1B11 Foundations of Astronomy Telescopes and instruments

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1B11 Foundations of AstronomyTelescopes and
instruments

• Liz Puchnarewicz
• emp_at_mssl.ucl.ac.uk
• www.ucl.ac.uk/webct
• www.mssl.ucl.ac.uk/

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1B11 Telescopes and instruments

• Telescopes collect photons and bring them to a
  focus.
• They operate over the full range of the
  electromagnetic spectrum, eg radio, microwave,
  IR, optical, UV, X-ray, g-ray and cosmic rays.
• Different techniques are used to collect the
  photons at different wavelengths.
• This module concentrates on optical telescopes
  (similar technologies are used in the IR and UV).

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1B11 Refracting (dioptric)
Collecting area pD2/4 Focal ratio f/D
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1B11 Refracting (dioptric)
focal length, f
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1B11 f-ratios
F-ratio Often written as f/f-ratio ie f/8
is an f-ratio of 8, ie the focal length is 8x the
lens (or mirror) diameter The smaller the
f-ratio, the brighter the image at the focus. So
for faint extended objects, the smaller the
f-ratio, the better. Telescopes with small
f-ratios are said to be faster. The amount of
light gathered is determined only by D. The
angular separation of objects may be magnified by
adding an additional lens, ie an eyepiece
(Galileo, 1609).
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1B11 The eyepiece
objective lens
eyepiece lens
NB the image is inverted!
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1B11 Reflecting (catoptric) telescopes
These use mirrors to reflect and focus light
(Newton, 1668).
prime focus
Primary mirror (paraboloid hyperboloid)
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1B11 Newtonian
flat mirror
Newtonian focus
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1B11 Cassegrain
Secondary mirror (hyperboloid)
For a given f-ratio, a Cassegrain telescope is
more compact. Also, the Cassegrain design lends
itself to mounting heavy equipment more easily.
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1B11 Diffraction effects
This effect places a fundamental limit on our
ability to distinguish two closely spaced objects
(eg stars). Only 84 of the light is concentrated
in the central spot, the rest falls in
surrounding rings.
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1B11 Diffraction patterns
I
n1
n1
n2
n2
Airy disk
distance
The constructive and destructive interference
patterns are described according to Huygens
Principle.
For a telescope with diameter D and light with
wavelength l, minima occur at positions given by
a
a
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1B11 Resolution
n is the number or order of the minimum and m is
the numerical factor for any given n (found by
integrating over the light pattern). Because a is
small, we
may write
The light contained within the radius (84 of the
total) defined by the first minimum is called the
Airy disk, where
and this is used to define the resolution
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1B11 Rayleigh Criterion
I
Airy disk
distance
1.22l/D
I
Rayleigh Criterion A point source is said to
be resolved if the closest peak of any other Airy
disk falls at least as far away as its own first
minimum.
1.22l/D
distance
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1B11 Telescope resolution
Telescope diameter large
Telescope diameter small
Airy disk small
Airy disk large
Resolution high
Resolution low
eg Keck 10m telescope, amin 0.014 arc sec (for
l5500A) (1p piece at 300km)
eg Fry 8inch(20cm) telescope, amin (1.22 x
5500x10-8)/0.20 3.36x10-6 radians
0.69 arc sec (for
l5500A)
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1B11 Atmospheric seeing
There are other factors which limit the
resolution obtainable in astronomical
observations. A major factor is the turbulence in
the Earths atmosphere which has the effect of
blurring stellar images. This what causes stars
to twinkle and is known as atmospheric seeing.
Measured in arcseconds.
atmosphere
Good site 0.3 - 1 (Hawaii, Chile) Generally 1
2 (diff lim for D20cm) qseeing propto l-0.2
adaptive optics
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1B11 Chromatic Aberration
Unique to refractors because the focal length
of the lens is wavelength-dependent.
Effect considerably reduced by using a compound
lens
Works for 2 colours
Crown glass, m low, convex
Flint glass, m high,
plano-concave
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1B11 Spherical aberration
Light rays which are parallel to the optical axis
of a lens or spherical mirror, but which lie at
different distances from the axis, are brought to
different foci.
(exaggerated for clarity)
centre focus
edge focus
Paraboloidal mirrors do not suffer from
aberration, but spherical mirrors have a wider
field of view, are coma-free and have low
f-ratios.
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1B11 Catadioptric telescopes

• Spherical aberration from mirrors may be
  corrected by adding corrective plates or lenses
  (eg Hubble Space Telescope).
• There are two main designs of telescope which use
  these corrective elements
• Schmidt telescopes where a thin corrector plate
  is placed at the centre of curvature of the
  mirror
• Maksutov cameras a spherical meniscus lens is
  inserted in the lights path.

