A Fourier Transform Spectrometer for SCUBA-2
Astronomical spectroscopy at submillimetre wavelengths holds much promise for fields as diverse as the study of planetary atmospheres, molecular clouds and extragalactic sources. While strong absorption bands of water vapour preclude ground-based observations over much of this spectral range, it is possible to observe in the atmospheric windows, regions where the earth's atmosphere has partial, though often variable, transmission. Fourier Transform Spectrometers (FTS) are an important class of spectrometers well suited to those applications which require broad spectral coverage at intermediate spectral resolution through these windows. For example, the study of pressure broadened absorption lines of constituents in planetary atmospheres, unbiased searches for, and identification of, spectral features in molecular clouds and measurements of the continuum emission in a variety of sources.
To date most astronomical FTS's (including the U of L FTS) have used a single detector element to obtain spectral information from one point on the sky. However, the rapid development of imaging detector arrays, which has resulted in such consumer products as hand-held video cameras and high resolution single-shot cameras, has paved the way for long wavelength cameras like SCUBA-2. By combining an imaging detector array with a spectrometer it is now possible to obtain, simultaneously, a spectrum from each point on the sky corresponding to individual pixels in the array. An imaging spectrometer therefore opens up a third dimension in astronomical observations by providing spectral information at each point in the object under study (e.g. galaxy, molecular cloud). While SCUBA-2 will provide unprecedented morphological information on, for example, a galaxy, the composition and physical conditions of the galaxy can only be determined through imaging spectral measurements.
Following the pioneering work of Maillard in astronomical imaging Fourier transform spectroscopy at near infrared wavelengths with the Canada France Hawaii Telescope, the rapid development of array detectors and the increased capabilities of modern computers have propelled this field to a mature level. This is evidenced by the SPIRE instrument (Spectral and Photometric Imaging REceiver) of ESA's Herschel mission, and proposals for an imaging FTS for NASA's Next Generation Space Telescope (NGST). An imaging FTS is therefore seen to be an ideal complement to SCUBA-2, providing broadband, intermediate resolution imaging spectroscopy.
Our group has over twenty five years experience in the development and use of infrared spectrometers on ground-based, balloon-borne and space-borne platforms. In particular, we have over 12 years experience in the development and use of FTS's at the JCMT. The U of L FTS played a pivotal role in the commissioning of SCUBA (the forerunner of SCUBA-2) by diagnosing a spectral leak that severely limited the performance of the camera. In addition to its use as an astronomical spectrometer, the FTS-2 will play a critical role during the commissioning and performance optimization phase of the SCUBA-2 instrument.
More recently, in collaboration with Prof. Peter Ade's group at Cardiff University, we have completed the development of a novel type of FTS which is ideally suited to imaging spectroscopy. This design uses two broadband intensity beamsplitters in a Mach-Zehnder configuration, which provides access to all four interferometer ports while maintaining maximum throughput and removing any polarization sensitivity. The performance of this design, on which the SPIRE instrument is based, has been demonstrated in the laboratory and recently tested in the field.
The Canadian Space Agency has funded participation in the SPIRE/Herschel project and Prof. Naylor is one of 5 Canadian Associate Scientists chosen for this work. The synergy between the FTS proposed for SCUBA-2 and the SPIRE instrument is evident. Both share a common FTS design and both face similar data processing challenges. Prof. Ade's knowledge of submillimeter detector, filter and beamsplitter technologies coupled with Prof. Naylor's expertise in Fourier spectroscopy represent a potent collaborative effort and position both groups to exploit this emerging field.
The SCUBA-2 imaging FTS will provide simultaneous, intermediate resolution, spectroscopy over the broad 450 and 850 µm spectral bands. The spectral resolution can be instantly adjusted for the scientific problem at hand. Three areas of interest are given below:
The submillimetre region is a particularly rich field of study because it is the region of maximum intensity for the rotational lines of many potential atmospheric constitutents. Spectroscopic measurements provide an inventory of molecular species and information on the physical and dynamical processes (e.g. internal heat sources) of the atmosphere. The FTS and small diffraction limited JCMT beam of 7" at 450 µm will allow, for the first time, submillimetre spectral mapping of the Jovian, Saturnian and Martian discs (which have angular sizes of 45", 19" and 16", respectively at opposition) and the study of hemispheric, zonal and polar differences and transport effects. Off-source pixels will provide a direct means of cancelling variations in telluric emission without the need for off-source pointing. Spectral mapping of Jupiter will also be practical at longer wavelengths where the diffraction limit reaches 14" at 850 µm.
The FTS-2 has the potential to have a broader impact on submillimetre astronomy by allowing direct measurements of the spectral energy distributions (SED) of sources detected at submillimetre wavelengths. Recent measurements obtained with the U of L FTS operating with a single detector at the JCMT have shown that Fourier spectroscopy is capable of differentiating continuum and line emission in complex regions like the Orion molecular cloud. Measurements of the spectral energy distributions (SED) of sources detected at submillimetre wavelengths would allow the determination of the dust contribution to emission seen in SCUBA-2 maps without the uncertainties associated with using complex dust models.
The extragalactic submillimetre community is engaged in projects that require some understanding of the Spectral Energy Distribution (SED) of the dust emission from Ultra-Luminous Infra-Red Galaxies (ULIRGs). Unfortunately, our knowledge is limited to only a handful of photometric measurements in even the best studied bright sources. In particular, the dust emissivity is determined essentially by fitting a line between fluxes measured with SCUBA at 450 and 850 µm. The problem with this approach is that 450 µm observations are typically difficult to obtain and calibrate, and the 850 µm fluxes have a non-negligible contamination from CO line emission. Also, the assumption used in these fits is that the SED is dominated by a single dust temperature/emissivity, which in many cases is known to be wrong. By using an FTS to measure the SED across a long wavelength band, one can escape these issues and determine the emissivity directly.
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Last modified on: Tuesday, 22-Jan-2008 11:39:48 MST
Some content for this page was provided by Dr. Michel Fich.