ESA Science & Technology14-Jul-2005 12:24:24
 

Instruments

Instruments in Brief

The Herschel scientific instrument complement comprises three instruments, two cameras (PACS and SPIRE) with additional imaging spectroscopy capabilities, and a very high-resolution heterodyne spectrometer (HIFI). Principal Investigator consortia provide the instruments.

Herschel Principal Investigators

HIFI

Thijs de Graauw, SRON (Groningen, The Netherlands)

PACS

Albrecht Poglitsch, MPE (Garching, Germany)

SPIRE

Matthew Griffin, University of Wales (Cardiff, United Kingdom)


Heterodyne Instrument for the Far Infrared (HIFI)

HIFI is a very high-resolution heterodyne spectrometer. The heterodyne detection principle involves translating the frequency range of the astronomical signal being observed to a lower frequency where it is easier to perform the required measurements. This is done by mixing the incoming signal with a very stable monochromatic signal, generated by a local oscillator, and extracting the difference frequency for further processing. HIFI observes in seven bands covering 480 to 1910 Ghz. Bands one to five, which give continuous coverage from 480 to 1250 GHz, use superconductor-insulator-superconductor (SIS) mixers. Bands six low and six high cover 1410 to 1910 GHz and use hot electron bolometer (HEB) mixers.

HIFI Frequency Bands

Band

Mixer type

Lower freq.

Upper freq.

1

SIS

480 Ghz

640 Ghz

2

SIS

640 Ghz

800 Ghz

3

SIS

800 Ghz

960 Ghz

4

SIS

960 Ghz

1120 Ghz

5

SIS

1120 Ghz

1250 Ghz

6L

HEB

1410 Ghz

1600 Ghz

6H

HEB

1600 Ghz

1910 Ghz

The difference signal from the heterodyne process is passed to the instrument spectrometers. There are two spectrometers, a Wide Band Spectrometer and High Resolution Spectrometer, each of which is capable of processing signals of both polarisations simultaneously.

HIFI Spectrometers

Spectrometer

Wide band

High resolution

Mode

N/A

Normal

High Resolution

Type

Acousto-optical

Autocorrelation

Bandwidth (GHz)

4

1

0.5

Resolution (MHz)

1

0.27

0.135

Velocity resolution (ms-1)

600 - 160

180 - 65

90 - 54


Photodetector Array Camera and Spectrometer (PACS)

PACS is an imaging photometer and integral field line spectrometer for wavelengths between 60 and 210 µm. It employs two bolometer arrays for imaging photometry and two germanium/gallium photoconductor arrays to perform imaging line spectroscopy.

In photometry mode, PACS will simultaneously image in two bands, one of either 60 - 90 µm or 90 - 130 µm together with 130 - 210 µm, over fields of view of 1.75 x 3.5 arcminutes with full beam sampling in each band.

In spectroscopy mode, PACS will image a field of about 50 x 50 arcseconds, resolved into 5 x 5 pixels, with an instantaneous velocity coverage of about 1500 kms-1 and a velocity resolution of between 150 and 200 kms-1.


Spectral and Photometric Imaging Receiver (SPIRE)

SPIRE comprises a three band imaging photometer and an imaging Fourier transform spectrometer. SPIRE employs arrays of spider-web bolometers with neutron transmutation doped (NTD) germanium temperature sensors as its detectors.

The photometer images a 4x8 arcminute field of view on sky in three bands simultaneously.

SPIRE Photometer Characteristics

Centre Wavelength (µm)

250

360

520

λ/Δλ

~ 3

~ 3

~ 3

Number of detectors

139

88

43

Detector array size (mm)

45 x 23

45 x 23

45 x 23

The photometer has three observing modes:

  • Point source photometry
  • Field mapping, with a maximum field size of 4 x 4 arcminutes
  • Scan mapping, with a field of view of 4 x 8 arcminutes

The SPIRE spectrometer is based on the Mach-Zehnder configuration. One input port receives the incoming beam from the telescope while the second port accepts a signal from a calibration source. The two output ports each have a detector array, one for 200-300 µm (37 detectors) and the other for 300-670 µm (19 detectors). The spectrometer will be operated in continuous scan mode. The spectral resolution can be adjusted in the range between 0.04 and 2 cm-1, corresponding to λ/Δλ of 20 to 1000 at 250 µm. The SPIRE spectrometer has a circular field of view 2.6 arcminutes across.



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