introduction to the Meade 0.25 meter telescope explains some basics of astronomy and optics.
discusses the limitations of a ground-based telescope and why
adaptive optics is necessary.
The stars in the sky are all pretty much like our Sun. Some are smaller, some are bigger. They may
also be cooler and more red in color, or hotter and more blue. Many
are actually two or more stars that orbit about each other. They all look like points of light because they
are so far away.
Because they are so distant, they also appear to be at fixed
locations on the sky.
are mapped on the sky using the equatorial coordinate system.
is equivalent to a latitude measured from the
Earth's equator. Right ascension is
like the Earth's longitude, measured on a circle starting
from a fixed point on the sky. Declination is measured in degrees, arcminutes, and
arcseconds. One arcminute is 1/60th of a degree. One arcsecond is 1/60th of an arcminute.
Right ascension is measured in hours,
minutes, and seconds.
Earth is spinning at a constant rate, making the stars appear to go in the opposite
direction. To counteract this, our telescope has an equatorial type mount. We align
the telescope mount with the north pole, and a motor
rotates it to counteract the motion of the Earth. This keeps the telescope pointed at the
same star in the sky.
Light produced from a point source, like a distant star, reaches Earth
essentially as flat wavefronts. A telescope focuses these waves to
form an image.
The aperture is the opening of the telescope which
allows the light to enter. However, some of the light is obstructed by
the secondary mirror and bracings. As a result, the
pupil is the net area of
wavefront that is collected by the telescope. Our telescope has an aperture
of 0.25 meters. About 20 percent of its collecting area is blocked by the secondary mirror.
There is a limit to
the sharpness of images a telescopes can provide. This is the
diffraction limit, and it is a fundamental property of finite telescope aperture. As
wavefronts from a point source pass through the pupil of the
telescope they are bent around the edges. This process is called diffraction. These
bent wavefronts interfere with each other and produce an image that consists of a
central bright source with light and dark circular fringes around it. The width of the central peak of the diffraction
pattern is the finest separation between two objects that the telescope can discriminate.
The diffraction pattern can be made smaller by increasing the diameter of the telescope.
The largest optical telescopes in the world are the twin Keck telescopes in Hawaii.
They each have an aperture of 10 meters, and a diffraction limit 40 times smaller than our 0.25 meter
Did you ever
notice that when you look down a straight stretch of road on
a hot day that objects in the distance appear to shake and shimmer? As hot air rises off
the road, different parts of the air are at different temperatures and, therefore, different densities.
These variations in density
cause the light to bend
and distort. The
same process affects light coming down from the stars, just not as
dramatically. Turbulent convective air currents cause variations in
density which in turn cause stars to appear to jiggle.
Cool, smooth flowing air will have fewer density variations and so
the star's image will jiggle less.
If the density variations are over a big enough scale, say, much larger than the
telescope aperture, the light is hardly effected at all.
However, if the
variations are all on very small scales, much smaller
than that of the telescope pupil, each section of the light will
be bent and distorted in complex ways - one part will go one way, another
part will go another way -
creating a blurry image.
The size of the regions of air that have the same density are measured by
(generally given the symbol ro)
and are usually about 0.05 to 0.2 meters.
is about the same as the telescope aperture, an image of star will look a lot like the
diffraction pattern. As ro gets smaller, the
look more blurry. Astronomers usually
refer to the full-width at half maximum (FWHM) of a
star when gauging its blurriness.
The FWHM is the width of the star image at half its peak brightness;
the smaller the FWHM, the better the image, which will make it easier to resolve two very close objects.
Adaptive optics (AO) provides astronomy with the possibility of building optical
systems that correct for image distortions caused by
the turbulence in the atmosphere. If the only effect of the
atmosphere was to jiggle the star around, one could build a device with
a steerable mirror to measure and correct for its motion. This
type of camera is called a tip-tilt wavefront corrector and it is the simplest kind
of AO system. As ro gets smaller relative to
the telescope aperture, more sophisticated adaptive optical components are needed to
flatten the wavefront.
Go to an introduction to the tip-tilt camera.
Tip-Tilt Camera |