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Deepsky Observer's Companion tutorial
Part 7: The atmosphere, weather, and seeing

In this section: the atmosphere, slow and fast seeing, twinkling, transparency, how to identify sources of seeing, weather indicators, the impact of light pollution.

The atmosphere which blankets us greatly influences our view of the universe. Weather conditions, too, play an important role in the quality of observing. Not to forget the growing problem of light pollution. Some of these complex topics are discussed here.

Schaefer (1993) writes:

"Under a clear sky the twinkling of stars creates an atmosphere of liveliness." MacRobert (1995) says that "viewed at high power from the bottom of our ocean of air, a star is a living thing. It jumps, quivers, and ripples tirelessly, or swells into a ball of steady fuzz." Schaefer continues: "This rapid change in a star's apparent brightness is termed scintillation. Even though stars subtend infinitesimal solid angles, they appear in a telescope as a finite disk with fuzzy edges. This image blurring is called seeing. When a star is viewed through a small telescope, the light appears to move around like a will-o'the-wisp dancing around a fairy. This effect is called image movement."

"All three phenomena are closely related manifestations of turbulence in the atmosphere. The correct idea was first advanced by Robert Hooke in 1665 when he suggested the existence of "small, moving regions of atmosphere having different refracting powers which act like lenses." The refractive index of the air varies slightly from point-to-point due to small changes in temperature and density caused by turbulent motions of the winds and heating from the ground. So the path of a beam of light passing through the atmosphere will be bent and kinked from the random scatterings imposed by the weak refractive prisms of air. An observer on the ground will be able to see light from a point source by looking in many directions at once. This spreading of the light into a 'seeing disk' is caused by many small angle scatterings, and hence has a two-dimensional Gaussian distribution. As the wind blows the eddies across the line of sight, the number and centroid of paths will shift randomly resulting in scintillation and image movement."

"The best introductory article on this topic is Young (1971) while Mikesell, Hoag and Hall (1951) provide a good discussion of the observational properties of scintillation. A review paper by Coulman (1985) gives a detailed technical discussion along with an extensive bibliography."

Slow and fast seeing

MacRobert (1995) notes that

"Telescope users recognize two types of seeing: 'slow' and 'fast'. Slow seeing makes stars and planets wiggle and wobble; fast seeing turns them into hazy balls that hardly move. You can look right through slow seeing to see sharp details as they dance around, because the eye does a wonderful job of following a moving object. But fast seeing outraces the eye's response time."

Twinkling

"An old piece of amateur folklore is that you can judge the seeing with the naked eye by checking how much stars twinkle. This often really does work. Most of the turbulence responsible for twinkling originates fairly near the ground, as does much poor seeing. But high-altitude fast seeing escapes this test. If the star is scintillating faster than your eye's time resolution (about 0.1 second), it will appear to shine steadily even if a telescope shows it as a hazy fuzzball."

Transparency

Transparency describes the clarity of the atmosphere. As the transparency worsens, faint stars begin to disappear. Extended objects such as nebulae suffer most from poor transparency, lunar and planetary detail from poor seeing, star clusters equally from both effects.

How to identify sources of seeing

MacRobert (1995) writes:

"Tube currents of warm and cool air in a telescope are real performace killers. Reflectors are notorious for their tube currents. Any open-ended tube should be ventilated as well as possible. Suspending a fan behind a relfector's mirror has bgecome a popualr way to speed cooling and blow out mixed-temperature air. It's easy to check whether tube currents trouble your images. Turn a bright star far out of focus until its a big, uniform disk of light. Tube currents will show as thin lines of light and shadow slowly looping and curling across the disk."

On the other hand, if the out-of-focus star disk swarms with wrinkles that scoot across the view, entering one edge and leaving the other, then there is local seeing near the telescope.

To combat local seeing: allow your telescope to cool down before observing; avoid tube currents (flows of warm and cool air in a telescope tube), keep body heat and breath out of the light path, and try to keep the telescope surroundings "thermally friendly" (grass is better than pavements; the flatter and more uniform the greenery the better; the higher off the ground, the better).

Weather indicators

The following bits of advice were gleaned from various authors; I really don't have a good understanding of the weather and its role in astronomy – any help would be welcome.

  1. Watch the colour of the daytime sky, especially near the horizon. The bluer the sky, the darker the night will probably be. The white haze in a blue sky consists of microscopic water droplets that have condensed on tiny solid particles, primarily sulphate dust from distant factories and power plants. These particles are the precursors of acid rain. They do just as good a job of scattering artificial light at night. A deep blue sky in the afternoon should mean a transparent sky after dark.
  2. If low humidity is predicted by the weatherman, that's a good sign.
  3. A windy cold front sweeping through a city can clear out local air pollution, leaving the night marvelously dark. The windiest city and suburban nights are often the darkest. A passing rainstorm or blizzard can also leave an unusually dark night in its wake.
  4. Poor seeing does seem more likely shortly before or after a change in the weather, in partial cloudiness, in wind, and in unseasonable cold. Any weather pattern that brings shearing air masses into your sky is bad news.
  5. After a cold front passes - often with a heavy rain or snowstorm - the sky usually becomes very dark and crystal clear but, unfortunately, very turbulent. These clear nights, when stars twinkle vigorously and the temperature plummets, may be great for deep sky observing but are usually worthless for the planets.

Impact of light pollution

A typical suburban sky today is about 5 to 10 times brighter at the zenith than the natural sky. In city centres the zenith may be 25 or 50 times brighter than the natural background.

Where there's no light pollution the limiting magnitude is usually assumed to be 6.5, though some people can see fainter. Under such conditions, the sky is packed with stars, the Milky Way is a mass of swirling, jumbled detail and any clouds appear blacker than the sky itself. At a limiting magnitude of 5.5, clouds are brighter than the sky because they are lit from below. The Milky Way is still easily visible but far less detailed. At limiting magnitude 4.5, the Milky Way is barely detectable as a faint, nearly featureless band. At a limit of 3.5 the Milky Way is completely invisible.

David W. Knisely posted an item to sci.astro.amateur (Subject: Measuring sky 'darkness', Date: Thu, 22 Jan 1998 19:52:21), in which he suggests the following guidelines for naked-eye limiting magnitudes and overhead light-pollution ratings:

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