1.1 Why Fourier Transform Spectrometry?

With the rapid improvement of the noise characteristics of infrared detectors the multiplex advantage of infrared spectroscopy with a classical Fourier transform spectrometer (FTS) is just as rapidly diminishing. However, the CFHT FTS has a number of instrumental characteristics that still make it very attractive for astronomical observations in the 0.9 to 5.5 range.

The FTS has a very high throughput, or étendue which is largely independent of the wavelength of the incident radiation. It has a circular entrance aperture rather than a slit and an inherently high efficiency. With an appropriate choice of beam splitter the overall telescope plus FTS combination can transmit on the order of 25% of the incident flux.

High signal-to-noise observations can be made routinely and is a consequence of several factors. In the case of the dual aperture, dual detector CFHT FTS, rapid modulation of the path difference places the data frequencies in the range of a few hundred Hz, lower than the frequencies associated with atmospheric scintillation, or telescope or building vibrations and hence reduces noise substantially. Since the two detectors always detect all of the flux in the passband of the filter, internal normalization of the data can be applied to further reduce sensitivity to source variations of any kind, including guiding errors. The dual input apertures provide continuous internal sky subtraction prior to the detection of any signal and can typically reduce the sky contamination (such as OH emission) by a factor of 50. At longer wavelengths beam switching can be applied to compensate for any residual sky contamination and can result in a total cancellation factor of to .

The quality of spectra obtained with the FTS can be superb. The instrumental profile is controlled by characteristics of the instrument and the observations, is very clean, and can easily be measured. Scattered light simply does not exist in an FTS, and wavelength precision of can be readily obtained throughout the passband. The flexibility of the instrument is remarkable - the resolving power can be tailored to the scientific program and the Doppler velocity resolution can approach .

It is also possible to perform low-resolution, 2-dimensional spectroscopy on extended objects by replacing the normal InSb detectors with one of the CFHT Redeye cameras - an instrumental configuration referred to as ``Bear''. In this mode interferograms are recorded for each pixel of the image on the NICMOS3 array. Spectra for each pixel or monochromatic images of the target can then be obtained by analysing the recorded data cube. This manual will not discuss this mode of operation and interested investigators should contact CFHT staff for more detailed information, or see the article by Maillard and Simons, ``First Results of an Imaging FTS with a NICMOS Camera'' (Proceedings of an ESA Workshop on Solar Physics and Astrophysics at Interferometric Resolution), p. 205-210.

The above characteristics and others lead some to refer to the CFHT FTS as the ``last word'' in astronomical Fourier transform spectrometers (Johnson, ``Fourier Transform Infrared: A Constantly Evolving Technology,'' [Ellis Horwood, 1991] p. 249.).

Please send comments and suggestions to: veillet@cfht.hawaii.edu