Key Concepts
- Telescopes use either a lens or
a mirror to gather and focus light.
- The primary purpose of a telescope is to gather
light.
- The secondary purpose of a telescope is to
resolve fine detail.
(1) Telescopes use either a lens or a mirror to
gather and focus light.
Refracting telescopes use a lens to
collect and focus light. The first telescope invented was a refracting
telescope. Light is bent, or refracted, when it moves
from a dense medium to a lighter one (glass to air, for instance), or when
it moves from a light medium to a denser one (air to glass). The angle
through which light is bent depends on the relative density of the glass
and air, and also on the WAVELENGTH of the light. For light passing
through a glass lens, short wavelength light is bent more than long
wavelength light.
Various optical gizmos use refraction in different
ways:
- Prism: a chunk of glass, usually
triangular in cross section, which spreads white light out into a
rainbow, or spectrum. Since the short wavelength light (purple &
blue) is bent through a sharper angle than the long wavelength light
(red & orange), the different colors are physically separated.
- Lens: a chunk of glass which is
convex in cross section will take parallel rays of light and bends
them until they focus at a point. This is the property which we want
to use when we build a refracting telescope.
A.1 Light Buckets
Stars are faint. Even the brightest stars appear 25 billion times fainter than the Sun,
and most interesting objects are much fainter than that.
The amount of light gathered
is proportional to the collecting area of the telescope (or the square of the diameter).
The primary purpose of a telescope is to gather
light.
Astronomers often refer to telescopes as ``light
buckets''. The human eye is a wonderful optical instrument, but it is
limited in size; even when fully dilated, the pupil of your eye is less
than a centimeter across. To gather up more light, you need a big lens or
mirror.
The number of photons collected each second by a
telescope is proportional to the area of its lens or mirror. For a
circular lens or mirror, the area is given by the formula:
Area = (pi/4)D2
By doubling the diameter of a telescope's mirror, you
quadruple the area, and quadruple the amount of light you gather from a
distant star in a given time.
The secondary purpose of a telescope is to resolve
fine detail.
Two stars are resolved if they are
seen as two separate points of light rather than a single blur. The
smallest angle you can resolve with your un-aided eye is roughly an arcminute.
Telescopes with large apertures can resolve smaller angles, and thus see
finer detail. The smallest angle resolved by a telescope is INVERSELY
proportional to the diameter of its mirror or lens. For instance,
Galileo's telescope had a diameter of 1 inch; the maximum possible
resolution at this diameter is 4.6 arcseconds. If Galileo's telescope were
twice as large, with a diameter of 2 inches, it would have been able to
resolve angles only half as large, 2.3 arcseconds instead of 4.6
arcseconds.
Magnification is a relatively unimportant purpose of
telescopes; a big fuzzy blur is no better than a small fuzzy blur.
Resolution is the smallest angle for which you can tell two objects are separate.
The limiting resolution is given by diffraction, so a larger aperture (diameter
of the main mirror or lens) gives better resolution.
The resolution angle (arcseconds) alpha = 0.002 lambda/d, where lambda = wavelength in
angstroms and d = diameter in cm. Note: 1 arcsecond = 1/60 arcminute. 1
arcminute = 1/60 degree.
e.g.. For yellow light, lambda = 5500Å:
- human eye: d = 0.4cm, alpha = 28''
Typical Binoculars: d = 3cm, alpha = 4''
Hubble Space Telescope: d = 2.4m (240cm), alpha = 0.05''.
In the radio, lambda = 20cm = 2 x 109Å
- Arecibo d = 300m = 30000cm, alpha = 133''.
B. Telescope Designs
Telescopes focus
light from a large area to a small area.
Telescope sizes determined by the diameter of the largest mirror or lens.
Light path is bent (refracted) by a lens
A primary lens focuses light onto a point, thereby increasing the magnification: an
optional second lens "straightens" the light beams for viewing.
The largest refracting telescope is 1m.
Problems at large sizes: need high quality glass
the lens sags under it's own weight
chromatic aberration
(different colors refracted by different amounts)
Light is reflected off of curved mirrors to a focus.
Largest is 10m (largest single mirror is 6m).
Mirrors can be more easily supported than lenses, only need good surfaces, and have no
chromatic aberration.
Almost all large telescopes are reflectors.
Four of the major reflector designs are:
- Prime Focus: detectors put at primary focus of main mirror
- Newtonian: light bounces off flat mirror out side of tube
- Cassegrain: light bounces off of curve mirror back down telescope and through
hole in main mirror
- Coudé: light bounces off secondary curved mirror to flat and out side of
telescope (often through telescope mounting to a separate room).
