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Lecture 3. The various contaminants to the SZ effect Telescopes and how they deal with the various issues: Single dishes (radiometers and bolometers) Examples (OCRA) Interferometers Examples (AMiBA) Future prospects (Planck etc). SZ Practicalities.

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Lecture 3

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lecture 3
Lecture 3
  • The various contaminants to the SZ effect
  • Telescopes and how they deal with the various issues:
    • Single dishes (radiometers and bolometers)
    • Examples (OCRA)
    • Interferometers
    • Examples (AMiBA)
  • Future prospects (Planck etc)
sz practicalities
SZ Practicalities
  • SZ is a tiny signal - requires sophisticated observing techniques
  • Various sources of contamination and confusion, which observing techniques deal with in different ways
    • Radio sources (galaxies, planets)
    • Atmospheric emission, ground emission
    • Primordial CMB fluctuations
radio sources
Radio Sources
  • If a radio source is present in the field of a galaxy cluster, it will ‘fill in’ the SZ decrement
  • This could be true for sources in front of / behind the cluster, or indeed member galaxies
  • Problem greater at low frequency: most sources are ‘steep spectrum’
  • Can choose to observe clusters with no sources - introduce bias
  • Better to ‘subtract’ effects
  • No high-freq. radio surveys - further complication
atmosphere ground
Atmosphere, Ground
  • Atmosphere is ‘warm’ - radiates.
  • Time variable emission
  • Ground also a source of thermal emission
  • Varies with pointing angle or telescope
  • Can minimise this using a ‘ground shield’
    • Various ways exist of dealing with these contaminant signals
primordial cmb
Primordial CMB
  • Primordial anisotropies look remarkably similar to the SZ effect on large angular scales (tens of arcminutes)
  • Seem unsurprising that telescopes such as the VSA and CBI (built to observe the primordial CMB) suffer drastically from this type of contamination....
  • .....We were still surprised!
sz telescopes
SZ Telescopes
  • 3 main types of instrument:
    • Single dishes. May have bolometric or radiometric receivers. Receiver arrays are becoming more common.
    • Interferometers - synthesise a large telescope using an array of small dishes
  • Each has its own advantages and disadvantages when it comes to dealing with sources of contamination and confusion
simplest case
Simplest Case
  • Measure signal received by single beam of solid angle 
  • Provides one ‘pixel’ of information
  • Records signal from the sky plus parasitic signals from the ground and atmosphere


  • Signals from ground and atmosphere are time variable
improvement beam switching
Improvement: Beam Switching
  • Receiver with 2 beams (horn feeds), A and B
  • Beam A positioned coincident with target (cluster) so measures:

PA = Cluster + Ground + Atmosphere

  • Beam B observes blank sky:

PB = Ground + Atmosphere

  • Differenced signal measures cluster signal only.

PA - PB = Cluster

  • Only spend half observing time on target, but uniform signals (ground, atmosphere, CMB) are subtracted
  • Configuration is asymmetrical. No problem for the atmosphere, put potentially fails to remove ground emission
  • Also susceptible to differences in receiver gain and beamshape
  • Need revised strategy....
improvement position switching
Improvement:Position Switching
  • Start with A on target, B on blank sky. Then swap
  • Switch in azimuth - comparable volumes of atmosphere
  • Differenced signal: twice cluster signal
  • Better at removing ground emission, but beware radio sources!
  • Array receivers
    • Multiple detectors offer increased sensitivity and imaging capabilities
    • Can apply similar, or more complex, switching
    • Observe ‘background’ fields either side of field containing cluster
    • Offers further subtraction possibilities
single dish pros cons
Single Dish Pros & Cons
  • Large dishes are very sensitive, particularly when fitted with array receivers
  • Array receivers required for imaging
  • Have to employ switching schemes to suppress systematics
  • Some limitations on cluster observability due to angular size vs switching strategy
  • Difficult to deal with radio sources - may have to avoid cluster centres
  • Radiometers
    • Incident radiation produces a voltage
    • Difficult to build at high freq., limited bandwidth
  • Bolometers
    • Thermal - measure temperature increase
    • Sensitive to all wavelengths - often used at high frequency, and for multi-frequency instruments
    • Technically challenging - require excellent cooling
example ocra
Example: OCRA
  • Torun 32-m telescope, Poland. 30GHz radiometric detectors
  • Currently, 2-beam receiver - prototype for focal plane array (OCRA-p)
  • Observed 4 well-known clusters (Lancaster et al 2007) to verify strategy
  • Now observing ‘sensible’ sample of 18 clusters
example ocra1
Example: OCRA
  • Next phase, OCRA-F, under construction.
  • 16 beam (8xOCRA-p)
  • Has the ability to make SZ images, resolved for the first time
  • Will also be capable of small blind surveys
  • Set to be mounted on the telescope later this year
bolometers acbar
Bolometers: ACBAR
  • 16-element bolometer mounted on VIPER
  • Requires excellent observing conditions - located at the south pole
  • Observing frequencies: 150, 220 and 275GHz.
  • Observed the decrement-null-increment (lecture 1)
  • Blind survey complete, analysis underway
others worth noting
Others worth noting
  • Radiometers:
    • Nobeyama 45-m, Japan. 46 GHz
    • OVRO 40-m / 5-m, USA. 30 GHz (Mason et al)
  • Bolometers:
    • CSO 10.4-m (SuZIE), USA. 150, 220, 280, 350GHz
    • JCMT 15-m (SCUBA), USA. 350GHz

