SPIRE - Spectral and Photometric Imaging Receiver

SPIRE comprises a three-band photometer, operating in spectral bands centred on 250, 350 and 500 µm, and an imaging Fourier-Transform Spectrometer (FTS), which provide low resolution spectra over the 195-670 µm band. Both instruments use germanium bolometers operating at 0.3 K and coupled to the telescope with hexagonally conical feddhorns. The photometer and the spectrometer are not designed to operate simultaneously.

SPIRE optical layout.

SPIRE Photometer Optical layout.
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The SPIRE photometer and spectrometer lie on the two sides of the optical bench panel.
The common input optics of the two instruments belong to the photometer and a pick-off mirror is used to direct the FTS field of view to the other side of the optical bench through a hole. The main element of the common optics is a Beam Steering Mirror (BSM), which is placed at an intermedate pupil position and is used both for photometer and FTS observation. The BSM can chop up to 4 arcmin amplitude along the long axis of the field of view and, simultaneously, it can chop in the orthogonal direction up to an amplitude of 30 arcsec. This two-axis motion allows chopping and jiggling to remove background emissions and fully sample the image of the sky. The optical bench panel, and all optical elements mounted directly from it, are at 4K.
In the photometer the beam is imaged simultaneously on the three bolometer arrays by using several reflecting mirrors and two dichroics. The last mirror (M9) and dichroics are within a 2-K box. The spectral passbands are defined by a sequence of metal mesh filters, and are centred at approximately 250, 350 and 500 µm with λ/Δλ of 3.3, 3,4 and 2.5 respectively.
An internal calibration source provides a repeatable signal for the bolometers. It is an emitting element at a temperature of about 80 K which radiates in the centre of the BSM. It is not designed to provide an absolute calibration and it operates at regular interval in-flight in order to check the responsivity of the bolometers.
For further information, see the section in the Observer Manual.

In the Fourier-Transform Spectrometer, the incoming radiation is separated by a beam divider in a Mach-Zehnder configuration into two beams, which travel inside a Mirror Mechanism Box (MMB) before recombining. Inside the MMB, a moving corner-cube mirror is used to change the Optical Path Difference of the two beams and the interference signal is directed onto the two spectrometer bolometer arrays. The passbands for each array (194-324 µm and 316-672 µm) are defined by a sequence of mesh filters.
A thermal calibration source is located at the second input port of the FTS, which provides a radiation roughly equivalent to the dilute 80-K black body emission spectrum of the telescope. The requirement of the calibration source is to be able to annull the central maximum of the interferogram.
For further information, see the section in the Observer Manual.



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Photometry with SPIRE.

The SPIRE photometric camera allows simultaneous observations in three large bands centered at 250 µm (PSW), 350 µm (PMW) and 500 µm (PLW) using three arrays with 139, 88 and 43 detectors respectively.
The SPIRE arrays are not CCD type matrices (as in the PACS), but they are feedhorn bolometers coupled with the instrument optics. For each band the feedhorn dimensions are optimized to the diffraction limit for the wavelength, so that the three arrays have a different number of pixels even though they exactly cover the same Field Of View (FOV 4’x8’); the FWHM of the single feedhorn is 18”, 25” and 36” in the three bands.
For further information, see the section in the Observer Manual.


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A scheme of the SPIRE photometr bolometer array. In the right side the overlapping of the three arrays.
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The overlap of the SPIRE spectrometer arrays on the sky. the 2.6' diameter circle shows the unvignetted beam footprint.
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Spectroscopy with SPIRE.

The SPIRE Fourier Transform Spectrometer contains two arrays with 37 and 19 detectors, operating in the 194-324 µm range (SSW) and in the 316-672 µm range (SLW) respectively.
Each detector is feedhorn bolometer coupled with the instrument optics and every detector is spaced by about two beam widths from the others, forming hexagonally modulated arrays. The detector dimensions are chosen to have images close to the diffraction limit and the FWHM beams are about 34 arcsec for SLW and 16 arcsec for SSW. The two arrays cover the same field of view with the central pixels of SSW and SLW perfectly coaligned. The unvignetted field of view has a diameter of 2.6 arcmin and contains 7 detectors for SLW and 19 detectors for SSW.
For further information, see the section in the Observer Manual.


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SPIRE Documentation.

The Herschel Observers' Manuals (Version 2.1) provides general information about the Herschel Observatory.
The SPIRE Observers' Manual (Version 2.1) describes the instrument characteristics, the available observing modes, the SPIRE scientific capabilities, how to perform observations and an overview of the data processing.
For an overview of the SPIRE instrument see the paper.
Click here to download the SPIRE flyer.

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