SPIRE The instrument The SPIRE spectrometer is an imaging Fourier Transform Spectrometer (FTS) that use bolometer arrays operating at 0.3 K. The FTS has spatially separated input and output ports. One input port views a 2.6 arcmin diameter field of view on the sky and the other is fed by an on-board reference source. Two detector arrays at the output ports cover overlapping bands of 194-324 m and 316- 672 m. The FTS spectral resolution is set by the total optical path difference, and can be adjusted between 0.04 and 1 cm-1 (corresponding to /D = 1000 - 40 at 250 m). Fourier-transform spectroscopy is based on autocorrelation using a Michelson interferometer: the incident radiation is separated by a beam splitter into two beams, which travel different optical paths before recombining. By changing the Optical Path Difference (OPD), an interferogram of signal versus OPD is created. This interferogram is the Fourier transform of the source spectrum. The spectrometer mirror mechanism (SMEC) scans the OPD and the interference signal is directed onto the two spectrometer bolometer arrays covering overlapping bands of 194-324 m and 316- 672 m. Performing the inverse Fourier transform thus produces the spectrum as a function of the frequency. Figure 1 show the layout of the FTS side of the instrument. Figure 1. FTS layout. The two spectrometer arrays contain 37 hexagonal close-packet detectors in the short-wavelength (SSW) and 19 detectors in the long-wavelength (SLW) each with its own individual feedhorn. The array feedhorn layouts are shown schematically in Fig. 2. The arrays are hexagonally packed with a spacing between pixels of ~2 beam widths (50.5" for SLW and 32.5" for SSW). Measured pixel FWHM are ~34" for SLW and ~16" for SSW. The two arrays cover the same field of view on the sky and are designed so that most of the SLW pixels are approximately coaligned with SSW pixels. Figure 2. SPIRE spectrometer bolometer arrays. The detectors co-aligned on the sky are shaded. Spectral resolution The spectrometer observing modes support three different spectral resolutions corresponding to three standard values of maximum OPD: low at = 1 cm-1, medium at = 0.25 cm-1 and high resolution at = 0.04 cm-1. The corresponding spectral resolution / for the three regimes are shown on Fig. 3. Figure 3. The unapodised resolving power of SPIRE FTS for three standard spectrometer resolutions. The short wavelength array SSW is shown in blue, while SLW in red. Spectrometer observing Modes In operation, the Spectrometer Mirror Mechanism (SMEC) is scanned continuously at constant speed over different distances to give different spectral resolutions. At least two scans of the SMEC are always done: one in the forward direction and one in the backward direction. Two scan pairs are also deemed essential for redundancy in the data. The desired integration time is set by increasing the number of scan pairs. The spectrometer can be used to take spectra with different spectral resolutions: High resolution: = 0.04 cm-1 which corresponds to λ/λ = 1000 at λ = 250 m. It takes 67.2 s to make one scan in one direction. Medium resolution: = 0.25 cm-1 (λ/λ = 160 at = 250 m) will be more suited to broad features, and will enable faster mapping with the spectrometer. One scan takes 24.4 s to make. Low resolution: = 1 cm-1 (/ = 40 at = 250 m). The SMEC is scanned symmetrically about Zero Path Difference. It takes 6.4 s to perform one scan at this resolution. High and low resolution: to make both line, and high S/N continuum spectra in a single observation. This mode allows the observer to observe a high resolution spectrum as well as spending more integration time to increase the S/N of the continuum than would be available from a high resolution observation on its own. These spectra can be measured in two different pointing modes: Single Pointing Mode: this is used to take spectra of a region smaller than the instrument field of view (2.0 arcmin diameter circle unvignetted). It is produced with one pointing of the telescope, hence only the field of view of the array on the sky is observed, this is combined with the image sampling to determine how well the field of view is covered. Raster Pointing Mode: this is used to take spectra of a region larger than the field of view of the instrument (2.0 arcmin diameter circle unvignetted). The telescope is pointed to various positions making a hexagonally packed map. At each position, spectra are taken at one or more beam steering mirror (BSM) positions depending on the image sampling chosen, determining how well the area is filled in. The dimensions of the area to be covered determines the number of pointings in the map. The distances between these are 116 arcsec along the rows and 110 arcsec between the rows. For either of these pointing modes, it is possible to choose a different sampling of the sky: Sparse image sampling: to measure the spectrum of a point or compact source well centred on the central detectors of the spectrometer. To provide sparse maps (either within the array with single pointing or large that the array with a raster). The BSM is not moved during the observation, producing a single array footprint on the sky. The result is an observation of the selected source position plus a hexagonal-pattern sparse map of the surrounding region with beam centre spacing of (32.5, 50.5) arcsec in the (SSW, SLW) bands. For a point source this requires accurate pointing and good source position knowledge to be sure to have the source well in the (central) detector beam. Intermediate image sampling: to produce imaging spectroscopy with intermediate spatial sampling (1 beam spacing). This gives intermediate spatial sampling without taking as long as a fully Nyquist sampled map. This is achieved by moving the BSM in a 4-point low frequency jiggle, giving a beam spacing of (16.25, 25.25) arcsec in the final map. At each of the 4 positions an even number of SMEC scans are performed to produce the spectra. Full image sampling: it allows fully Nyquist sampled imaging spectroscopy of a region of sky or extended source. This is achieved by moving the BSM in a 16-point jiggle to provide complete Nyquist sampling (1/2 beam spacing) of the required area. The beam spacing in the final map is (8.13, 12.66) arcsec. At each position an even number of SMEC scans are performed to produce the spectra. To define an observation, one needs to select a spectral resolution, a pointing mode and an image sampling. Spectrometer Sensitivity Estimates The predicted FTS line sensitivities (unresolved line; point source) are shown in Fig. 4, and the point source continuum sensitivity estimates are shown in Fig. 5 for low-resolution mode. For an FTS, the continuum sensitivity is proportional to the spectral resolution, so for medium and high resolution, the rms flux limits shown in Fig. 5 should be multiplied by 4 and 25, respectively. Figure 4. 5-, 1 hour point source line flux limit vs. wavelength for SSW (top) and SLW (bottom) for an unresolved spectral line. The operational limits defined for the bands are indicated by the vertical lines. These plots apply both to High and Medium resolution modes. Figure 5. Low resolution mode 5-, 1 hour point source flux density limit vs. wavelength for SSW (top) and SLW (bottom). The operational limits defined for the bands are indicated by the vertical lines. For Medium and High resolution modes, the limits must be multiplied by 4 and 25 respectively.
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