Transcript Telescopes and instruments

1B11
Foundations of Astronomy
Telescopes and instruments
Liz Puchnarewicz
[email protected]
www.ucl.ac.uk/webct
www.mssl.ucl.ac.uk/
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, gray 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).
1B11 Refracting (dioptric)
focal plane
focal length, f
lens (diameter, D)
Collecting area = pD2/4
Focal ratio = f/D
optical axis
image
1B11 Refracting (dioptric)
focal plane
optical axis
q
q
focal length, f
Image scale (plate scale), d=fq
For q in degrees:
(for q in radians)
q (deg)
180

deg/mm
d(mm) pf
1B11 f-ratios
focal length f

F-ratio =
diameter
D
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).
1B11 The eyepiece
focal plane
optical axis
b
a
fo
objective
lens
Angular magnification =
b fo

a fe
fe
eyepiece
lens
NB – the image
is inverted!
1B11 Reflecting (catoptric) telescopes
These use mirrors to reflect and focus light (Newton, 1668).
prime focus
Primary mirror
(paraboloid + hyperboloid)
1B11 Newtonian
flat mirror
Newtonian focus
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.
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.
1B11 Diffraction patterns
I
n=2
n=1
n=1
n=2
Airy disk
distance
The constructive and destructive interference patterns
are described according to Huygen’s Principle.
a
a
For a telescope with diameter
D and light with wavelength l,
minima occur at positions
given by:
ml
sin a n 
n
D
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,
n
m
we may write:
mnl
1
1.22
an 
D
The light contained within
the radius (84% of the total)
defined by the first minimum
is called the Airy disk, where
2
3
1.22 l
a
D
2.23
3.24
and this is
used to
define the
resolution
1B11 Rayleigh Criterion
I
Airy disk
1.22l/D distance
I
1.22l/D
distance
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.
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 l=5500A)
(=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 l=5500A)
There are other factors which limit the
resolution obtainable in astronomical
observations. A major factor is the
turbulence in the Earth’s atmosphere
which has the effect of blurring stellar
images. This what causes stars to
“twinkle” and is known as atmospheric
seeing. Measured in arcseconds.
Good site: 0.3 - 1” (Hawaii, Chile)
Generally: 1 – 2” (diff lim for D=20cm)
qseeing propto ~ l-0.2
adaptive optics
atmosphere
1B11 Atmospheric seeing
1B11 Chromatic Aberration
Unique to refractors – because the focal length of the lens is
wavelength-dependent.
Effect considerably reduced by using a compound lens
Crown glass,
m low, convex
Flint glass,
m high, plano-concave
Works for
2 colours
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.
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:
1. Schmidt telescopes – where a thin corrector plate is
placed at the centre of curvature of the mirror
2. Maksutov cameras – a spherical meniscus lens is
inserted in the light’s path.
Hubble Space Telescope
1B11 Schmidt telescope
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,
Spherical mirror
UK Schmidt survey.
Corrector lens
Focal
surface
1B11 Maksutov cameras
The meniscus lens
has a negative longfocus and spherical
surfaces. It produces
its own spherical
aberration which
cancels out the
mirror’s. 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
1B11 Coma
Point sources which do not lie along the axis of a lens or nonspherical 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.
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.
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.
1B11 Reflectors vs refractors
REFLECTOR
REFRACTOR
Fewer optical surfaces
More optical surfaces
No chromatic aberration
Some chromatic aberration
Small/medium telescope
efficiencies 50-60%. Large
reflectors more efficient.
Aluminium mirrors effective
over all visible range.
Small/medium telescope
efficiencies 50-60%. Losses
high in large objectives
Absorption l-dependent. Most
UV is absorbed.
Supported across back and
around the rim.
Supported only around the
rim – large refractors suffer
from strain.
Temp changes less critical.
Subject to temp changes.
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:
1. Equatorial – one axis is aligned with the celestial
poles so you need only track in one axis (ie in HA)
2. Altazimuth – telescope moves in (local) altitude and
azimuth, so is simpler to design but has to be
tracked in two axes
1B11 Equatorial mounts
NCP
equator
HA
declination
f
f = latitude
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.
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.
Cassegrain
focus
Coude focus
1B11 Altazimuth mounts
Horseshoe
mount
Altitude
axis
Nasmyth
focus
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
1B11 Instruments and detectors
Instruments and detectors analyze and record the light
focussed by a telescope.
instrument
telescope
Typical instruments:
• Camera
• Spectrograph
• Polarimeter
• Photometer
astronomer
detector
Detectors:
• Charge-coupled devices
(CCDs)
• Photographic plates
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.
1B11 Spectrograph layout
slit in telescope
focal plane
beam
from
telescope
collimator
mirror
CCD
q
diffraction
grating
For normal incidence:
nl
sin q 
d
imaging lens
n = order number
l = wavelength
d = groove spacing
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.
1B11 Structure of a CCD
electrodes
photon
pixel
f3
f2
f1
0.1mm insulator (SiO2)
electron
p-type semi-conductor
hole
(silicon crystal)
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
pd’s of about 10V and attract the electrons, which accumulate
under the electrodes until the CCD is read out.
1B11 CCD readout
f3
f2
f1
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.
current
induced
charge
accumulated
incident
photons
image
reconstruction
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.
photons detected
Q.E. 
incident photons
Q.E. is a function of wavelength but may be as high as
80-90% over much of the visible spectrum.
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 Earth’s 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
1B11 Radio astronomy
D
Cassegrain
detector
prime
focus
Recall that the
telescope resolution,
a, is given by:
1.22 l
a
D
In the radio, l is large, so D
must be large for reasonable
resolution. eg the Jodrell
Bank 76m dish has a
resolution of 0.19 degrees.
Telescope resolution
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.
1B11 Interferometry
With a minimum of
two telescopes,
can map the
pattern, but results
are 1-D and
limited.
The Rayleigh
Criterion (ie the
resolution for this
technique) is:
Using multiple
telescopes and/or
the Earth’s
rotation, can map
out the other
dimension as well
– this is called
aperture
synthesis.
q min  l 2L
where L is the spacing
of the telescopes and
qmin is the resolution.
1B11 Basic interferometry
q
a
L
signals
combined
q
The effective
baseline,
L = a cos ZD
(where a is the
distance between
the telescopes, and
ZD is the zenith
distance of the
source.)
1B11 A brief history of interferometry
• The success of radio interferometry was first demonstrated
in the 1940’s.
• It was based on experiments into optical interferometry first
developed by Michelson in 1890.
• Australian and British astronomers further developed the
technique in the 1950’s and 1960’s. 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!
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).
1B11 X-ray astronomy
X-rays are absorbed by the Earth’s 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.
NASA’s Chandra X-ray observatory
ESA’s XMM-Newton
The Rossi X-ray Timing Explorer
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.
If an X-ray hits a mirror at a very high angle of incidence, it
will be reflected.So if you construct a conical mirror which is
almost cylindrical, it will bring the X-rays to a focus.
These are called “grazing incidence” mirrors.
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.
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.
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.