"UNDER A CLEAR
SKY" writes Schaefer (1993), "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
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) "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 telescope 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 - telescope
surroundings should be "thermally friendly' - grass is better than
pavements; the flatter and more uniform the greenery the better; the higher off
the ground, the better. |