Transcript ISA_lecture02

The tools of astrophysics
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Virtually all information about the external
Universe is received in the form of
electromagnetic radiation.
The EM spectrum covers a range >1020 in
wavelength.
The Planck-Einstein relation
E  hf 
hc
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implies higher energy = shorter wavelength
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The EM spectrum
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Radio
Millimetre
Microwave
Infrared*
Visible
Ultraviolet*
X-rays*
-rays*
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*Note: The atmosphere is opaque (or partially so) for radiation
in these bands. They can only be observed from high
altitude observatories, balloons, rockets or satellites.
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Different ‘astronomies’
Astronomy/Astrophysics today gathers its information
from across the EM spectrum, but we still sometimes
talk about different ‘astronomies’ (optical astronomy,
radio astronomy, X-ray astronomy) because
 Atmospheric transmission varies
 Telescopes and detector vary
 Different parts of the spectrum reveal different objects
and different kinds of information…..
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HST visible
For example …
Combined - HST visible (blue-cyan),
Spitzer 3.6-4.5 m (green) and 8.0 m ( red)
Spitzer IR
3.6 (blue), 4.5 (green), 5.8 (orange), and 8.0 (red) m
M104 Sombrero Galaxy.
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© NASA/HST and Spitzer
© NASA ADF - http://adc.gsfc.nasa.gov/mw/mmw_sci.html
Milky Way at many wavelengths
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Telescopes
Telescopes at many
wavelengths are
basically similar.
Important factors are:
 Configuration lens/mirror,
paraboloids, prime
focus, cassegrain,
grazing incidence…
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Telescopes - 2
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Surface materials - glass, metal sheet, chicken wire,..
Surface accuracy - ‘diffraction limited’ is < /8 (p-p
in the surface) or  /4 in the wavefront
Magnification - not very important
Collecting area - light gathering power (sensitivity)
 D2 with possible ‘secondary obstruction’
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W.M. Keck Observatory - Hawai’i
© NASA/JPL-Caltech
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Keck primary mirror
© NASA/JPL-Caltech
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Parkes radio telescope
© CSIRO/ATNF ?
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Sensitivity
Factors affecting sensitivity:
 Atmospheric transmission
 Collecting area
 System throughput
 Detector quantum efficiency
 Observing time
 Background - e.g. scattered light. As well as natural
sources, man-made pollution is a major problem for
astronomy. At optical wavelengths for example….
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Light pollution
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
© Pearson Education 2007
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© unknown
Resolution
The final important factor is resolution
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Theoretical resolution - Rayleigh’s criterion: min
1.22 
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D
In practice, this is limited (for optical, IR) by ‘seeing’ - practical
limit is 0.3 ~ 1.0 arcsec.
At radio wavelengths, telescope sizeis the limiting factor.
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Resolution - single telescopes
Band
UV
Optical
Near IR
mm
cm
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Typical
min.surface
accuracy
for D=10 m actual telescopes
100 nm
500 nm
2 m
1 mm
21 cm
13 nm
63 nm
250 nm
0.13 mm
26 mm
0.0025” 0.010” (HST 2.4 m)
0.013”
(Keck 10 m)
0.050”
(Keck 10 m)
25”
(JCMT 10 m)
1.5°
9’ (Greenbank 100m)
Resolution
Now, concentrating on the optical for a moment…….
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Adaptive optics
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Active Optics:
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slow image correction (f < 1 Hz), to correct mirror and
structural deflections
Adaptive Optics:
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fast image correction (f ≥ 1 Hz), primarily to correct random
phase fluctuations of wavefronts caused by atmospheric
turbulence - resulting image motion and blurring
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Where does Seeing arise?
Turbulence in the atmosphere
leads to refractive index variations.
Contributions are concentrated into
layers at different altitudes.
© John O’Byrne
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Scidar measurements at SSO
10 minutes
of data
refractive index
structure
constant (Cn2 )
v. altitude
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QuickTime™ and a
GIF decompressor
are needed to see this picture.
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© John O’Byrne
Seeing parameters
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Fried parameter ro(,z) = 0.185  6/5cos3/5 z(∫ Cn2dh)-3/5
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Seeing disk FWHM without AO ≈  /ro for large telescopes
So at ~500nm, ro ≈ 10 cm for 1 arcsec FWHM seeing
At 2.5 m, this corresponds to ro ≈ 70 cm and
0.7 arcsec seeing
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Essentials of an
AO system
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Wavefront sensor
Computer
Phase modulator
© John O’Byrne
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AO example
© University of Hawaii ?
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Keck - Io
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Upper Left:
Keck AO; K-band,
2.2micron.
Upper Right:
Galileo; visible light.
Lower Left:
Keck AO; L-band,
3.5micron.
Lower Right:
Keck without adaptive
optics.
© NASA/JPL-Caltech
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Interferometry
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If EM waves from two or more apertures are coherently
combined, the resolution is set by the “baseline” B between the
apertures.
Interferometry first proposed by Fizeau but first successful
astronomical interferometer was due to Michelson (1891
Galilean satellites).
In 1921 Michelson & Pease measured angular diameter of
a Orionis (Betelgeuse).
1950s: Discovered by radio astronomers!
Now widely used in radio, difficult at optical/IR.
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© University of Sydney
Basic principle of an optical interferometer - the
Sydney University Stellar Interferometer (SUSI)
at Narrabri is a 2-dimensional example
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Resolution
- interferometers
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Baseline
Typical
max.
SUSI 400 nm
ATCA 6 cm
VLBI 6 cm
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640 m
~20 km
~5000 km
Resolution
0.0002”
2.5”
0.003”
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© CSIRO/ATNF