Hubble Space Telescope
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1B11 Schmidt telescope
Corrector lens
In the Schmidt design, a thin corrector plate or
lens is placed at the centre of curvature of the
(spherical) mirror. The features of the plate
shown in the diagram are exaggerated for
clarity. Most suitable as a camera and often used
as survey instruments, eg Palomar Observatory Sky
Survey, UK Schmidt survey.
Focal surface
Spherical mirror
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1B11 Maksutov cameras
The meniscus lens has a negative long-focus and
spherical surfaces. It produces its own spherical
aberration which cancels out the mirrors. It can
also be made achromatic so a Maksutov camera can
be free of spherical and chromatic aberrations.
meniscus correcting lens
focal surface
spherical mirror
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1B11 Coma
Point sources which do not lie along the axis of
a lens or non-spherical mirror, will look
comet-like (or fan-like) this effect is called
coma and is because the focus of off-axis light
depends on the path it takes through the lens (or
where it falls on the mirror).
Consider the lens as a series of annuli. Each
annulus produces an annular image but these lie
at different foci.
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1B11 Coma (cont.)
The coma may point towards the axis (positive
coma) or away from it (negative coma). Spherical
mirrors do not suffer from coma because the
mirror always presents the same geometry to the
point source, irrespective of off-axis
angle. Parabolic mirrors, which do not have
spherical aberration, do suffer from coma, so are
only effective in a narrow field around the
optical axis. In Ritchy-Chretien (modified
Cassegrain) telescopes, spherical aberration and
coma are both removed, by using hyperbolic
primary and secondary mirrors.
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1B11 Astigmatism
If the mirror or lens is stressed or poorly
machined, the focal length along one axis of the
mirror may be different to focal length along
another, resulting in a spread of the image in
the focal plane.
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1B11 Reflectors vs refractors
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1B11 Telescope mounts

• Telescopes must be free to move about two
  mutually perpendicular axes in order to cover the
  whole sky.
• There are two main types
• Equatorial one axis is aligned with the
  celestial poles so you need only track in one
  axis (ie in HA)
• Altazimuth telescope moves in (local) altitude
  and azimuth, so is simpler to design but has to
  be tracked in two axes

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1B11 Equatorial mounts
equator
HA
NCP
declination
f latitude
f
SCP
Only need to track in HA and there is no field
rotation (direction of North in the focal plane
is fixed). But it is complex and expensive to
build especially for large telescopes.
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1B11 Coude focus
Polar axis
Dec axis
The Coude focus is accessed by taking the beam
out through the Dec axis. Useful for very large
and massive instruments, eg spectrographs.
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1B11 Altazimuth mounts
Horse-shoe mount
Altitude axis
local vertical
The design is simple, but it must track in both
axes and the field rotates. The largest (8-10m)
telescopes have altaz mounts with Cassegrain and
Nasmyth foci
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1B11 Instruments and detectors
Instruments and detectors analyze and record the
light focussed by a telescope.
instrument
astronomer
detector
telescope

