No abstract

We present far-infrared spectra and maps of the DR21 molecular cloud core between 196 and 671 μm, using the Herschel-SPIRE spectrometer.Nineteen molecular lines originating from CO, 13 CO, HCO + and H 2 O, plus lines of [N ii] and [CI] were recorded, including several transitions not previously detected. The CO lines are excited in warm gas with T kin ∼ 125 K and n H 2 ∼ 7 × 10 4 cm −3 , CO column density N(CO) ∼ 3.5 × 10 18 cm −2 and a filling factor of ∼12%, and appear to trace gas associated with an outflow. The rotational temperature analysis incorporating observations from ground-based telescopes reveals an additional lower excitation CO compoment which has a temperature ∼78 K and N(CO) ∼ 4.5×10 21 cm −2 .

Fourier Transform Spectrometers (FTS) are commonly operated in a rapid-scan (RS) mode, in which an interferogram of an astronomical source is obtained as quickly as possible, followed by one of a nearby background position. In an alternate operating mode, known as step-and-integrate (SI), the optical path difference in the interferometer is incremented in discrete steps, and the signal is integrated only when the interferometer mirrors are stationary. This mode requires some other means of modulating the signal, such as chopping the secondary mirror so that the detector alternately views source and background. The noise bandwidth in the SI mode (typically ~1 Hz) is much smaller than in the RS mode (~1 KHz), which in principle can lead to an increase in overall sensitivity. The main problem with the SI mode is that it takes much longer (~30x) to acquire an interferogram. At submillimetre wavelengths, through the use of narrowband optical filters, which are matched to regions of low atmospheric opacity, it is possible to sample the interferogram at less than the interval determined from the DC band limited Nyquist frequency (a condition known as aliasing) and still unambiguously recover the spectral information. We describe in detail the aliased, SI mode of operation of an FTS at the JCMT and present first results of astronomical spectra obtained using this mode. The resulting spectra are compared and contrasted to data obtained in the RS mode.

The design and performance of a dual polarizing bolometer detector system for use with a polarizing Fourier transform spectrometer to conduct broadband astronomical spectroscopy at submillimeter wavelengths is presented. The system features a fully differential electronics design which virtually eliminates common mode noise. The optical design efficiently rejects unwanted radiation, both spectrally and spatially, while minimizing the effects of resonant optical cavities. The system is cooled by an efficient, closed cycle He4–3He refrigerator which is cycled under computer control. The noise performance of the system is determined from analysis of electrical, optical, and spectral measurements, and the results are compared with a theoretical bolometer model.

The submillimetre atmospheric transmission spectrum above Mauna Kea has been measured at a resolution of 0.005 cm−1 (150 MHz) with a Fourier transform spectrometer at the James Clerk Maxwell Telescope, using the Sun as a source. Column abundances of O2, H2O and O3 determined from these spectra are found to be in excellent agreement with independent measurements. The derived column abundances have been used as inputs to the atmospheric spectral modelling program fascod. The synthetic transmission spectrum is found to be in excellent agreement with the measured spectrum, and provides a template for submillimetre observations from the JCMT.

The Far-Infrared Surveyor (FIS) onboard the AKARI satellite has a spectroscopic capability provided by a Fourier transform spectrometer (FIS-FTS). FIS-FTS is the 1 first space-borne imaging FTS dedicated to far-infrared astronomical observations. We describe the calibration process of the FIS-FTS and discuss its accuracy and reliability. The calibration is based on the observational data of bright astronomical sources as well as two instrumental sources. We have compared the FIS-FTS spectra with the spectra obtained from the Long Wavelength Spectrometer (LWS) of the Infrared Space Observatory (ISO) having a similar spectral coverage. The present calibration method accurately reproduces the spectra of several solar system objects having a reliable spectral model. Under this condition the relative uncertainty of the calibration of the continuum is estimated to be ±15% for SW, ±10% for 70-85 cm −1 of LW, and ±20% for 60-70 cm −1 of LW; and the absolute uncertainty is estimated to be +35/ − 55% for SW, +35/ − 55% for 70-85 cm −1 of LW, and +40/ − 60% for 60-70 cm −1 of LW. These values are confirmed by comparison with theoretical models and previous observations by the ISO/LWS.

No abstract

Astronomical spectroscopy at submillimeter wavelengths holds much promise for fields as diverse as the study of planetary atmospheres, molecular clouds and extragalactic sources. Fourier transform spectrometers (FTS) represent an important class of spectrometers well suited to observations that require broad spectral coverage at intermediate spectral resolution. In this paper we present the design and performance of a novel FTS, which has been developed for use at the James Clerk Maxwell Telescope (JCMT). The design uses two broadband intensity beamsplitters in a Mach-Zehnder configuration, which provide access to all four interferometer ports while maintaining a high and uniform efficiency over a broad spectral range. Since the interferometer processes both polarizations it is twice as efficient as the Martin-Puplett interferometer (MPI). As with the MPI, the spatial separation of the two input ports allows a reference blackbody to be viewed at all times in one port, while continually viewing the astronomical source in the other. Furthermore, by minimizing the size of the optical beam at the beamsplitter, the design is well suited to imaging Fourier transform spectroscopy (IFTS) as evidenced by its selection for the SPIRE instrument on Herschel.