We're awed by the magnificent images from the Hubble Space Telescope. Of course, it's unencumbered by vision impairments of the Earth's atmosphere, while ground-based observers do constant battle with their environment. In particular, very few amateur astronomers have the luxury of a permanent observing site on a mountain to minimize the amount of atmosphere through which we have to peer.
If you observe only from your backyard, then you're stuck with whatever conditions exist in your neighborhood. As amateur astronomers become more involved with observing, they usually venture out with their telescopes in search of better sites. With a large telescope, this becomes imperative if you hope to exploit its capabilities to the fullest. A darker site is the primary goal. But a dark site still doesn't guarantee that you'll come away with a totally satisfying observing experience. The Earth's atmosphere has the final word.
First, let's look at something called seeing. This refers to the steadiness of the atmosphere - a condition where the bright planets don't shimmer and where their images are crisp and detailed. Surprisingly, a slightly hazy sky will often produce exceptionally good seeing conditions. Here, water vapor in the atmosphere works to our advantage. In fact, some of the best seeing occurs near large bodies of water where laminar airflow also helps create very steady conditions. As one example, the Big Bear Solar Observatory in California is located on a small island in the middle of a lake. This is especially important because there is increased atmospheric instability during daylight hours. Thus, for amateur astronomers, the sun is best observed with a small instrument, which is not as severely affected by instability as is a larger telescope.
Some people claim that due solely to atmospheric effects, 300x is generally the highest usable magnification for any telescope. The best seeing I've ever experienced was from my backyard on a humid, windless early winter night with above normal temperature. Jupiter and Saturn revealed amazing detail at the highest magnification my scope could muster. If your main interest is observing the moon and planets, this just proves that you don't need to get away to a high altitude location. In fact, a site in the mountains can be counterproductive for the planetary observer. Randy Moench, of the Northern Colorado Astronomical Society, sent me an e-mail in which he said: "I live west of town (Fort Collins, CO) up in the foothills and have pretty dark skies, 5.9 or so. The trouble with our skies is the seeing. The mountains really tend to churn up the atmosphere. Planetary work is not too good. I rarely seem to get steady skies, but I keep trying."
If you're interested in deep sky observing (galaxies, nebulae, star clusters) then that's another matter. An atmosphere with a bit of smoke, dust and/or water vapor that might produce good seeing also filters the faint light from deep sky objects. In this case, we don't need good seeing, we want transparency, which literally means the clearest possible sky, with the least amount of particulates.
Nights with good transparency typically occur shortly after the passage of a cold front, but long enough after the front goes by so that the atmosphere has had a chance to settle down a bit. Stars will twinkle, but not dance all over the place. This generally would be an unproductive night on which to observe the planets. There are often short-term variations in transparency over the course of a night due to passing atmospheric cells, which are more apparent in large telescopes.
The search for transparency is what drives deep sky observers far afield. It's best at higher altitudes and drier climates. Locally, observers look for dark sites away from a body of water and on a hill or slight rise to reduce the humidity that settles in low areas and fogs our optics. Randy Moench also noted: "Had a chance to observe from the Shrine Pass area near Vail a couple of summers ago... I'll always remember the naked eye view of the Milky Way from that site. High and cold, even in July. I was just out to Mauna Kea this fall and I can't say there was a big difference." Of course, this is also why LCAS sponsors the annual trip to New Mexico.
If you keep a record of the seeing and/or transparency on your observing nights, the generally accepted method is to rate them on a ten-point scale, with 10 being the best. The record first shows the rating, then the scale; thus, an average night of seeing or transparency would be shown as 5/10. The perfect night of wintertime seeing that I experienced would be recorded as 10/10.
All plans aside, what usually happens here in northern Illinois is that observers watch for any clear night to haul out the telescope. If you're a deep sky fan and the transparency looks pretty good on a moonless night, you might head for a local darker sky site. But our fickle weather means we can't be too particular. The critical thing is to understand how the atmosphere will affect the type of observing you intend to do, then make the most of it. If we waited for just that perfect night, we'd hardly ever get out to observe at all!Published in the March 2001 issue of the NightTimes