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, 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

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.

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 × 23	45 × 23	45 × 23

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

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.

Introduction

____________________________________
Last Update: 26 Feb 2009