PH507
Astrophysics
Dr Dirk Froebrich
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Lecture 1: Introduction to telescopes
Lecture Outline.
Lecture #1: Introduction to telescopes
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Basic optical principles
Refractor
Reflector
Advantages and disadvantages of each
Discussion of F-ratios/focal lengths/speed.
Lectures #2, #3: Introduction to the E-M spectrum
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General discussion of E-M spectrum from radio -> gamma rays
Heterodyne receivers
Bolometers & Golay cells
Photoconductors
Hand out assignment #7.
Lecture #4 and #5: Visible and beyond detectors.
• CCDs
• Grazing incident focussing (X-rays etc.)
• ‘Exotic’ detectors (neutrino, gravity wave etc.).
Lecture #5: Space and ground based observatories
• Descriptions of ground based observatories and instruments (JCMT,
SCUBA2, ALMA etc.).
• Descriptions of space based observatories and instruments (SIRTF,
Spitzer, Kepler etc.).
• List of on-line resources.
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Astrophysics
Dr Dirk Froebrich
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Telescopes.
There are two basic types of telescopes, refractors and reflectors. The part
of the telescope that gathers the light, called the objective, determines the
type of telescope.
A refractor telescope uses a glass lens as its objective. The glass lens is at
the front of the telescope and light is bent (refracted) as it passes through
the lens. A reflector telescope uses a mirror as its objective. The mirror is
close to the rear of the telescope and light is bounced off (reflected) as it
strikes the mirror.
Refractor Telescopes
The refractor telescope uses a lens to gather and focus light. The first
telescopes built were refractors and the invention of the refracting telescope
is generally ascribed to Galileo in 1609, although it is recorded that a Dutch
lens maker called Hans Lippershey built one in 1608. The small, cheap toy
telescopes sold in shops are generally of the refractor type.
Refracting Telescopes focus light using lenses and the Principle of
Refraction.
Note: ‘n’ is a function of the wavelength of the incident radiation.
Generally, as wavelength increase, ‘n’ decreases. So shorter wavelength
light (e.g. blue) is ‘bent’ more than longer wavelength light (e.g. red).
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Astrophysics
Dr Dirk Froebrich
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The Refractive Index of Air is very nearly 1 (nair = 1.0003, depending on
Temperature & Pressure, but astronomers need to take this into account
when calculating wavelengths in the earth's atmosphere). The refractive
index of water is nH2O = 1.33 and refractive indices for various kinds of
glass vary from about n = 1.5--1.8. A diamond's luster is partially due to its
high refractive index, n = 2.4.
In order to look through a telescope you need two lenses, the objective,
which is the principal lens of the telescope, and an eyepiece. The image
scale in the focal place is determined by the focal length of the objective; if
you look through the telescope, the magnification will be determined by the
ratio of the focal lengths of the objective and the eyepiece.
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Astrophysics
Dr Dirk Froebrich
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The sensitivity of the telescope is determined by the collecting area of the
objective lens (or primary mirror) which is proportional to the square of the
diameter of the primary lens or mirror:
d
Area = π ⋅ ⎛⎜ ⎞⎟
2
⎝2⎠
Though excellent refractors are still made, the disadvantages of the
refractor telescope have blocked the construction of very large refractors
for use in astronomical research.
Advantages
1. The glass surface inside the tube is sealed from the atmosphere so it
rarely needs cleaning.
2. Since the tube is closed off from the outside, air currents and effects
due to changing temperatures are eliminated. This means that the
images are steadier and sharper than those from a reflector telescope
of the same size.
3. Refractor telescopes are rugged. After the initial alignment, their
optical system is more resistant to misalignment than the reflector
telescopes.
4. Generally produce a sharper image than reflectors due to the lack of
secondary mirror support structure.
Disadvantages
1. All refractors suffer from an effect called chromatic aberration
(``color deviation or distortion'') that produces a rainbow of colors
around the image. Because of the wave nature of light, the longer
wavelength light (redder colors) is bent less than the shorter
wavelength light (bluer colors) as it passes through the lens. This is
used in prisms to produce rainbows, but can it ruin an astronomical
image!
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Astrophysics
Dr Dirk Froebrich
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There are a couple of ways to reduce chromatic aberration. One way
uses multiple compensating lenses to counteract chromatic
aberration. The other way uses a very long objective focal length
(distance between the focus and the objective) to minimize the effect.
This is why the early refracting telescopes were made very long.
2. How well the light passes through the lens varies with the
wavelength of the light. Ultraviolet light does not pass through the
lens at all.
3. How well the light passes through decreases as the thickness of the
lens increases.
