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Herschel
Space
Observatory

An
ESA
Mission
with
Participation
from
NASA

 

NASA
Herschel
Science
Center's
Portal
to
the
Cool
Universe

Science Instruments

The telescope will focus light onto three instruments: PACS and SPIRE, which each contain a photometer and low-to-medium resolution spectrometer, and HIFI, an extremely high-resolution heterodyne spectrometer.

They are designed for deep, wideband photometry (with high spatial resolution thanks to Herschel's large 3.5 meter mirror) and full spectral coverage, making Herschel the first space facility to completely cover the far infrared and submillimeter range. Together, PACS and SPIRE are capable of detecting light from 57-670 microns in wavelength. HIFI covers 480-1250 and 1410-1910 GHz (which corresponds to about 157-625 microns).

The three instruments are designed to compliment each other. SPIRE and PACS have imaging spectrometers that provide spatial information, while HIFI resolves with very high spectral resolution, but only one line at a time and in only one beam on the sky.

The emission lines of the main interstellar cooling agents are predominantly in the PACS range, but fall into the SPIRE wavelength range for very distant and hence highly redshifted objects. The combination of both instruments will provide the full range of information needed to determine SEDs (spectral energy distributions - the total amount of power radiated by cosmic objects, measured in watts), photometric redshifts, total luminosity, and accurate positions.

SPIRE and PACS will both conduct deep surveys with their photometers to find galaxies from the early Universe, and follow up with spectroscopic studies of the most interesting objects.

Observing light in that range means that Herschel will detect subtle "heat" emissions from very cold objects, such as vast clouds of interstellar dust. The spacecraft has to keep its instruments cold because their electronics work only at frigid temperatures, and to prevent the instruments' own infrared radiation from drowning out the faint signals they're trying to detect.

So the instruments are housed in a cryostat filled with more than 2,000 liters of superfluid helium. Based on technology developed for the Infrared Space Observatory (ISO), it will keep the instruments at a temperature of less than -271° Celsius, which is less then three degrees above absolute zero. The bolometers aboard PACS and SPIRE will be chilled even further, to -273.3° Celsius, just a few tenths of a degree above absolute zero.

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 × 3.5 arcminutes with full beam sampling in each band.

In spectroscopy mode, PACS will image a field of about 50 × 50 arcseconds, resolved into 5 × 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 4 × 8 arcminute field of view on sky in three bands simultaneously. The photometer has three observing modes: Point source photometry; Field mapping, with a maximum field size of 4 × 4 arcminutes; Scan mapping, with a field of view of 4 × 8 arcminutes

SPIRE Photometer Characteristics

Center Wavelength (µm)

250

360

520

λ/Δλ

~ 3

~ 3

~ 3

Number of detectors

139

88

43

Detector array size (mm)

45 × 23

45 × 23

45 × 23

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.

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, or the wavelength range 157-625 µm. 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

1703 Ghz

6H

HEB

1703 Ghz

1910 Ghz

The difference signal from the heterodyne process is passed to the instrument spectrometers, housed in the service module. 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

0.25

0.25

Resolution (MHz)

1

0.27

0.14

Velocity resolution (ms-1)

600 - 160

180 - 65

90 - 54