Many telescopes appear tilted so that the frame lines up with the celestial poles. This
makes steering and `tracking the sky' easier.
For telescopes, then, BIGGER IS BETTER.
Larger diameter lenses or mirrors enable you to see fainter objects and
finer detail.
The world's largest telescopes are all reflectors (using mirrors) rather
than refractors (using lenses). This is because very large lenses are
plagued by a number of problems:
- Lenses absorb light, dimming images.
- Lenses sag, distorting images.
- Lenses suffer from chromatic
aberration.
A few words about chromatic aberration: because short wavelength (violet)
light is refracted more than long wavelength (red) light, violet light is
focused close to the lens and red light is focused farther from the lens. No
matter where you place your detector (be it your eye, or a photographic
plate, or an electronic photon counter), you can't focus all the light
simultaneously. You will end up with a sharp focused violet image surrounded
by a red blur, or with a sharp focused red image surrounded by a violet
blur.
The World's Largest Refracting Telescope:
- Yerkes Observatory,
Wisconsin
- 1 meter diameter lens
- Completed in 1897 (over a century ago!)
The World's Largest (Fully Steerable) Reflecting Telescopes:
- Keck
Telescopes, Mauna Kea, Hawaii
- 10 meter diameter mirror
- Completed in 1993, with an identical `clone' of the original telescope
built in 1996
The limits of lens technology were reached a century ago. Now, all large
telescopes use mirrors rather than lenses to gather light. (Small amateur
telescopes, though, frequently use lenses; at small scales, their problems
are not crippling.)
A number of gargantuan `new-generation' telescopes, using advanced
technology and highly computerized control systems, are currently under
construction. Ohio State University,
which has long been among the leaders in astronomical research, is involved
in a project to build one of these new-generation telescopes. OSU is part of
a consortium building the Large Binocular Telescope, on
Mount Graham in southeastern Arizona. When completed, the telescope will
contain, as its name implies, TWO mirrors. Each of the mirrors will be 8.4
meters (27 feet!) in diameter. The total light-gathering power of the Large
Binocular Telescope will be 40 percent greater than that of the Keck
Telescope. An engineering drawing of the completed telescope is shown below.
The two aqua disks are the main mirrors.
Once you build a telescope, you can't leave it sitting out in the open
air; it's a delicate optical instrument and needs protection from the
weather. An artist's rendition of the observatory building protecting the
Large Binocular Telescope is shown below. (A pair of cars parked at the base
of the building indicate the scale. As you can see, the Large Binocular
Telescope really is large.
The first mirror of the Large Binocular Telescope was cast in 1997; the
second mirror was cast in 2000. The observatory building is completed, and
awaits installation of the telescope. For a picture of the building as it
currently appears, click
here. ``First light'' for the Large Binocular Telescope is currently
scheduled for spring in the year 2004; big telescopes are not built in a
day.
Why are Arizona and Hawaii so popular for telescope sites? Why not Ohio?
The problem with Ohio, as you have probably noticed, is that it is
frequently cloudy, it rains a lot, and there are lots of people around who
insist on having street lamps and other sources of ``light
pollution''.
A good telescope site has:
- clear weather (clouds absorb light)
- dry air (water vapor absorbs light, particularly at long wavelengths)
- dark skies (the glare of city lights drowns out faint astronomical
sources)
- smooth air flow (turbulent air makes stars twinkle and blurs their
images)
Isolated mountains, such as Mauna Kea in Hawaii and Mount Graham in Arizona,
are usually good sites to build telescopes.
C. Recent Advances
In recent decades, several new ideas have been tried to solve old and new problems.
Seeing =
apparent size of a point-like image.
- Light passing from one air parcel to another is refracted. Since the air parcels move,
the light path bends erratically and the image appear blurred (several arcseconds).
- Traditionally this has been solved by putting the telescope onto a mountain (0.5 to 1
arcseconds), or more recently into space.
- A new technique is adaptive optics. The mirrors move to keep up with changes in the
atmosphere.
Sky Brightness (and light pollution)
- Artificial and natural light scattered into the telescope, usually by dust in the
atmosphere.
- Sky brightness limits the faintest objects that can be studied.
- Traditional solution is mountain sites.
- Newer solution is to get cities to use low pressure sodium street lights, which only
glow in a few wavelengths.
Weather: some nights it's cloudy.
