Astronomical Observational Techniques
and Instrumentation
RIT Course Number 1060-771
Professor Don Figer
Other wavebands: gamma-ray, FIR, submm, mm
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Gamma Ray Imaging
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Introduction
• Gamma rays are photons with energies above 105 eV
• Gamma rays can be detected directly in space or as secondary
showers on the ground.
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Gamma Ray Astronomy
• There are ~100 known gamma ray sources.
• Crab is the standard candle (2.1 × 10−11 photons cm−2 s−1
above 1 TeV).
• Today’s technology gives sensitivity limit of ~1% Crabs.
• Typical objects
– supernova remnants
– pulsar wind nebulae
– broad-lined active galactic nuclei (AGN with jet aligned along the line
of sight)
– pulsars
– binaries
– a few starburst galaxies
– a few radio galaxies
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Typical Sensitivity (VERITAS)
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Gamma Ray Results: Pulsar Wind Nebula
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Gamma Ray Detectors
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Gamma Ray Detectors
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Scintillators
Solid State Detectors
Compton Scattering Detectors
Pair Production Detectors
Cherenkov Radiation Detectors
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Scintillators
• A scintillator is a material that emits low-energy (usually in
the visible range) photons when struck by a high-energy
charged particle.
• The scintillator emits photons that are then detected by
photomultiplier tubes (PMTs).
• The most common scintillators are sodium iodide (NaI) or
cesium iodide (CsI).
• Scintillators have been used as gamma-ray detectors aboard
many space-based missions to observe sources of cosmic
gamma-radiation. These missions include: the Compton
Gamma-Ray Observatory (CGRO), the first High Energy
Astrophysical Observatory (HEAO-1), and the Rossi X-Ray
Timing Explorer (RXTE).
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Solid State Semiconductor Detectors
• Solid state detectors use semiconductors, e.g. germanium or
cadmium zinc telluride (CdZnTe).
• The gamma ray deposits energy in the semiconductor, thus
promoting many electron/hole pairs into the conduction band.
• These materials often use coded aperture masks or Compton
scatter type configurations.
• Most of the advanced materials being considered for future
missions have the problem that the arrays are small.
• High energy resolutions, E/dE of 500, can be achieved with
germanium detectors. Other materials can routinely match or
improve on the resolution available with scintillators.
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Compton Scattering
•
Compton scattering occurs when a photon "hits" an electron with some of the photon energy being
transferred to the charged particle. The Compton Scatter Telescope uses this interaction as the basis of its
detection scheme.
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Compton scatter telescopes are typically two-level instruments. In the top level, the cosmic gamma-ray
Compton scatters off an electron in a scintillator. The scattered photon then travels down into a second level
of scintillator material which completely absorbs the scattered photon. Phototubes viewing the two levels
can approximately determine the interaction points at the two layers and the amount of energy deposited in
each layer.
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Pair Production
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A high energy photon (like a gamma ray photon) can create an electron/positron
pair (especially in the presence of a “converting” nucleus).
The standard instrument design is to have a layered telescope, with converter layers
interleaved with tracking material.
The converter is typically a high Z material (ex. heavy metal such as lead) which
provides the target for creating the initial pair while the tracking material detects the
pair.
Once the electron/positron pair has been created in one of the converter layers, they
traverse the chamber, ionizing the gas. Triggering the detector causes the wires to
be electrified, attracting the free electrons which provides the detected signal. The
trail of sparks essentially provides a three dimensional picture of the the e+/epaths.
By reconstructing the tracks of the charged pair as it passes through the vertical
series of trackers, the gamma-ray direction and therefore its origin on the sky are
calculated.
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Gamma Ray Detection Airshowers
• It is possible to detect gamma rays by the presence of their byproducts produced in Earth’s atmosphere.
• Ground-based gamma ray telescopes actually detect
Cherenkov radiation emitted by high energy particles produced
through the interaction of the gamma rays and atmospheric
particles.
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Cherenkov Technique
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A hadronic cosmic ray or high energy γ-ray incident on the
Earth’s atmosphere triggers a particle cascade, or air shower.
Relativistic charged particles in the cascade produce
Cherenkov radiation, which is emitted along the shower
direction.
Cherenkov light is produced throughout the cascade
development, with the maximum emission occurring when
the number of particles in the cascade is largest, at an altitude
of ~10 km for primary energies of 100 GeV–1 TeV.
