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4. Telescopes
• Light gathering power and resolution
• Optical and radio telescopes
• Limitations of Earth’s atmosphere and satellite
missions.
• Instruments (prism spectrographs and diffraction
gratings), detection devices (CCDs) and methods
(photometry and spectroscopy)
4.1 What is a
telescope?
 A telescope
gathers and
focuses
electromagnetic
radiation.
Optical telescope
designs - we use
reflecting
telescopes today
Telescopes with lenses work because of refraction: at the
interface between two transparent media the light path
changes because the speed of light depends on the medium
that it is passing through.
Snell' s Law :
n1 sin 1  n2 sin  2
Problems for refracting telescopes: the degree of refraction
depends on wavelength - can lead to chromatic aberration
- wavelength dependent focal point.
Telescopes with mirrors
work because of
reflection:
angle of incidence = angle
of reflection
This effect is wavelength independent and can be exploited
to bring light of all wavelengths to the same focus with
concave or convex mirrors.
4.2 Why a BIG Telescope? Light Gathering Power
Think of a telescope as a
bucket sitting in a rain of
photons: increasing the
aperture (diameter) of a
telescope its light
gathering power (it can
capture more photons in the
same amount of time
because it is bigger). I.e.
LGP  D
2
E.g. Two pictures of the Andromeda galaxy taken with the
same exposure time…...
4.3 Why a BIG telescope? - Resolution and the
Diffraction Limit
Light from the same object incident on different portions
of the telescope’s mirror can interfere constructively and
destructively to form a pattern of light and dark Airy
rings (similar to the double slit experiment).
Airy rings around a single star.
The interference pattern
produced as the Airy rings of
two stars overlap each other.
The larger the aperture size of the telescope, the smaller or more
compact the Airy rings.
The smaller the aperture size of the telescope, the larger or more
extended the Airy rings.
No image can be smaller than the innermost Airy ring, and
this sets the diffraction limit of a telescope to be
 min  1.22(206,265)

D
where D is the telescope' s aperture
 diameter of the telescope' s mirror
(in same units used to measure the wavelength).
 min (the above expression gives the answer
in arcseconds) sets a limit to the
the image.
resolution of
E.g. The Andromeda
galaxy seen by a small
telescope………
……vs the Andromeda
galaxy seen by a
telescope with a larger
aperture.
4.4 Limitations of Ground-based observing Effect of the Atmosphere
Cells of air
Ground
level
Seeing typically limits the
resolution of ground-based
telescopes to no better than 1
arcsecond
Limitation 1:
turbulence in the
Earth’s atmosphere
causes starlight to
be blurred - an
effect known as
seeing.
Solution 1 to limitation 1 - go to space!
 Because the Hubble Space Telescope (HST) is
above the atmosphere, it is not affected by
atmospheric turbulence that blurs ground-based
observations.
Ground-based Image
HST
Solution 2 develop
adaptive
optics to
correct for
effect of
Earth’s
atmosphere
Limitation 2 - The Earth’s atmosphere is not transparent
to all wavelengths, so IR, UV, X-ray and gamma-ray
observations require satellite missions (the only
solution).
Space-Based Observing
The Chandra X-ray
satellite…….
Ground-based observatories tend to be located on tops of
mountains with low rainfall to limit atmospheric affects
Cerro Tololo Inter-American
Observatory - in the Chilean Andes
near La Serena, Chile
Ground-based observatories tend to be located on tops of
mountains with low rainfall to limit atmospheric affects
Kitt Peak National Observatory - near Tucson, Arizona
Ground-based Observing
La Campanas Observatory in Chile.
Telescopes
The twin 10 meter
Keck reflecting
telescopes.
Telescopes
The 10-meter Keck primary mirror consists of many
smaller mirror segments which fit together precisely to
create the reflecting surface.
Just one of the Keck
mirror segments…..
Ground-based Observing
Astronomers
no longer
look through
the eyepieces
of telescopes
and draw
pictures rather they
collect data
using
instruments
and analyze
them on
computers
Going observing can be quite a magical experience….
4.5 Detecting the Light
 We no longer use our eyes to look
through telescopes for the purpose of
astronomical study.
 Up until the early-1980’s, the
principal light detector that optical
astronomers used was the
photographic plate. This consisted of a
flat piece of rectangular glass about
1/4 of an inch thick which was coated
on one side with a photographic
emulsion.
 Photographic plates were extremely inefficient! For every 100
photons that fell on the plate, only one was detected - 1% efficiency.
 Nowadays, we use sensitive cryogenically cooled
solid state light detectors known as Charge Coupled
Devices (CCDs).
 Most CCDs are over 90% efficient at detecting
photons.
(a)
(b)
(c)
(a) A negative print (black stars on white sky) of a photographic image.
(b) A CCD image. Notice the many faint objects that are revealed.
(c) This color picture was created by combining a series of CCD
images taken through different filters.
 CCD’s are made up of light sensitive diodes laid out in a square array.
 These diodes are known as pixels (picture elements).
# of photons
Star on a CCD
y
0
x
 (curve)dx
x
x
Photometry - by calculating the
area under this curve, we can
measure the apparent brightness of
this star.
Photometry - measuring the apparent brightness of
this star. Astronomers also use filters to measure
brightness in a specific range of wavelengths (e.g. red
light vs blue light).
Spectroscopy - using a diffraction grating or prism
astronomers split the light into a spectrum and
measure the intensity of light at each wavelength.
The wavelength dependence of refraction allows prisms of
glass to to split light into spectra:
At the first interface (air/glass) the light rays are bent
towards the normal perpendicular to the surface.
At the second interface (glass/air) the light rays are bent
away from the normal.
Used in prism spectrographs to split light into spectra.
4.5 Radio telescopes
The diffraction limit means that
telescopes to detect longer wavelengths
have to be larger to maintain a
reasonable angular resolution. Radio
telescopes achieve this either with huge
single dishes, or by synthesizing signals
from more than one dish:
Radio Observatories
Arecibo Observatory
- Puerto Rico
Very Large Array
(VLA) - near Socorro,
New Mexico