APOGEE Spectrograph
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| A schematic illustration of the APOGEE spectrograph fiber link and optical layout |
APOGEE is a high-resolution near-infrared spectrographic survey of ~100,000 stars in the Milky Way galaxy. The spectrograph will work in H band, from 1.52 to 1.69 microns, with a resolution of order 20,000. The APOGEE spectrograph uses 300 low-OH ("dry") fused silica fibers with a FOV of 2" that will transfer light from the plug plate on the SDSS-III telescope focal plane to a room in the adjacent support buiding housing the bench-mounted APOGEE spectrograph. Each one of the 300 fiber trains consist of two fiber runs in series. A 2-m fiber run (so-called "fiber harness") goes from the plug plate to a "gang-connector" just below the telescope. A 50-m fiber run ("fiber link") connects to the 2-m run at the 'gang-connector' and routes the light via a vacuum feed-through at the cryostat wall to the instrument pseudo-slit inside the cryogenically cooled instrument. The final 10 meters of the fiber link are contained within the cold volume (housed within the "racetrack") to mitigate transmission of thermal backgrounds into the instrument from the warm fiber core.
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| The APOGEE camera fabrication |
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 pseudo-slit actually conforms to a curved focal surface in two dimensions. The pseudo-slit also curves laterally to counteract the natural slit curvature seen in long-slit spectrographs. Therefore, all pseudo-slit images are nearly vertical (spectra are horizontal) to maximize detector real-estate. 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.
The spectrograph will use an innovative 3-panel mosaic Volume Phase Holographic (VPH) grating as the disperser. The spectra will be recorded on three JWST-style H2RG detectors on loan from the JWST NIRCam project.