Following absorption and scattering in the atmosphere, the
Cherenkov light at ground level peaks at a wavelength, λ ≈
300–350 nm.
The photon density is typically ~100 photons/m2 at 1 TeV,
arriving in a brief flash of a few nanoseconds duration.
This Cherenkov pulse can be detected from any point within
the light pool radius by using large reflecting surfaces to
focus the Cherenkov light on to fast photon detectors
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Cherenkov Technique
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VHE gamma-ray sources status ICRC 2007
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Current generation Cherenkov telescopes
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Gamma Ray Facilities
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Recent X-/Gamma Ray Facilities
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Recent X-/Gamma Ray Sensitivities
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Fermi Gamma-ray Large Area Space Telescope
(GLAST)
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GLAST LAT
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GLAST Science Goals
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Recent X-/Gamma Ray Sensitivities
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Far-infrared/mm Imaging
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Introduction
• Far-infrared/mm covers 30 um to 1 mm wavelengths
• Most of this waveband is only accessible from
space/stratosphere, except for partial transmission over small
portions of the range at high altitude/dry observing sites (e.g.
south pole)
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FIR/mm Detectors
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FIR/mm Detectors
• FIR/mm detectors:
– bolometers
– transition edge sensors
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Bolometers
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Bolometers convert light into thermal energy (heat).
They have been used for ~40 years for FIR/mm astronomy.
Most previous applications used single element detectors, but imaging arrays are now
being implemented.
A signal is detected as a change in voltage due to the change in resistance in the
thermometer for a constant bias current.
Performance is characterized in terms of the Noise Equivalent Power (NEP).
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Transition Edge Sensors
• TES materials have extremely high sensitivity, i.e. change in resistance
versus temperature, at the transition between normal and superconducting
modes.
This is a picture of an 8 x 8 array of
TES microcalorimeters built at the
NASA Goddard Space Flight Center.
Each Pixel is 250 μm on a side.
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FIR/mm Facilities
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FIR/mm Facilities
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Caltech Submillimeter Observatory (CSO)/Bolocam, SHARC
James Clerk Maxwell Telescope (JCMT)/SCUBA
SOFIA
Herschel
Planck
ALMA
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Caltech Submillimeter Observatory (CSO)
• CSO has a 10.4m primary dish.
• SHARCII has 350, 450, 850um
passbands, 12x32, 2.6x1amin field.
• Dry nights lead to better sensitivity
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James Clerk Maxwell Telescope (JCMT)
• JCMT has a 15m primary dish.
• SCUBA is the primary instrument, 450/850um with dichroic,
2.3amin field diameter
Frequency Wavelength Beamwidth Atmos.
(GHz)
(um)
(arcsec)
trans.
150
2000
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0.97
230
1300
21
0.96
345
870
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0.88
492
610
12
0.43
690
435
8
0.44
870
345
6
0.53
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Stratospheric Observatory for Infrared Astronomy
(SOFIA)
• SOFIA has 2.5m mirror.
• It has a variety of instruments (see below) covering optical to FIR.
• HAWK is being upgraded with new detectors and polarimeters.
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Herschel
• The Herschel telescope is a Cassegrain
design with a 3.5m primary. The three
scientific instruments are:
– HIFI (Heterodyne Instrument for the
Far Infrared), a very high resolution
heterodyne spectrometer
– PACS (Photodetector Array Camera
and Spectrometer) - an imaging
photometer and medium resolution
grating spectrometer
– SPIRE (Spectral and Photometric
Imaging Receiver) - an imaging
photometer and an imaging Fourier
transform spectrometer
• Covers 60-670 um.
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Herschel: RCW 120 and Triggered Star Formation
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Herschel Compared to Spitzer
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Planck
• The Planck telescope has an offaxis 1.5m primary. The
scientific instruments are:
– LFI (Low Frequency Instrument),
a High Electron Mobility
Transistor based radio receiver.
– HFI (High Frequency Instrument),
a bolometer based imaging array
• Covers 300um to 1.2cm.
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ALMA
• The Atacama Large Millimeter/submillimeter Array
• Covers 300um to a few cm
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ALMA
• Science Goals
– Detect spectral line emission from CO or CII in a normal galaxy like
the Milky Way at a redshift of z = 3, in less than 24 hours of
observation.
– Image the gas kinematics in protostars and in protoplanetary disks
around young Sun-like stars at a distance of the nearest star-forming
clouds. Provide precise images at an angular resolution of 0.1 arcsec.
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