SPIRE by asafwewe


More Info
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
      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

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

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