- Traditionally, use mountain sites (above most clouds). Otherwise, go to bed early.
- All atmosphere problems disappear if you have a telescope in orbit; however, this is a
very expensive solution.
Wavelength limitations: even in good weather, the atmosphere is only clear in two
"windows": optical (and near infrared) and radio (longer than 1 cm).
- Before spacecraft and very high altitude balloons, no astronomy was done outside of
these windows. Now, space-based astronomy us very important in far infrared and UV-Gamma
Ray regions.
C.2 Stuctural Limits
C.2 Stuctural Limits
A 5m mirror is difficult to support and move around.
As a large mirror moves, it sags out of shape.
From 1949-1978, Mt. Palomar 5m was largest telescope on Earth.
New designs are being tried to get around this:
Multiple mirrors (few large mirrors, e.g.. MMT)
Segmented mirrors (many small interlocking pieces, e.g.. Keck)
Meniscus mirrors (thin mirror)
Liquid mirrors (spinning liquid mercury) D. Detectors
D. Detectors
A telescope is only as good as the instruments that record the information. Most
astronomers don't ``look through'' their telescopes.
Three classes of detectors: photometers, spectrometers, and imagers.
Photometers record the flux of light in a given range of wavelengths from a given
part of the sky.
- Filters in the light path allow selection of the wavelengths studied.
- Photometers are especially useful for studying objects that vary in brightness,
especially in short periods of time.
Spectrometers
break light into component wavelengths.
- Usually use a diffraction grating, which bounces light off of a finely ruled mirror.
Different wavelengths diffracting off the lines are bent by different
amounts, producing a
spectrum.
- Spectrometers are best for doing detailed analysis of an object.
Imagers produce a 2-dimensional image of the sky.
- These are best for determining shapes of objects and finding the relationships between
different objects.
Different types of detectors can be combined: e.g., imaging spectrometer produces a
spectrum of every star in an image.
Before the late-19th century, all astronomy was done by drawing what the astronomer saw.
Through most of the 20th century, astronomers took photographs of spectra and images.
More recently, charge coupled devices (like in VCR's) allow digital recording of
images and spectra. E. Beyond the Optical
E.1 Radio Astronomy
E. Beyond the Optical
E.1 Radio Astronomy
The other large spectral ``window'' in the atmosphere is from wavelengths of 1cm to 10m.
In 1931, Karl Jansky discovered a radio hiss from Sagittarius: later found to be the
galactic center.
After WWII, many radio telescopes built.
Designed like a prime focus reflecting telescope, with a radio receiver at the
focus (acts like a photometer).
Mirrors only have to be smoother than the light wavelength, so radio dishes can be quite
rough.
Radio telescopes are especially useful for ``non-thermal'' radio sources such as:
synchrotron radiation - electrons accelerated in a magnetic field
molecular transitions - molecules change energy levels like electrons
fine structure transitions - very small electron transitions, esp. hydrogen 21cm line.
Radio normally has poor resolution (because of the long wavelengths).
Interferometry = combine light from several dishes to produce an image with
resolution defined by separation of dishes, not dish size.
e.g.. Very Large Array (VLA) = 27 dishes up to 35km apart.
Very Long Baseline Interferometry (VLBI) = multiple telescopes on several continents.
Achieves resolution of 0.001''. E.2 Infrared
The near-IR (7000Å < lambda < 100000Å) is only partially blocked by water
vapor in the Earth's atmosphere.
Some optical telescopes at high altitudes can also observe in the
near-IR.
Detectors must be kept very cool. Otherwise they glow in the IR.
For work between 100000Å and 1mm, we need to get out of the atmosphere.
Infrared Astronomical Satellite (IRAS) mapped sky at 4 wavelengths in 1983.
IR is good for seeing ``cool'' objects (T < 2000o),
e.g.. dust. Also some
molecular transitions. E.3 Ultraviolet
For lambda < 3000Å, the atmosphere is opaque, so all astronomy is done from above
it.
The International Ultraviolet Explorer is a small UV telescope that has been in orbit
since 1979.
HST also does UV work.
Hot stars and high energy electron transitions are observed in the UV. E.4 X Rays
and Gamma-rays
Several small satellites have studied the high energy sky (especially Einstein).
ROSAT (X-ray) and Compton Gamma Ray Observatory presently working.
Shuttle acts as platform for Astro telescopes.
Gamma rays and X-rays are used to observe high energy objects,
e.g.. neutron stars, black
holes, synchrotron radiation from intense magnetic fields, matter-antimatter
annihilation.
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