Near Infrared Imaging Spectrograph (NIRIS)
NIRIS is a long-slit spectrograph. Like NIC, it incorporates NICMOS3 FPAs. It uses two separate channels to provide nearly continuous coverage between 0.8 and 2.5 microns. A common slit and field lens are followed by a beamsplitter that reflects the light shortward of 1.38 microns to the "blue" channel and transmits the longer wavelengths to the "red." The crossover point is chosen to fall within a deep telluric H 2 O vapor absorption band. Each channel has its own collimator, diffraction grating, camera, and detector array. The collimators and cameras use refractive, antireflection-coated elements. The detectors are 2-quadrant NICMOS3 devices providing 256 channels in the spectral dimension and 128 in the spatial. A 600-line/mm grating blazed at 1.0 micron operates in the "blue" channel while a 300-line/micron blazed at 2.0 microns services the red. Each channel has a nearly constant dispersion-- 14 Å for the blue channel and 36 Å for the red, when using a slit that is 2-arcsec wide.
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NIRIS mounted to the instrument rotator at the cassegrain focus of the 120-inch Shane telescope of Lick observatory. |
The cameras reduce the image to a scale of 1 arcsec/pixel at the arrays, providing for a 2-arcmin slit length at the telescope focus. Each channel has approximately 24 mm of free spectral range to be covered by a 10-mm array. To accomplish this, the arrays are attached to translational stages with a single degree of freedom in the spectral direction. These stages are moved to cover the full spectral range using externally mounted linear actuators capable of positioning to 1-micron accuracy. A CCD camera provides for direct slit guiding. The spectrograph mounts to the same platform that supports the camera, either of which can be selected by a mirror that pivots to redirect the telescope beam. NIS is presently being modified to accept the (1024 x 1024) Hawaii 1 array.
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Four years after outburst, Nova Cas 95 remains bright and displays a remarkable high excitation spectrum. Its emission line spectrum has evolved to include species of higher and higher excitation states. The coronal line [Si VI] l19629 is the second strongest feature in the spectrum (next to He I l10830). Two other strong coronal features are marked. In addition to these very uncommon features, the spectrum displays the recombination line spectrum of hydrogen, showing both strong Paschen and Brackett series, and very strong lines of He II. Forbidden line of S+ and S+2 indicate that there are large regions with densities < 106 cm-3. The presence [Si VI] l19629 requires ionizing photons with energies in excess of 205 eV. [Si VII] l24827 at the extreme red edge of the spectrum needs even higher energies (> 246 eV). The absence of even higher excitation features at 1.07 and 1.25 mm brackets the energy of the photon field and constrains the temperature of the nova remnant. |
IR spectroscopy of Nova Oph 1998. This was an extremely fast nova and the outburst was essentially over at the date of the October measurement. What remains is a blue continuum and weak lines of H and He II. Note the broad base under He I l10830 and the P Cygni absorption 6000 km/sec from line center in the June 19 spectrum. Nova Oph displayed distinct broad and narrow components, the width for the latter matching the highest velocities observed in classical novae. |
The infrared spectrum of the type II supernova SN 1998bl. A type II supernova is formed by the explosion of a massive star. The characteristic spectral signature of a type II is emission from hydrogen. The line at 1.28 microns is the hydrogen line Paschen Beta. The other strong features are due to neutral helium and singly ionized calcium. The absorptions on the blue wings of these features are caused by gas ejected in the explosion and moving toward us at several thousand kilometers per second. |