4. It is difficult to make a glass lens with no imperfections inside the
lens and with a perfect curvature on both sides of the lens.
5. The objective lens can be supported only at the ends. The glass lens
will sag under its own weight. Note: A 40” (~1 meter) diameter
refractor lens weighs approximately 1.5 – 2 tons!
Because of these disadvantages, the largest refractor telescope ever built is
one at the Yerkes Observatory, which was built more than 100 years ago! It
has an objective 1.02 meters (40 inches) across at one end of a 19.2-meter
(63 feet) tube. The two largest refractors are shown below. The first picture
is the 40-inch refractor at Yerkes Observatory. The second picture shows
an astronomer (Kyle Cudworth) next to the objective to give you an idea of
the size of the telescope. Notice the size of the people in the first picture!
The third picture is the 0.91-meter (36-inch) refractor at Lick Observatory.
Notice the astronomer at the lower left. The last picture is E.E. Barnard at
the eyepiece of the Lick 36-inch.
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Astrophysics
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.
Reflector Telescopes
The reflector telescope uses a mirror to gather and focus light. All celestial
objects (including those in our solar system) are so distant that all of the
light rays coming from them reach the Earth as parallel rays. Because the
light rays are parallel to each other, the reflector telescope's mirror has a
parabolic shape. The parabolic-shaped mirror focuses the parallel lights
rays to a single point. All modern research telescopes and large amateur
ones are of the reflector type because of its advantages over the refractor
telescope.
Advantages
1. Reflector telescopes do not suffer from chromatic aberration because
all wavelengths will reflect off the mirror in the same way.
2. Support for the objective mirror is all along the back side so they can
be made very BIG! The larger reflecting telescopes have primary
mirrors >8 meters in diameter; compare that to 1 meter for refractors.
This gives a factor of 64 improvement in light gathering power.
3. Reflector telescopes are cheaper to make than refractors of the same
size.
4. Because light is reflecting off the objective, rather than passing
through it, only one side of the reflector telescope's objective needs
to be perfect.
Modern reflecting telescopes use a parabolically shaped primary mirror
coated with a thin film of aluminum.
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Astrophysics
Dr Dirk Froebrich
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Disadvantages
1. It is easy to get the optics out of alignment.
2. A reflector telescope's tube is open to the outside thus thermal
differences in the tubes can effect the ‘seeing’. Also the optics need
frequent cleaning.
3. Often a secondary mirror is used to redirect the light into a more
convenient viewing spot. The secondary mirror and its supports can
produce diffraction effects: bright objects have spikes (the
``Christmas star effect'').
Two famous reflector telescopes are shown below. The first picture is of
the 5-meter (200-inch) Hale Telescope at Palomar Observatory. The
number refers to the diameter of the objective (almost 17 feet across!). The
telescope is the vertical piece in the middle with the mirror close to the
floor. The huge diagonal piece is used to balance the telescope. Until
recently it was the world's largest optical/infrared telescope.
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Astrophysics
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All modern optical/infrared telescopes are reflecting telescopes, because:
1. Reflecting telescopes do not suffer from chromatic aberration.
2. Figuring a mirror requires polishing only one precise surface rather
than two (or four for a compound lens).
3. Mirrors are easier to support because they can be supported on the
sides and the back; large lenses tend to sag because they can only be
supported on the perimeter.
Reflectors are also much more versatile than refractors because they can be
used at several different foci.
The world's largest optical/infrared telescopes are the twin 10-meter Keck
Telescopes operated by the University of California and Caltech on the
13,700ft dormant volcano, Mauna Kea, Hawaii. These are amongst the
principal research instruments of optical and infrared astronomers.
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Astrophysics
Dr Dirk Froebrich
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The Keck Telescopes employ hexagonal segmented primary mirrors, each
made out of 36 hexagonal segments, 1.8m in diameter. In order to maintain
a precise optical surface the positions of the segments are monitored by
sensors which relay signals to a computer which drives precision actuators,
keeping each segment in proper alignment.
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Astrophysics
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Mauna Kea is probably the world's best observatory site because of its
stable atmosphere, maintained by the island's marine layer, and its altitude.
Over 20 astronomical telescopes from the US, Britain, Canada, France and
Japan are in operation or under construction.
The second picture shows the path light travels in the 10-meter Keck
Telescope at the W.M. Keck Observatory. The objective is composed of 36
hexagonal mirrors put together to act as one large mirror 10 meters across.
The small image next to it shows the 10-meter objective. The person in the
red clothing at the center gives you a sense of scale.
