One-Meter Fourier Transform Spectrometer

(Staff contacts: Mike Dulick, Claude Plymate, Jeremy Wagner)

Maximum path difference:     1 meter
Minimum resolution element:  0.005 1/cm (0.010 symmetric)
Spectral range:              550 1/cm to 45000 1/cm (2200  to 18 micrometers)
Throughput:                  1 cm diameter at f/50 for R = 600,000
Position accuracy:           1.5 millifringe RMS at 6328
Typical wavenumber accuracy: 10^-3 cm to 10^-4 cm
Dynamic range:               19 bits (5*10^5)

The McMath-Pierce 1-meter Fourier Transform Spectrometer (FTS) is a folded Michelson interferometer housed in a vacuum vessel. The instrument is available for use either in conjunction with the McMath's main beam or with laboratory sources. It should be considered when very accurate line positions are needed, when broad spectral coverage is required, when a stable instrumental profile is desirable, or when working in the infrared. In general, full time run support is provided for visitors by the observatory staff for the duration of FTS runs.

Several optical and detector configurations are necessary to accomplish the broad spectral coverage described above. The tables below describe the beam splitters and detectors that are currently available. Although detector changes can be made in a matter of minutes, beam splitter changes are considerably more involved and are therefore scheduled in blocks of time. Observing programs should be designed so that a single observing run confines itself to the range of one of the beam splitters listed on the next page.

The resolution of the instrument is limited in the infrared only by the maximum path difference of the interferometer (1 meter). When the KCl beam splitter is installed, the FTS is configured so that asymmetric interferograms are obtained, permitting resolutions to 0.005 1/cm. All other beam splitters are installed for symmetric operation, which limits the resolution to 0.01 1/cm. In the visible and near infrared, the resolution may be further reduced by practical limitations on the size of the interferogram which can be transformed. In general, the resolving power of the instrument does not exceed 3*10^6.

The Fourier transforms of the interferograms obtained by the FTS are performed by the observatory staff in Tucson following the observing run. The raw spectra are then shipped to the visitor on magnetic tape within a few weeks of the observations.

Solar Observations

The solar image from the main telescope can be focused on a rotating vertical table at the entrance aperture of the FTS. Limb guiders can be attached to this table to provide guiding on any solar area. The entrance apertures are circular with diameters ranging from 0.5 mm (1.2 arcsec) to 10 mm (25 arcsec). A variable width slit-shaped mask is also available for limb work.

A single, full resolution scan requires from 3 to 14 minutes to complete depending on the spectral region. In general, several such scans are co-added to improve the signal-to-noise ratio in the interferogram. For broad-band (several thousand wavenumbers) solar observations, signal-to-noise ratios in excess of 103 can be achieved in 30 minutes of integration. The amount of time required for a single resolution scan can be reduced to as little as 10 seconds if the optical band width is sufficiently narrow. This is done by aliasing the interferogram and is useful for time-series observations of the Sun. Successive scans can be made with a cycle time of roughly 15 seconds. These techniques require special electronic data filters, however, and the NSO staff should be consulted before proposing this type of observation.

Solar FTS Polarimetry

(Staff contact: Jack Harvey)

The FTS polarimeter can currently be used to obtain simultaneous spectroscopic observations of the Stokes I, Q and V components of a source. A variable retarding element is driven sinusoidally to modulate the circular and linear components of the source at 20 kHz and 40 kHz respectively. These signals are then synchronously demodulated in electronics which follow the detectors and heterodyned down into unused portions of the FTS frequency domain prior to the analog-to-digital conversion of the data. The optical transmission of the modulator (fused silica) limits the effectiveness of the device to wavelengths shortward of approximately 2.2 micrometer. The heterodyning techniques also limit the total optical bandwidth of a given observation to roughly 30% of the highest frequency present in the optical passband. A passive, telescope-polarization compensator is mounted in front of the modulator. This reduces telescope polarization to levels under 1%.

Beam Splitters

Designation  Material	                Coating             micrometers       1/cm
UV           Optosil Fused Silica	Aluminum            0.22 to 1.5	  6500 to 45000
Visible      Infrasil Fused Silica	Silver	            0.4 to 2.5    4000 to 27000
CaF2         Calcium Fluoride           Gallium Phosphide   1.1 to 8.0    1250 to  9000
KCl          Potassium Chloride         Gallium Phosphide   3.3 to 18.0   550  to  3000


Detector                       micrometers           1/cm
Silicon PIN Diode              0.22 to 1.1       9000 to 45000
Indium Antimonide Diode        0.67 to 5.4       1850 to 15000
Arsenic Doped Silicon          5.4  to 20.0       500 to 1850

Laboratory Observations

The FTS is also used as a laboratory instrument in support of astrophysically interesting laboratory problems. A 6-meter White cell capable of path lengths up to 432 meters is available for absorption work, as well as a number of other shorter cells. Continuum sources are also available which are appropriate for absorption spectroscopy throughout the usable wavelength range of the FTS. Considerable emission work is also done. In this area, the visiting investigator typically takes responsibility for the operation of the source, although supporting equipment such as power supplies and water cooling is available. A complete description of the facilities provided for laboratory work at the FTS is given in the FTS Laboratory Users Guide.

Other FTS Web Pages

Last updated by Claude Plymate on Tuesday, 4 December 2001.