Cryogenic Spectrograph Research Articles

The Apache Point Observatory Galactic Evolution Experiment (APOGEE)

AbstractThe Apache Point Observatory Galactic Evolution Experiment (APOGEE) is a large scale, high-resolution, near-infrared spectroscopic survey of Milky Way stellar populations and one of the four experiments in the Sloan Digital Sky Survey III (SDSS-III). APOGEE will be based on a new multi-fiber cryogenic spectrograph, currently under construction, expected to begin survey observations on the 2.5 m Sloan telescope in the Spring of 2011. APOGEE will measure high-precision radial velocities and elemental abundances for ~15 elements for ~ 105 stars, and is expected to shed new light on the processes that led to the formation of the Galaxy.

High-precision CTE measurement of aluminum-alloys for cryogenic astronomical instrumentation

We are completing the construction of GIANO, a high resolution near-infrared cryogenic spectrograph for the Telescopio Nazionale Galileo (TNG). Most of the optics are made of aluminium and operate at cryogenic temperature. We evaluated the optical degradation due to mis-matches between the thermal expansion coefficients of the different aluminium parts of the instrument. We performed accurate measurements of the relative thermal expansion coefficients (CTE) of Al-6061 and Al-6082 over the 300–77 K temperatures range. We find that the two alloys have identical thermal expansion coefficient within a maximum (3σ) uncertainty of Δα/α < 0.28%. Our results show that it is possible to overcome the problem of the alignment of a cryogenic instrument, manufacturing the curved optics, the optics’ holders and the optical bench with different metallic alloys with small CTE mismatch (Al-6061 and Al-6082). This conclusion has also been confirmed by the results of the optical tests with the instrument cooled in the laboratory, showing no significant image quality degradation.

SpeX: A Medium‐Resolution 0.8–5.5 Micron Spectrograph and Imager for the NASA Infrared Telescope Facility

ABSTRACTWe present the design, construction, and performance of SpeX, a medium‐resolution 0.8–5.5 μm cryogenic spectrograph and imager, now in operation at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea. The design uses prism cross‐dispersers and gratings to provide resolving powers up to R∼2000 simultaneously across 0.8–2.4, 1.9–4.2, or 2.4–5.5 μm, with a 15″ long slit. A high‐throughput low‐resolution R∼200 prism mode is also provided for faint‐object and occultation spectroscopy. Single‐order 60″ long‐slit modes with resolving powers up to R∼2000 are available for extended objects. The spectrograph employs an Aladdin 3 1024 × 1024 InSb array and uses narrow slits and a spatial scale of 0.″15 pixel -1 for optimum sensitivity on point sources. An autonomous infrared slit viewer is used for object acquisition, infrared guiding, and scientific imaging in the wavelength range 0.8–5.5 μm. The imager employs an Aladdin 2 512 × 512 InSb array that covers a 60′′ × 60′′ field of view at 0.″12 pixel -1. SpeX was successfully commissioned on IRTF during 2000 May, June, and July. Astronomical observations are presented to illustrate performance.

Very Cool L-Dwarfs

ASTRONOMY Dwarfs are low mass, very cool, main-sequence stars, and recent large-scale star surveys have identified a new spectral class, called the L-dwarfs. The low estimated masses of L-dwarfs suggest that most of these objects are brown dwarfs that are rich in hydrides rather than methane. McLean et al. have obtained high-resolution spectra of six L-dwarfs that range in temperature from 2200-2000 K to 1600-1250 K with the near-infrared cryogenic spectrograph (NIRSPEC) on the Keck II telescope. As the temperature of the star decreases, the spectrum changes systematically: the neutral potassium band broadens and weakens, but persists in the lowest temperature L-dwarf; the iron hydride (FeH) band weakens and disappears at about 1600 K; the water band strengthens, as do the neutral alkali metal bands of Na, Rb, and Cs. These spectral features are consistent with models of a cooling atmosphere in which the iron hydride condenses into iron metal. The less refractory alkali metals can persist as neutral species to lower temperatures than the iron, and because they are present in the L-dwarf spectra to about 1200 K, they will form sulfides rather than chlorides when they do condense.— LR Astrophys. J. 533 , L45 (2000).

J-Band Infrared Spectroscopy of a Sample of Brown Dwarfs Using NIRSPEC on Keck II.

Near-infrared spectroscopic observations of a sample of very cool, low-mass objects are presented with higher spectral resolution than in any previous studies. Six of the objects are L dwarfs, ranging in spectral class from L2 to L8/9, and the seventh is a methane or T dwarf. These new observations were obtained during commissioning of the near-infrared spectrometer (NIRSPEC), the first high-resolution near-infrared cryogenic spectrograph for the Keck II 10 m telescope on Mauna Kea, Hawaii. Spectra with a resolving power of R approximately 2500 from 1.135 to 1.360 µm (approximately J band) are presented for each source. At this resolution, a rich spectral structure is revealed, much of which is due to blending of unresolved molecular transitions. Strong lines due to neutral potassium (K i) and bands due to iron hydride (FeH) and steam (H2O) change significantly throughout the L sequence. Iron hydride disappears between L5 and L8, the steam bands deepen, and the K i lines gradually become weaker but wider because of pressure broadening. An unidentified feature occurs at 1.22 µm that has a temperature dependence like FeH but has no counterpart in the available FeH opacity data. Because these objects are 3-6 mag brighter in the near-infrared compared with the I band, spectral classification is efficient. One of the objects studied (2MASSW J1523+3014) is the coolest L dwarf discovered so far by the 2 Micron All-Sky Survey (2MASS), but its spectrum is still significantly different from the methane-dominated objects such as Gl 229B or SDSS 1624+0029.

Spectra of Cool Stars in the [ITAL]J[/ITAL] Band (1.0–1.3 μ[CLC]m[/CLC]) at Medium Resolution

A spectroscopic survey of 103 cool S-, C-, and M-type stars was undertaken with the Kitt Peak cryogenic spectrograph on the 1.3 m telescope to make a detailed search for new molecular bands in the 1.0–1.3 μm J-band region. While the spectra have high signal-to-noise ratios and good repeatability, no strong new features were found. Higher resolution spectra of a small sample of representative stars observed with the 4 m Fourier transform spectrometer were invoked in order to identify the features in these spectra. With few exceptions, the details of these spectra are well understood. Laboratory spectra were obtained to aid in the identification of some weak features. Spectral features from dominant lines of Ti, Fe, Al, Si, Mn, Na, and K, and molecular bands due to TiO, TiS, ZrO, ZrS, VO, H2O, and CN, have been identified in the spectra. Several weak unidentified bands are found.