• Typical instruments
• Camera
• Spectrograph
• Polarimeter
• Photometer

• Detectors
• Charge-coupled devices (CCDs)
• Photographic plates

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1B11 Spectrographs
Narrow-band filters can be used to examine a
small wavelength range of a source, but to look
over a wide wavelength range in very fine detail,
a spectrograph is used. Light at the focal plane
is dispersed and focussed onto a detector. A
narrow slit is used to select the region of the
image for analysis. The light is collimated using
a mirror or lens, and directed towards a
dispersing element (diffraction grating or
prism). The dispersing element spreads out the
light into a spectrum, which is then focussed
onto a detector (usually a CCD) by a lens.
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1B11 Spectrograph layout
slit in telescope focal plane
collimator mirror
beam from telescope
CCD
diffraction grating
imaging lens
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1B11 Charge-coupled device (CCD)
CCDs are solid state detectors with typically1k x
1k light sensitive elements (pixels) each of
which is usually about 10-20mm square. The pixels
are electrically insulated from each other and a
charge accumulates in each one. The amount of
charge is proportional to the intensity of light
falling on it. The charge distribution is the
same as the light distribution.
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1B11 Structure of a CCD
electrodes
photon
pixel
f3
f2
f1
0.1mm insulator (SiO2)
p-type semi-conductor (silicon crystal)
electron
hole
A photon strikes the silicon crystal and an
electron, which normally settles in the valence
band, is excited into the conduction band. Here
the electrons are free to migrate, leaving behind
a hole. The electrodes are maintained at pds
of about 10V and attract the electrons, which
accumulate under the electrodes until the CCD is
read out.
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1B11 CCD readout
While collecting data, only f2 is held high, so
electrons accumulate there. To read out the chip,
the voltages are cycled through the electrodes so
that the electrons shift along (or down) until
they reach the edge.
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1B11 CCD quantum efficiency
The quantum efficiency of a CCD is defined to be
the ratio of recorded (ie detected) photons to
the number of incident photons.
Q.E. is a function of wavelength but may be as
high as 80-90 over much of the visible spectrum.
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1B11 UV and IR Telescopes
Telescopes in the ultraviolet and infra-red are
similar in concept to optical telescopes, but are
orbiting space observatories launched beyond
the Earths atmosphere which is opaque at these
wavelengths. Conventional CCDs are not
sensitive, particularly at wavelengths longer
than 1mm, so different types of crystal are used,
eg indium antimonide and gallium-doped
germanium. Hubble Space Telescope Imaging
Spectrograph Near-IR imaging on the UK Infra-Red
Telescope (UKIRT) The next generation of space IR
observing with the JWST
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1B11 Radio astronomy
D
Telescope resolution
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1B11 Interferometry
Radio telescopes get around this problem and
produce the most finely detailed images of the
sky at any wavelength using the technique of
interferometry.
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1B11 Interferometry
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1B11 Basic interferometry
The effective baseline, L a cos
ZD (where a is the distance between the
telescopes, and ZD is the zenith distance of the
source.)
a
L
signals combined
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1B11 A brief history of interferometry

• The success of radio interferometry was first
  demonstrated in the 1940s.
• It was based on experiments into optical
  interferometry first developed by Michelson in
  1890.
• Australian and British astronomers further
  developed the technique in the 1950s and 1960s.
  Martin Ryle and Antony Hewish obtained the 1974
  Nobel Prize for Physics for their work on Earth
  aperture synthesis.
• Radio telescopes around the world join together
  to form enormous Earth-size telescopes this is
  Very Long Baseline Interferometry.
• There are plans to extend the baseline further
  into space!

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1B11 More information on radio astronomy
The National Radio Astronomy Observatory (NRAO)
The NRAO guide to radio astronomy The Jodrell
Bank Observatory in Manchester Probably the
biggest astronomical telescope in the world. The
Very Long Baseline Array. On the Development of
Radio Interferometry (by Bob Tubbs, Cambridge).
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1B11 X-ray astronomy
X-rays are absorbed by the Earths atmosphere, so
we need to go into space to make observations.
Originally, sounding rockets were used to make
the first X-ray observations, but these were
limited in their accuracy and could only make a
relatively short programme of observations,
before it fell back into the atmosphere. Today,
we have a powerful array of X-ray space
observatories, and X-ray astronomy is a very
fast-moving science. NASAs Chandra X-ray
observatory ESAs XMM-Newton The Rossi X-ray
Timing Explorer
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1B11 X-ray imaging
If an X-ray hits a mirror, it will pass straight
through unless it hits the mirror at a very high
angle of incidence.
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1B11 X-ray mirrors
X-ray mirrors are conical, almost cylindrical.
The design currently used is the Wolter mirror,
whose profile is parabolic in the wider section
and hyperbolic in the narrower section.
paraboloid
hyperboloid
This diagram is exaggerated for clarity in
XMM-Newton the mirrors are typically 0.3-0.7m in
diameter and the focal point lies 7m away.
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1B11 Nesting mirrors
Because they are almost cylindrical, X-ray
mirrors present a very small collecting area to
incoming radiation. To improve the
light-collecting area, many mirrors are nested
together, eg each telescope module in XMM-Newton
contains 58 mirrors, stacked like Russian dolls.
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1B11 Focal plane instruments
Chandra and XMM-Newton are imaging X-ray
telescopes and carry an array of instrumentation,
including special X-ray sensitive CCDs and
grating spectrometers. On XMM-Newton, there are 7
600x600 pixel MOS CCDs in each MOS array,
arranged to match the focal plane of the
telescope. There is also a pn CCD a fixed
format X-ray CCD chip which uses a different kind
of technology.
Chandra imaging resolution is about 1arcsec
rivalling that of the best ground-based optical
telescopes. XMM-Newton is sensitive enough to
reach to 1Myr after the Big Bang.