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Gemini-South's Near-Infrared Spectrograph Gets 3-D vision

April 27, 2004

The new GNIRS Integral Field Unit (IFU) arrived on Cerro Pachón (Gemini South) in February in a deceptively small package. The new IFU is about the size of a large paperback, but contains some 66 miniature optical elements. Designed and built by the University of Durham (UK), the IFU fits in the GNIRS slit slide mechanism, providing an alternative input "aperture" to the variety of traditional slit masks that feed the long-slit and cross-dispersed configurations of the instrument. With the IFU in the optical path, a 2-dimensional input field of roughly 3” x 5” is divided into 21 slices of 0.15” x 4.9” each. Then these are arranged end-to-end to form a pseudo-slit that is dispersed by one of the spectrograph's three gratings as if it was a normal long-slit. This gives GNIRS the ability to form spectra of each 0.15” x 0.15” sample over the entire field without gaps.

Schematic of the GNIRS IFU showing key elements highlighted in the accompanying text.

Unlike the IFUs built for both Gemini Multi-Object Spectrographs by the same Durham group, the GNIRS IFU dispenses with optical fibers (each GMOS IFU has 1500 fibers*), in favor of a complex set of multifaceted optics which split the field into 21 slices. Each slice requires a chain of three mirror facets (see illustrations):

  1. the slicing mirror itself (S1) which sends the light from each piece of sky off at a different angle;
  2. a pupil mirror (S2) which re-images the slice onto the pseudo-slit, and;
  3. a slit mirror (S3) which re-images the pupil (itself re-imaged by S1 onto S2) onto the spectrograph pupil stop.

The GNIRS IFU installed showing key elements discussed in the text

The ability of each mirror to form images makes it possible to adapt the system to an existing beam-fed spectrograph design, unlike its pioneering predecessor - the 3D instrument devised by the Max-Planck-Institute for Extraterrestrial Physics (MPE, Munich) -- which uses flat optics. The versatility, compact size and high performance of the Advanced Image Slicer**  comes at the price of requiring tiny, close-packed optics with precisely shaped, ultra-smooth surfaces. This complexity was the major challenge facing the Durham team.  Their solution was to diamond-turn each of the 21 optical surfaces required in every stage on the same aluminum substrate.  This technique greatly reduced the problem of alignment since the special process places each facet correctly relative to its neighbors. Furthermore, the all-metal, monolithic construction means that the system remains aligned at the cryogenic temperatures inside the GNIRS cryostat where it is located. For extra efficiency, the optics are also gold-coated.

The GNIRS IFU was installed in GNIRS while it was in the instrument lab on Cerro Pachón in mid-March by a joint team from Durham  and NOAO-Tucson together with Gemini engineers and scientists.  It was immediately clear that all of the fabrication team's hard work had paid off when the installation went very smoothly and the IFU fit perfectly on the first try.  Proper alignment and co-focality with the slit mask were confirmed while the instrument was still in the lab.  The newly enhanced GNIRS was returned to the telescope in early April and was ready for its first commissioning run on April 6-11, 2004.   

Estimates of the throughput of the IFU relative to that of a matched long-slit are 90%, 80%, 73% and 66% at K, H, J and X (~1 um) respectively averaged over each band, confirming the theoretical expectation based on the optical surface quality measured in the lab. The wavelength dependency is expected since surface scattering off the various mirror surfaces is the dominant loss mechanism.  A complex series of baffles ensures that the scattered light does not reach the detector.

Scientific commissioning to establish the limiting performance included diverse targets such as the Galactic center, bipolar planetary nebulae and ultra-luminous IR galaxies implicated in recent mergers.  For example, the twin nuclei of NGC 6240 reveal a clear kinematic signature and a strong velocity shear near the supermassive black hole at the center of our galaxy was clearly observed with the GNIRS IFU.

GNIRS IFU will be offered to the user commuinty in semester 2005A.

* J. Allington-Smith et al. 2002, PASP, 114, 892

** Content, R., 1997. Proc. SPIE, 2871, 1295