HIFI has four spectrometers, one Wide Band Spectrometer (WBS) and one High Resolution Spectrometer (HRS) per polarization. These may all be used simultaneously. When all spectrometers are in use frame times are 4 seconds each. Shorter frame times are possible when only one type of spectrometer is used (1 or 2 seconds).
The high resolution spectroscopy modes available with HIFI are most useful for observing faint details and to separate adjacent spectral lines from each other. The contrast between higher and lower resolution data is illustrated in Figure 3.3, “Example of the use of high resolution spectroscopy in the Orion-Irc2 region.” which shows spectra for the Orion-Irc2 region.
The Wide Band Spectrometers have a single resolution (1.1MHz) with pixels of width around 0.54MHz (varies slightly across the IF bandwidth). A total contiguous IF bandwidth of 4GHz is covered by 4 linear CCDs that cover 1GHz bandwidth each. Precise frequency calibration is available via an internal comb generation, supplying a signal providing a regular line spectrum with lines 100MHz apart. Two buffers are available for source and reference spectra.
The High Resolution Spectrometers have configurations with a variable resolution that is user selectable (see Table 3.2, “List of HRS configurations available in each polarization ”). Between one and four subbands of 230MHz of 460MHz bandwidth can be centred anywhere within the 4GHz intermediate frequency range made available to the spectrometers. Frequency calibration comes from the internal local oscillator frequency settings for the spectrometer. Two buffers are available for source and reference spectra.
Table 3.2. List of HRS configurations available in each polarization
Mode | Number of bands per polarization x bandwidth | Number of lags | Number of offset channels | Spectral resolution (kHz) -Hanning type apodisation-. | Channel spacing (kHz) |
|---|---|---|---|---|---|
High resolution | 1 x 230MHz | 1 x 4080 | 16 | 125 | 64 |
Nominal resolution | 2 x 230MHz | 2 x 2040 | 16 | 250 | 125 |
Low resolution | 4 x 230MHz | 4 x 1020 | 16 | 500 | 250 |
Wide resolution (band) | 4 x 460MHz (x2) | 4 x 510 (x2) | 16 | 1000 | 500 |