Download PHY2083 ASTRONOMY

PHY2083
ASTRONOMY
Non-optical astronomy
Signal-to-noise
�
σ(γ) = Nγ
�
Nγ
S
=�
= Nγ
N
Nγ
S
Nγ (obj)
= �
2
N
Nγ (obj) + Nγ (sky) + Ndark + Nread
Angular Resolution
θmin
θmin
1.22λ
=
radians
D
1.22 × 206, 265λ
=
arcsec
D
The E/M spectrum
•
Black-body spectrum
The E/M spectrum
Radio Telescopes
Radio waves are produced by a variety of
physical mechanisms e.g. the interaction of
charged particles with magnetic fields.
Provides a window into processes not
accessible at other wavelengths
Radio waves interact differently with matter c.f.
visible light => detector + telescope design
different
76-m Lovell Telescope: Jodrell Bank
Work out the resolution of this telescope at a wavelength
of 10cm
The parabolic
dish reflects
the radio
energy of the
source to an
antenna. The
signal is then
amplified and
processed to
make a radio
map.
305-m Arecibo Telescope:Puerto Rico
Calculate the
diameter a
single-dish
telescope would
need to have to
obtain a
resolution of 1’’
at 21cm
• An advantage of working at such long λ is
that small deviations from an ideal
parabolic shape are not crucial.
• Can tolerate ~ λ/20 of perfect shape e.g.
variations of 1cm ok when observing at
21cm
Comparison between optical and radio telescopes
• Remember: angular resolution of a telescope
∝ λ /
D
• Optical telescopes: ~25 milli-arcseconds (D = 5m, λ = 500nm
• Radio telescopes: 1 arcmin (D = 100m λ = 2.8cm)
• Distant radio sources have fine-scale structure < 1
milli-arcsec. 1 milli-arcsec @ λ = 2.8cm => D ~ 6000
km!
Solution: Interferometry (c.f.Young’s doubleslit experiment)
• Instead of building one huge dish: make
several smaller telescopes and place them far
away from each other
• Combine the signal from these telescopes
• In effect have 1 large telescope!
Radio Interferometers
P � = L sin θ
L = nλ
P
θ
P2
P1
Radio Interferometers
Very Large Array,
New Mexico
Jupiter at radio wavelengths
λ = 13 cm
λ = 22 cm
8-m Gemini Telescope,
Hawaii
SOFIA
(Stratospheric Observatory For Infrared Astronomy)
Spitzer
Space
Telescope
JWST
(James Webb Space Telescope)
Jupiter at mid-IR
wavelengths
λ = 5 µm
Jupiter at near-IR
wavelengths
λ = 1 − 2 µm
HST
(Hubble Space Telescope)
Jupiter at optical
wavelengths
λ = 400 − 600 nm
Jupiter at UV
wavelengths
λ = 150 nm
High energy astrophysics
•Below ~ 300nm, the Earth’s atmosphere blocks all
radiation
=> ultraviolet (uv) and xray astronomy not possible
from the ground
• Xray and gamma-ray photons cannot be easily
reflected from any kind of surface (pass straight
through / absorbed)
Grazing incidence optics for high energy photons
At grazing angles, some reflection is possible. Series of
nested cylindrical mirrors to bring the photons (keV) to
a focus
XMM-Newton
Gamma-rays
• For gamma-rays, grazing incidence optics doesn’t
work.
=> point in a direction, and count the number of
gamma-ray photons received
Note: traditional photographic plates / CCDs do
not work for high-energy photons. Use other
electronic detectors. Spatial resolution very low ~
30” or worse.
Gamma-ray
observatories
INTEGRAL
(ESA)
FERMI
(NASA)
MAGIC: Cosmic Ray Telescope
These are high-energy protons, electrons, and atomic
nuclei that reach the Earth. Highest energies ~ 1020
eV; source unknown
Ice-Cube
neutrino telescope
Ice-cube Neutrino Telescope
Gravitational Wave Observatories
LISA (Laser
Interferometer
Space Antenna)
2020?