Ryle Telescope

  • Traditionally used to synthesise a large telescope with an array of small dishes, thus increasing resolution
  • Useful in this context for suppressing systematics - various advantages over single dish experiments
    • Can deal neatly with radio sources
    • Can filter out contaminant signals

●2 Dishes, separated

by a distance D

● Same resolution as

a large dish of

diameter D

● Resolution ~/D

● But only samples

this angular scale

Need many baselines for aperture synthesis

  • Measure the product (‘correlation’) of the voltages of the two antennas
  • A bit like reverse diffraction - the response for this interferometer will be cos2 fringes (remember Young’s slits?)
  • Actually measures the Fourier transform of the sky (again, like diffracction):
real interferometer
Real Interferometer
  • In practice, have many pairs of antennas, usually referred to as baselines.
  • n telescopes give n(n-1)/2 baselines.
  • Baseline of length D is sensitive to angular scale =/D(resolution)
  • Long baselines sensitive to small scales
  • Short baselines sensitive to large scales
  • Evaluate response for each baseline in order to find response of whole telescope
radio sources1
Radio Sources
  • Short baselines (large angular scales) sensitive to SZ (large angular scales) + radio sources (all angular scales)
  • Long baselines (small angular scales) sensitive to radio sources (all angular scales)
  • Measure source flux on long baselines and subtract from short baseline data
    • ‘Spatial Filtering’ - used to good effect by many experiments (Ryle, OVRO/BIMA)

High res...

Low res, after source subtraction...

Colourscale - 30GHz

Contours - 1.4GHz

Colourscale - Xrays

Contours - SZ

cmb anisotropies
CMB Anisotropies
  • The problem of contamination by primordial CMB anisotropies is most relevant for instruments working on large angular scales
  • e.g. the VSA, also CBI
  • Need multi-frequency experiments for spectral subtraction
  • However, less important on smaller angular scales (i.e. only really affects low-redshift samples)

- 30GHz, longest baseline 3m

- Dedicated source subtractor

- Suffers badly with CMB

- Some clusters ‘drowned out’


15 arcmin

Power in primordial anisotropies falls off with decreasing angular scale

interferometer pros cons
Interferometer Pros & Cons
  • Can subtract signals from radio sources via spatial filtering
  • Lack angular dynamic range - may be difficult to produce higher resolution images
  • Problem of CMB contamination persists unless multi-frequency measurements are available (but only a large problem for a few instruments)
example amiba1
Example - AMiBA
  • Currently 7 60cm antenna, 90GHz, Hawaii. Baselines chosen for optimum sensitivity to SZ effect. Small enough scales for the primordial CMB to be suppressed.
  • Radio sources are significantly less problematic at this frequency, although we currently have a large problem with ground emission....
  • Huge potential as a survey instrument - will generate extensive cluster catalogues with well understood selection functions. Expected to find tens of clusters per square degree
  • Will eventually also produce detailed images

Simulated maps



All clusters found are too distant to be detected by current X-ray or Optical telescopes

next generation
Next Generation
  • Survey Instruments
    • e.g. AMI, AMiBA, SZA, ACBAR, SPT, APEX
  • Improved imaging / surveying
    • e.g. OCRA, AzTEC
the next big thing planck
The next big thing.... Planck
  • CMB experiment - primordial anisotropies including polarisation
  • Will ‘solve cosmological paradigms’
  • Will also detect many thousands of SZ clusters - less deep than other studies but will survey the entire sky
  • Launch.....2007?.....2008?...........
summary 3
Summary 3
  • Measuring the SZ effect is difficult. Requires specialist techniques to eliminate various sources of contamination and confusion
  • Two basic types of instrument - single dishes and interferometers. Single dish detectors may be radiometers or bolometers
  • Plethora of survey instruments ‘under construction’. Will yield vast cluster catalogues and revolutionise the field.