APOGEE Spectrographs

Optical Engineer Paul Maseman (Univ of Arizona) checks the laser alignment of the APOGEE spectrograph fore-optics during commissioning.
Optical Engineer Paul Maseman (Univ of Arizona) checks the laser alignment of the APOGEE spectrograph fore-optics during commissioning.

APOGEE-1 and APOGEE-2 constitute a high-resolution near-infrared spectroscopic survey of several hundred thousand stars in the Milky Way Galaxy. Spectra are observed with custom-built, multi-object spectrographs, which record the spectrum of 300 targets simultaneously across the H-band wavelength regime with an approximate resolution of 22,500.

The first APOGEE spectrograph was constructed and assembled primarily at the University of Virginia. It is situated at the 2.5-m Sloan Foundation Telescope at Apache Point Observatory and was commissioned in August 2011 during the SDSS-III APOGEE-1 Survey period. The second APOGEE spectrograph had a similar build process and is almost analogous to the first. It is located at the 2.5-m Irenee du Pont Telescope at Las Campanas Observatory and was commissioned three years into the SDSS-IV APOGEE-2 Survey, during February and March of 2017.

At both sites, light is transmitted telescope focal plane to the pseudo-slit within the cryogenically cooled instrument, located in the adjacent warm support building, via 300 low-OH (“dry”) fused silica fibers which have a 2″ field of view on the sky. Each of the 300 fiber runs consist of two fiber assemblies connected in series. A 2-m fiber run (so-called “fiber harness”) goes from the plug plate to a “gang-connector” just below the telescope and plug plate cartridge. Multiple cartridges are used throughout the night to observe different parts of the sky and each cartridge has its own set of 300 fiber harnesses. An innovative gang-connector allows the simultaneous connection of all 300 fibers from a specific cartridge to the single 40-m fiber run (“fiber link”) which transmits the light from the telescope and cartridges over to the adjacent building and into the apogee spectrograph. To avoid throughput losses from the use of another fiber coupling, the fibers are fed through an epoxy-sealed vacuum feed-through without break at the cryostat wall. Lastly, fibers which make up the fiber link terminate at the instrument pseudo-slit inside the cryogenically cooled instrument.

Diagram of the fiber routing and the 300-fiber gang connector which allows quick change of light source to the instrument.
Diagram of the fiber routing and the 300-fiber gang connector which allows quick change of light source to the instrument.

An ‘uncorrected’ Schmidt camera, used in reverse, collimates the light of each of the fibers. Thus the fiber tips are carefully positioned on a curved pseudo-slit. The pseudo-slit and spherical collimator have a common center of curvature near the system pupil which is also the approximate position of the spectrograph grating. The design is on-axis so the pseudo-slit is an obscuration in the collimated beam. Two-fold mirrors are used for efficient packaging of the optics train within the cryostat.

A schematic of the instrument optics.
A schematic of the instrument optics.

The dispersive optic is a transmissive 3-panel mosaic Volume Phase Holographic (VPH) grating fabricated by Kaiser Optical Systems Inc., the first ever of its kind deployed in a cryogenic astronomical instrument (note for the instrument in the South, there is a slight departure in the VPH is 2-panel). Due to its size the grating area of the VPH was recorded in multiple steps (panels) and then processed and capped as a single unit. A challenging 6-element refractive camera fabricated by New England Optical Systems focuses the various wavelengths of light onto the detectors. The camera features elements of mono-crystalline silicon and fused silica, the largest of which are nearly 400 mm diameter. Three JWST H2RG near-infrared detectors, on-loan from the University of Arizona, are mounted side-by-side to record the blue, middle and red portions of the spectrum. An Astronomical Research Camera (so-called Leach) controller operates all three detectors in sample-up-the-ramp mode.

While the nominal full-width half-maximum is approx. 2.3 pixels wide, the blue end of the spectrum is sampled with less than 2 pixels. To recover optimal sampling, the detector mount is translated (spectrally dithered) between sets of frames with a custom, precision single-axis actuator.

Main Instrument Characteristics

Spectral resolution
22,500
Wavelength coverage
1.51 – 1.70 μm
Fiber diameter
2 arcsec
Throughput
~15% H broad-band efficiency (including atmosphere)
Sensitivity
S/N~100/pixel for H < 12.2 and 3-hour integration
Detectors
Three JWST H2RG (2048 x 2048) Near-Infrared HgCdTe Detectors with 18 micron pixels
The APOGEE mosaic Volume Phase Holographic (VPH) grating is installed during instrument assembly.
The APOGEE mosaic Volume Phase Holographic (VPH) grating is installed during instrument assembly.
The 6-element, 250 lb, APOGEE refractive camera undergoes interferometric null-testing at New England Optical Systems, Inc.
The 6-element, 250 lb, APOGEE refractive camera undergoes interferometric null-testing at New England Optical Systems, Inc.
APOGEE fibers terminate in v-groove blocks. Each of the 10 v-groove blocks contain 30 fibers.
APOGEE fibers terminate in v-groove blocks. Each of the 10 v-groove blocks contain 30 fibers.
The 2-ton APOGEE instrument is lowered to the concrete pad in front of its room in the warm building next to the telescope.
The 2-ton APOGEE instrument is lowered to the concrete pad in front of its room in the warm building next to the telescope.