Seeing
In this example three
pictures taken, in the vicinity of a globular cluster, show the
seeing or clarity available to three different clases of telescopes.
From left to right the Hubble telescope produces the clearest
picture then progresively less clear are the best ground based
telescopes, like the twin Kecks in Hawaii, and then one example
of what a smaller telescope at sea level is capable of.
Most of the stars in this picture are very faint stars, about
30th magnitude, making them hard to see. The less light
distortion the better the seeing is. Distortion comes from
a number of different sources namely weather patterns, light pollution,
refraction, and absorbtion. As light passes through the
atmosphere it gets bent and absorbed making the atmosphere
the biggest contributor to bad seeing. Light pollution is
caused by man made light which reflects off of haze or light clouds
needless to say clouds in general make it hard to see the stars.
The Hubble Space Telescope
orbits high above the majority of the Earths atmosphere effectively
eleminating all distortion. It is also above the light pollution
and weather patterns which can cause bad seeing. On a whole
the Hubble telescope circumvents most problems and can take the
highest resolution images.
The second best way
to avoid as much bad seeing as possible is to build a very
large telescope on top of a mountain and perhaps even make it
with adaptive optics. A well chosen mountain is both high,
thus avoiding most of the atmosphere, and in the right prevailing
weather system to avoid the minuscule amounts of clouds that there
might be at high altitudes. Most mountain peaks are also
far away from civilization reducing light pollution. Large
mountain based sites combat atmospheric effects by being large,
the larger the surface area of the main mirror the more light
is collected. In order to have a very high resolution the
twin Kecks each have a primary mirror aproximately twice as large
as the Hubble. The Kecks also use adaptive optics to adjust
for atmospheric effects changing the shape of the mirror 670 times
a secound to adjust for the current seeing.
At sea level you simply
dont observe when there is bad seeing. The whole atmosphere
has a chance to distort the light before it hits the telescope.
Atmospheric effects play a big role in when you can take data.
If it is really hot out during the day then the air wont cool
down for much of the night making the air "noisy."
Pictures taken when its been hot have a grany unsettled texture
to them, because the light has to pass through hot unsettled air.
Humidity can have a similar effect, water droplets cause the light
to be bent and distorted causing noise. Also our telescope
here at BG is smaller so it doesnt gather in as much light as
do the Hubble or the Kecks. This allows faint objects whose
light is absorbed more quickly and whose light has to go through
much more atmosphere to reach us not be seen at all or to diffuse
into the glow like the picture on the right. Being right
here in town our telescope suffers from much light pollution.
The light of the city can very easily bounce off of a very few
clouds and prevent us from getting good data.
In Space
On a mountain
Sea level
Hubble
Keck
Bowling Green
