these things are generally bad for astronomers, too, but perhaps the worst thing is the fact that the air has an index of refraction that is not 1.0000. It’s almost 1.0000 (actually about 1.0003 on a good day), and since most optical work is done in air, it is usually taken to be exactly 1.0000, but it really isn’t.
That, and the fact that the air is made out of relatively heavy gas molecules (the light ones having escaped long ago), conspires to create the condition known as “bad seeing.”
When the temperature and pressure and chemical content of the air changes abruptly, it affects its index of refraction. The effect would seem not to be very great. For example, the heat waves rising from the hood of a car on a hot summer day are changing the index of refraction of air in the seventh decimal place. Nevertheless, for some reason, we can see these changes clearly. Very clearly. Punishment for misdeeds perhaps?
Moreover, the heaviness of the gas molecules makes them move slowly, so they don’t readily get mixed up and even things out. The result is blobs of air at the same STP (that’s Same Temperature and Pressure, not what first came to your mind, which was the thick goop you add to your engine oil to get up enough compression to drive it to the junk yard). These blobs of air have a given index of refraction, but it’s not necessarily the same index of refraction as the blob on either side of them. So each blob acts like a lens, sort of like a shower door, moving fast across the sky, and like a shower door, it’s usually between you and what you’re trying to see.
We’d really be better off if we had an atmosphere that was composed strictly of Hydrogen and Helium. Fill your telescope tube with that and you can kiss tube currents good-bye, as long as you can keep the stuff in there. The lighter gasses have indices that really are almost 1.0000, and they mix together quite well, thank you.
Failing that, we have to settle for observing when the blobs are at their most languid, or least disturbed. If there is not a lot of heat being input to them, or radiated from them, they may have time to merge and form large (up to 8 to 12” diameter) bobs, which bob like waves in a tidal pool, and only cause us to refocus our eyes, instead of cursing our luck.
When does this happen? When the daytime and nighttime temperatures are about the same. Say, on some days in the spring, or in October.
P.S. My son heartily recommends the movie of the same name. Go see it, and also, see the real thing while you’re out there in that strange mixture of gasses.