Herschel: An Introduction

1
Herschel An Introduction

• Dave Clements
• Imperial College
• Matt Griffin
• Cardiff University

2
What Herschel Means to you

• Far-infrared and submm observatory in space
• Much more sensitive than ground or airborne
  observatories
• Photometry, imaging and spectroscopy modes
• Long duration mission - at least 3 years

3
Herschel Mk. 1
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The Herschel Satellite

• Telescope diameter 3.5 m
• Telescope WFE 6 ?m
• Telescope temp 70-90 K
• Pointing lt 3.7 (1.5)
• Operational lifetime gt 3 years
• Helium capacity 2200 ltr
• Height 9 m
• Launch mass 3300 kg
• Orbit L2
• Launch date ?Aug? 2007
• Launch vehicle Ariane 5
• 3 instruments HIFI, PACS, SPIRE
• Photometry and spectroscopybetween 50 and 670 ?m

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Launch with Planck to L2
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Advantages and Limitations of Herschel

• Access to FIR/submillimetre for unbiased surveys
• Cold low-emissivity telescope
• Big aperture - much bigger than Planck or SIRTF
• No atmospheric attenuation or noise
• Large amount of observing time 21 hrs/day for gt
  3 years
• Multi-wavelength observations with wide coverage
  
• Deep photometry 75 - 500 ?m
• Spectroscopy 57 - 670 ?m
• Well adapted for large area deep surveys

• Small aperture
• much smaller than ground-based telescopes
• Confusion limited
• Low positional accuracy angular resolution
• Instruments designed now
• Need for flexibility in capabilities and
  observing modes

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Primary Mirror
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Herschels Instruments

• PACS (57 - 210 ?m)
• Imaging photometer
• Grating spectrometer
• SPIRE (200 - 670 ?m)
• Imaging photometer
• Fourier transform spectrometer
• HIFI (157- 212 ?m and 240 - 625 ?m)
• Heterodyne spectrometer

SPIRE
PACS
HIFI
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PACS - Photoconductor Array Camera and
Spectrometer

• 3-band imaging photometer
• 75?m 110?m 170?m
• FWHM (arcsec.) 6 8 12 ?/D? 2.5 2.8
  2.1
• Simultaneous observation at 170?m and (75 or 110)
  ?m
• 3.5 x 1.8 arcmin. field of view
• 300-mK Si bolometer filled arrays
• Integral field line spectrometer
• Field of view (arcmin.) 0.8 x 0.8
• Wavelength range 57 - 210 ?m
• ?/d? 1000- 2000 (150 300 km s-1)
• 2-K photoconductor array

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PACS FPU Layout
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PACS Integral Field Unit

• Optical image slicer re-arranges 2-D field of
  view of 47 x 47 (5 x 5 pixels) along 1-D slit
  (1 x 25 pixels)
• Grating spectrograph disperses the light
• The dispersed slit image is projected onto a 2-D
  detector array
• 16 spectral channels are recorded simultaneously
  for each spatial element
• Data are recorded with the grating set at a range
  of angles to construct the spectrum over a wider
  range
• Spectral resolution 125 300 km s-1
• Instantaneous coverage 1500 km s-1

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PACS Observing Modes

• Dual-band photometry
• Full spatial sampling in each band
• Long-wave bolometer array 130 - 210 µm
• Short-wave bolometer array 60 - 85 or 85-130 µm
• Line spectroscopy
• Observation of individual lines
• Long-wave array 105 - 210 µm
• Short-wave 57 - 72 or 72-105 µm
• Range spectroscopy
• Observation of extended wavelength range with
  continuous scan (full resolution) or steps (SED
  sampling)
• Pointing modes
• Stare/raster/line scan
• With/without nodding

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PACS Performance
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HIFI - Heterodyne Instrument for the Far-Infrared

• Seven-channel heterodyne receiver
• Frequency coverage
• 480 - 1250 GHz (625 - 240 ?m)
• 1410 - 1910 GHz (212 - 157 ?m)
• Frequency resolution 140 kHz - 1 MHz
• Near-quantum noise limited sensitivity (goal lt
  3h?/k)
• Instantaneous IF bandwidth 4 GHz
• Calibration accuracy 10 (3 goal)

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HIFI Operating Modes Performance

• Total power
• Telescope moves between source and reference
  positions every 100 s
• Suitable for large maps
• Beam switching
• HIFI chopper switches beam on the sky at 1 Hz
• Suitable for point sources
• Frequency switching
• LO freq switched at 1 Hz
• Suitable for very narrow lines

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HIFI - CII at High Redshift
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SPIRE Instrument Summary

• 3-band imaging photometer
• 250, 350, 520 ?m (simultaneous)
• ?/d? 3
• 4 x 8 arcminute field of view
• Diffraction limited beams (18, 25, 36)
• Imaging FTS
• 200 - 670 ?m
• 2.6 arcminute field of view
• Ds 0.04 cm-1
• (?/d? 20 - 1000 at 250 ?m)
• Sensitivity limited by thermal emission from the
  telescope (80 K e 4)

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SPIRE detector arrays
Photometer
Spectrometer
PLW 43 detectors
PMW 88 detectors
PSW 139 detectors
SLW 19 detectors
SSW37 detectors
22 mm
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Observing with SPIRE

• Feedhorn arrays do not provide a filled focal
  plane
• Like SCUBA, jiggling is necessary for proper
  sampling
• For point sources, need a 7-point jiggle unless
  the position is certain
• For imaging, need jiggle-maps or scanning at
  magic angles

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Point Sources 7-Point Jiggle Map

• Chopping 126
• 7-point jiggle pattern
• Angular step ? 4 - 6
• (gt pointing or positional error)
• Total flux and position can be fitted
• Compared to single accurately pointed
  observation, S/N for same total integration time
  is only degraded by
• 20 at 250 ?m
• 13 at 350 ?m
• 6 at 500 ?m

Signal loss for blind pointing
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Field (jiggle) Mapping

• Telescope pointing fixed or in raster mode
• Chopping up to 4 amplitude in Y direction
• 64-point jiggle pattern for full spatial
  sampling
• Available fov 4 x 4
• Single fields or moderate raster maps (40x40)

2 arcmin maximum.
Y
Z
X
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Scan Mapping

• Telescope in line scanning mode
• Scan rate 20-30/sec.
• Map of large area is built up from overlapping
  parallel scans
• Most efficient mode for large-area surveys

Scan directions for instantaneous full sampling
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Photometer Sensitivities
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Spectrometer Modes

• High resolution mode for studying specific
  spectral lines ?/d? 1000
• Low resolution SED mode ?/d??????
• Can be jiggled to provide full spatial sampling,
  and can be rastered for maps

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Spectrometer Sensitivity
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Observing speed vs. Telescope Temperature
Qdes Qexp/2 Qdes Qexp Qdes 2Qexp
Normalised Observing Speed
Actual/Expected Background
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Herschel Capabilities Arp 220 at various
redshifts
? Astro-F ? SIRTF ? Herschel ? SCUBA ? Planck ? AL
MA or BOLOCAM on LMT
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3x Arp220 At Various redshifts
? SIRTF ? Herschel ? SCUBA ? BOLOCAM on CSO
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6x Arp220 at various Redshifts
? SIRTF ? Herschel ? SCUBA ? BOLOCAM on CSO
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10x Arp220 at Various Redshifts
? SIRTF ? Herschel ? SCUBA ? BOLOCAM on CSO
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Conclusions

• Herschel very capable for studies of the high z
  universe
• Both small and large, key, programmes are catered
  for in the application process
• Key programmes come up first, which is why were
  all here