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We describe the design and performance of the near-infrared (1.51-1.70 μm), fiber-fed, multi-object (300 fibers), high resolution (R = λ/Δλ ∼ 22,500) spectrograph built for the Apache Point Observatory Galactic Evolution Experiment (APOGEE). APOGEE is a survey of ∼10 5 red giant stars that systematically sampled all Milky Way populations (bulge, disk, and halo) to study the Galaxy's chemical and kinematical history. It was part of the Sloan Digital Sky Survey III (SDSS-III) from 2011 to 2014 using the 2.5 m Sloan Foundation Telescope at Apache Point Observatory, New Mexico. The APOGEE-2 survey is now using the spectrograph as part of SDSS-IV, as well as a second spectrograph, a close copy of the first, operating at the 2.5 m du Pont Telescope at Las Campanas Observatory in Chile. Although several fiber-fed, multi-object, high resolution spectrographs have been built for visual wavelength spectroscopy, the APOGEE spectrograph is one of the first such instruments built for observations in the near-infrared. The instrument's successful development was enabled by several key innovations, including a "gang connector" to allow simultaneous connections of 300 fibers; hermetically sealed feedthroughs to allow fibers to pass through the cryostat wall continuously; the first cryogenically deployed mosaic volume phase holographic grating; and a large refractive camera that includes mono-crystalline silicon and fused silica elements with diameters as large as ∼400 mm. This paper contains a comprehensive description of all aspects of the instrument including the fiber system, optics and opto-mechanics, detector arrays, mechanics and cryogenics, instrument control, calibration system, optical performance and stability, lessons learned, and design changes for the second instrument.

Volume-phase holographic (VPH) gratings show great potential as an alternative dispersing element to the classical surface-relief (SR) gratings presently used in most astronomical spectrographs. We present an introduction to this technology and give the results of an evaluation of three di †erent VPH gratings : a 300 line mm~1 grating optimized at 1064 nm, a 1200 line mm~1 grating optimized at 532 nm, and a 2400 line mm~1 grating optimized for operation at 532 nm.

A diffraction grating technology based upon volume-phase holograms shows promise of enhanced performance for many applications in astronomical spectroscopy over classical surface-relief grating technology. We present a discussion of the underlying physics of a volume-phase grating, give some theoretical performance characteristics, present performance data for a real volume-phase grating, and discuss some potential applications for this grating technology.

been proposed. The interrogation system has the peculiarities of small size, low power consumption, and high accuracy. Experimental results show that the interrogation can be used to interrogate the FBG sensors with wavelength bandwidth of 42 nm, SNR of 43dB, wavelength accuracy of Ϯ10 pm, and wavelength ability of Ϯ15 pm. The interrogation system based on the InGaAs linear image sensor is easy to be integrated and has a good prospect for smart sensing.