In both the reflector and refractor telescopes, the focus is before the
eyepiece, so the image in astronomical telescopes is upside down.
Telescopes used to look at things on the Earth's surface use another lens to
re-invert the image right-side up. Most reflector telescopes will use a
smaller secondary mirror in front of the large primary mirror to reflect the
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light to a more convenient viewing spot. Isaac Newton used a flat
secondary mirror at a 45° angle to reflect the light to an eyepiece at the side
of the telescope tube near the top. Such an arrangement, called a newtonian
design is used by many amateur telescopes.
Many reflector telescope use another light path design called the cassegrain
design to reflect the light back through a hole in the primary mirror, so that
detectors or the eyepiece can be conveniently placed behind the telescope.
Most of the large telescopes used for research, including the Hubble Space
Telescope, are of this design. Some of the largest telescopes like the Hale
Telescope and the Keck Telescope have places to put detectors at the prime
focus, where the light from the primary mirror first comes to a focus. The
images in reflector telescopes do not have holes or shadows in them
because the light rays from the unblocked parts of the primary mirror are
all added together when they are focused together. Even though part of the
primary mirror is blocked or missing, there is still plenty of usable primary
mirror space to gather the light.
Both types of telescope can suffer from a defect called spherical
aberration so that not all of the light is focused to the same point. This can
happen if the mirror is not curved enough (shaped like part of a sphere
instead of a paraboloid) or the glass lens is not shaped correctly.
Hubble Space Telescope: a classic case of spherical aberration.
The Hubble Space Telescope objective suffered from this (it was too flat by
2 microns, about 1/50 the width of a human hair) so it used corrective
optics to compensate. The corrective optics intercepts the light beams from
the secondary mirror before they reach the cameras and spectrographs.
Fortunately, the Hubble Space Telescope's spherical aberration was so
perfect, that it was easy to correct for!
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Astrophysics
Dr Dirk Froebrich
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Even before the servicing mission that installed the corrective optics 2.5
years after the Hubble Space Telescope was put in orbit, astronomers were
able to get significant results from the telescope. The images were
computer-enhanced to correct for the spherical aberration to produce
sharper images than from any ground-based telescope. Also, astronomers
were able to observe ultraviolet light from celestial objects and fainter
objects than could be seen from the ground. However, the computer
processing took a long time and the aberration prevented the focusing of
most of the light. This meant that astronomers could not see the very faint
(and distant) objects they were looking for.
M100 a few days before (left) and after (right) the corrective optics
(COSTAR) were installed in December 1993.
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Astrophysics
Dr Dirk Froebrich
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Summary of optical telescopes.
The principle part of the telescope is called the collecting aperture, acting
as a means to focus light and to produce a primary image.
Note that the arriving light comes into the front of the telescope as parallel
rays. This is generally true for all astronomical objects as they can be
considered to be at infinity in comparison to the dimensions of any
telescope.
As all the objects to be investigated are effectively at infinity, the distance
of the primary images from the collecting aperture is defined as the focal
length, F, of the telescope. A plane through this point and at right angles to
the optic axis is defined as the focal plane. It is often convenient to describe
a telescope in terms of its focal ratio (f-ratio), f, which is the ratio of the
focal length of the collector to its diameter, D:
f =
F
D
This definition is synonymous with the ‘speed’ of an optical system when
applied to general photography.
The quantity of energy which is collected per unit time by the telescope is
proportional to the collecting area.
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Astrophysics
Dr Dirk Froebrich
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When a telescope is directed at the sky, an image of part of the celestial
sphere is produced in the focal plane. The relationship between the size of
this image and the angular field which is represented by it is governed by
the focal length of the telescope. In the lower of the pictures above, rays are
drawn for two stars separated by an angle θ. For convenience, one star has
been placed on the optic axis of the telescope. Rays which pass through the
centre of the system are not deviated, thus the separation, s, of the two
images in the focal plane is given by:
s = F tan θ
As θ is normally very small, this can be re-written as:
s=Fθ
where θ is normally expressed in radians.
The correspondence between an angle and its representation in the focal
plane is known as the plate scale of the telescope, which can be seen to be
given by:
dθ 1
=
ds F
Normally this is expressed in arc seconds per mm, and in this case the plate
scale is given by:
dθ 206265
=
ds
F
where F and s are expressed in mm and in seconds of arc respectively, with
the numerical term (206265) referring to the number of arc seconds in a
radian.
The telescope and the collected energy.
The apparent brightness of an astronomical object is increased as the square
of the diameter of the collector. Stellar brightness is normally termed Flux,
F, which is the energy received per unit area per unit wavelength interval
per unit time, and often expressed in units of W m-2 Å-1.
