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Register a new userAstronomical imaging Fourier spectroscopy at far-infrared wavelengths
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Added by
Matthijs H.D. van der Wiel
Publication date
2013
fields
Physics
and research's language is
English
Authors
David A. Naylor
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The principles and practice of astronomical imaging Fourier transform spectroscopy (FTS) at far-infrared wavelengths are described. The Mach-Zehnder interferometer design has been widely adopted for current and future imaging FTS instruments; we compare this design with two other common interferometer formats. Examples of three instruments based on the Mach-Zehnder design are presented. The techniques for retrieving astrophysical parameters from the measured spectra are discussed using calibration data obtained with the Herschel SPIRE instrument. The paper concludes with an example of imaging spectroscopy obtained with the SPIRE FTS instrument.
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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 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 $pm$ 15% for SW, $pm$ 10% for 70-85 cm^(-1) of LW, and $pm$ 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.
Double-Fourier interferometry is the most viable path to sub-arcsecond spatial resolution for future astronomical instruments that will observe the universe at far-infrared wavelengths. The double transform spatio-spectral interferometry couples pupil plane beam combination with detector arrays to enable imaging spectroscopy of wide fields, that will be key to accomplishing top-level science goals. The wide field of view and the necessity for these instruments to fly above the opaque atmosphere create unique characteristics and requirements compared to instruments on ground-based telescopes. In this paper, we discuss some characteristics of single-baseline spatio-spectral interferometers. We investigate the impact of intensity and optical path difference noise on the interferogram and the spectral signal-to-noise ratio. We apply our findings to the special case of the Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII), a balloon payload that will be a first application of this technique at far-infrared wavelengths on a flying platform.
Interstellar polarization at far-infrared through millimeter wavelengths (0.1 - 1 mm) is primarily due to thermal emission from dust grains aligned with magnetic fields. This mechanism has led to studies of magnetic fields in a variety of celestial sources, as well as the physical characteristics of the dust grains and their interaction with the field. Observations have covered a diverse array of sources, from entire galaxies to molecular clouds and proto-stellar disks. Maps have been generated on a wide range of angular scales, from surveys covering large fractions of the sky, down to those with arcsecond spatial resolution. Additionally, the increasing availability of observations at multiple wavelengths in this band allows empirical tests of models of grain alignment and cloud structure. I review some of the recent work in this field, emphasizing comparisons of observations on multiple spatial scales and at multiple wavelengths.
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