Support:
Dennis Carr
Tom Gorman
Izzy Lewis

We have built a series of instruments to test and demonstrate imaging Fourier Transform interferometry as a viable astronomical spectrographic technique.  We evolved these visible band prototype imaging Fourier transform spectrograph (IFTS) instruments from other LLNL projects.  We have started designing and building a visible-band instrument and a mid-infrared instrument for use on 4-meter and 10-meter class ground-based telescopes.  At the same time, these interferometers will also be used to promote IFIRS, an IFTS for Next Generation Space Telescope (NGST).  Our group is working in a collaboration, led by Prof. James Graham at UC Berkeley, performing a NASA pre-Phase A study for an IFTS for the NGST.
 

Livermore Collaborators : Charlie Bennett, Kem Cook, Edward Wishnow, Ron Wurtz

Academic and Government Partners : A. Dey, J. Najita, NOAO; J. Carr, NRL; S. Morris, DAO/HIA

Industry Partners : M. Abrams, ITT; A. Villemaire, Bomem
 

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 datacube -- "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 intereferogram yields a deep panchromatic image. When designed properly with two cameras, no photons are lost -- the instrument is simultaneously an imaging camera and high throughput spectrograph.

The first astronomical imaging Fourier transform spectrograph is a near-infrared device named BEAR at Canada-France-Hawaii Telescope. It is customarily used for high resolution spectroscopy studies of extended regions. 

Livermore has designed, built, and run an IFTS for low background astronomical use as a testbed for the proposed IFTS for the NGST. The relatively low background in the optical allows us to mimic the long dwell, step scan operation of the proposed infrared NGST FTS.

The first testbed IFTS reflects our current design for IFIRS.  It is a four-port (two input, two output) Michelson interferometer with two 45 degree, self-compensating beamplitters and cube-corner retro-reflectors. The interferometric laser metrology system was attached to the moving mirror's motion stage, but below the science interferometer beam.

This system was taken to the 1.5-m McMath-Pierce Solar Observatory (MPSO) in March 1999. Because our first prototype was sensitive to a changing gravity vector, we picked off the MPSO's horizontal focal plane projected onto a (de)rotating table. We collected data at one output port with an off-the-shelf PixelVision CCD camera with a 1024x1024, thinned SITe chip thermoelectrically cooled to 235K. Our final plate scale was about 0.4 arcsec/pixel with an unvignetted field of about 5 arcmin. 
 

We presented results from two MPSO datacubes at the June 1999 meeting of the American Astronomical Society in Chicago. The first shows the IFTS as a low resolution photometer for hundreds of field stars.
 
 

The second is a medium resolution datacube of an emission line region. These are the first ever visible band astronomical datacubes presented from an IFTS. 

Since March 1999, we have evolved the instrument once again. The new design will incorporate an interferometer built by Bomem, one of the industry partners. In addition, we are including a dispersing prism for higher resolution studies, and we will be using a camera on the second output port for common-mode rejection as well as increasing the throughput by a factor of two. We are planning for an engineering run scheduled for September 1999 at a 3.5 meter telescope.