The number of photons arriving at the telescope aperture can be written as:
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NT =
π
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Astrophysics
λ1
D 2 × Δt ∫ λ
λ2
Dr Dirk Froebrich
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Fλ
dλ
hc
Where λ1 and λ2 are the cut-on and cut-off wavelengths defined by the
filter that is used, and Δt is the integration time.
The arrival of photons is a statistical process. When the arriving flux is low,
fluctuations are clearly seen on any recorded signal, and the measurements
are said to suffer from photon shot noise. If no other sources of error are
present, the uncertainty in any measurement is given by N T . Hence any
record and its error may be represented by N T ± N T . In these
circumstances, the signal-to-noise ratio (S/N) of the observation is given
by:
NT
NT
= N T ∝ D 2 Δt → D Δt
Example: At 6300Å (630nm) the flux from a source is 10-18 W m-2 Å-1.
Determine the photon rate passing through a telescope with a diameter,
D=2.2 meters over a wavelength interval of 100 Å. Calculate the best S/N
ratio of a measurement with an integration time of 30 seconds.
NT =
π
4
2.2 2 × 30 ×
630 × 10 −9
× 10 −18 × 100 = 36121
−34
8
6.63 × 10 × 3 × 10
and if the noise is purely limited by shot noise, the S/N ratio is simply
= 782. Therefore the accuracy is about a part in 782, or about 0.1%
NT
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Astrophysics
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Chromatic aberration.
Chromatic aberration corrected using a doublet of 2 differing
refractive index glasses.
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Spherical aberration.
Dr Dirk Froebrich
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Dr Dirk Froebrich
The Newtonian and Schmitt-Cassegrain optical system.
1) “Celestron” 8-inchNewtonian reflector
2) “Meade” 10-inch Schmidt-Cassegrain
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Dr Dirk Froebrich
3) “Skywatcher” 6-inch refractor
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Radio Telescopes
Radio astronomy has its roots back in the 1930's when Karl Jansky
accidentally detected radio emission from the center of the Milky Way as
part of his research on the interference on transatlantic phone lines. The
British advanced radio antenna technology in their development of radar
technology to fight the German warplanes in World War II. After the war,
astronomers adapted the technology to detect radio waves coming from
space.
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Astrophysics
Dr Dirk Froebrich
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A radio telescope uses a large
metal dish or wire mesh, usually
parabolic-shaped, to reflect the
radio waves to an antenna above
the dish. An example of a mesh
is shown at left. This was the
mesh of the parabolic dish for
the former 100-meter radio
telescope at Green Bank, West
Virginia (photos courtesy of
National Radio Astronomy
Observatory). Looking from
underneath the radio telescope, a person could see the clouds in the sky
overhead but to the much longer wavelength radio waves, the metal mesh
was an excellent reflector.
The signal from the antenna is sent to an amplifier to magnify the very faint
signals. At the last step, the amplified signal is processed by a computer to
turn the radio signals into an image that follows the shape of the radio
emission. False colors are used to indicate the intensity of the radio
emission at different locations. An example is shown below for Jupiter.
Charged particles in its magnetic field produce a large amount of radio
energy in doughnut-shaped regions around its center. A visible band image
of Jupiter is shown below the radio image.
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Astrophysics
Dr Dirk Froebrich
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Radio telescopes may be made much larger than optical/infrared telescopes
because the wavelengths of radio waves are much longer than wavelengths
of optical light. A rule of thumb is that the reflecting surface must not have
irregularities larger than about 1/5 the wavelength of light that is being
focused. By that criterion a radio telescope is several hundred thousand
times easier to figure than an optical telescope of the same size.
The Arecibo Observatory radio telescope (below) is a 305 meter reflecting
surface in a natural limestone sinkhole in NW Puerto Rico. Because the
telescope cannot be pointed independently the telescope is "steered" by
tilting the instrument housing supported over the telescope.
Interferometry and aperture synthesis.
(Aperture synthesis: making a very large telescope out of lots of smaller
ones).
Because radio signals are detected as waves, signals from different
telescopes can be added to simulate or synthesize the resolving or pinpointing capability of a much larger telescope. The National Radio
Astronomy Observatory's Very Large Array (VLA) near Socorro, NM,
consists of 27 radio telescopes, each 25 meters in diameter which are
deployed on a Y-shaped track which may be extended up to 36km. The
VLA has the resolving power of a 36km telescope (but only the collectingarea and sensitivity of a 130m telescope).
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Astrophysics
Dr Dirk Froebrich
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