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NOAO Newsletter - NOAO Highlights - December 1999 - Number 60

Livermore Imaging Fourier Transform Spectrograph at the McMath-Pierce Telescope

Ron Wurtz (LLNL)

A team from Lawrence Livermore National Laboratory (LLNL) and a collaboration investigating an imaging Fourier transform spectrograph (IFTS) for the Next Generation Space Telescope (NGST) obtained data cubes using an IFTS on the main spectrograph table at the McMath-Pierce Solar Telescope Facility. The McMath-Pierce was chosen for several reasons: long experience with Fourier transform spectroscopy, a de-rotated horizontal field, and eagerness for supporting novel user instruments.

Figure 1. Panchromatic image of edge of M4, made by co-adding the cube of interferometric sample images. The field of view is four arcminutes in diameter. The interferograms of 454 stars were extracted.

LLNL is building a series of instruments to test and demonstrate imaging Michelson interferometry as a viable astronomical technique for multi-object spectroscopy and multi-band imaging. (BEAR at CFHT is the first facility instrument astronomical IFTS. It has been running with two InSb arrays since the mid-1990s.) The LLNL team evolved these visible-band prototype IFTS instruments from other LLNL projects. The long-range goal is to migrate to 4-meter and 10-meter class ground-based telescopes, in the mid-IR as well as the visible. At the same time, these interferometers will help to promote and design IFIRS, an IFTS for the NGST. The LLNL group is working in the IFIRS collaboration, an NGST pre-phase A study led by James Graham (Berkeley).

The advantage of an IFTS for astronomy is that it obtains an interferogram for every pixel. These interferograms can be transformed into spectra resulting in a true data cube—"a spectrum for every pixel." No field objects need to be pre-selected, no slits need to be placed, and co-adding all the frames of the interferogram yields a deep panchromatic image. When designed properly with two cameras, very few photons are lost—the instrument is simultaneously an imaging camera and a high throughput spectrograph.

The operation of a ground-based optical IFTS leads to better understanding of the proposed NGST IR instrument. Although NGST is an IR facility, the observing environment of NGST will be quite different from IR observing on the ground. Its orbit at L2 will feature comparatively low backgrounds and long dwell-times. The relatively low background in ground-based optical observing allows mimicking of the long-dwell, step-scan operation of the proposed infrared NGST FTS.

The LLNL instrument brought to the McMath-Pierce facility is a two-input, two-output Michelson interferometer, similar to the team's current design for the NGST instrument. Because this first prototype was sensitive to a changing gravity vector, the team picked off the McMath-Pierce's horizontal focal plane projected onto a (de)rotating table. In the ideal configuration, there are science cameras on both output ports, but this one used a guide camera on the second port. Science data was collected with an off-the-shelf PixelVision CCD camera with a 1024 x 1024, thinned SITe chip thermoelectrically cooled to 235K. The final plate scale was about 0.4"/pixel with an unvignetted field of about 4'. After some modifications to the environment in the spectrograph room, the best seeing was about 1.2".

The IFTS ran for nine, mostly clear nights and obtained data cubes for well-known objects including globular clusters, open clusters, spiral galaxies, elliptical galaxies, and nebular regions. The collaboration presented results from two McMath-Pierce data cubes at the 1999 meeting of the American Astronomical Society in Chicago. The first shows the IFTS as a low resolution photometer for hundreds of stars in the globular cluster M4. The second is a medium resolution data cube of the emission line region around the trapezium in M42. These are the first-ever visible band astronomical data cubes presented from an IFTS. See Figures 1 and 2.

Figure 2. A set of representative interferograms for M4 stars over a range of 7.5 magnitudes. Note that the interferograms are plotted with a logarithmic y-axis.

The LLNL team consists of C. L. Bennett, J. Bixler, D. Carr, K. H. Cook, E. Wishnow (also of SSL), and R. Wurtz. Members of the NGST IFIRS collaboration who made significant contributions to the run were M. C. Abrams (ITT), A. Dey (NOAO), J. R. Graham (UCB), and A. Villemaire (Bomem).

Special acknowledgment goes to the NSO/Kitt Peak staff, particularly Claude Plymate, for helping to make the run a success.

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