BOSS is a spectroscopic survey that measures redshifts of 1.5 million luminous red galaxies and Lyman-alpha absorption towards 160,000 high redshift quasars. Its two identical spectrographs are rebuilt from the original SDSS spectrographs. Each spectrograph has two cameras, one red and one blue, with a dichroic splitting the light at roughly 6000 Angstroms.
1,000 holes are drilled in an aluminum plate (a plug plate, which subtends 3 degrees on the sky), each hole corresponding to an astronomical object such as a quasar, a galaxy, a standard star, or a random blank area on the sky (to measure and subtract the foreground sky emission). The plug plate is then mounted on a cartridge that can be quickly affixed to the telescope. 1,000 optical fibers with 2 arcsec diameters are plugged into the holes. The fibers send the light from each object through a beamsplitter with a coating that reflects the blue part of the spectrum while allowing the red part through. Each spectrum is thus split into two parts, blue and red, and the two parts are recorded on separate CCDs. The red spectrograph uses thick, fully depleted 4Kx4K LBNL CCDs that have a high quantum efficiency in the near-infrared and much reduced fringing. The blue side has blue-sensitive e2v CCDs (CCD231-84).
Principal upgrades from the original cameras
- New, higher efficiency volume holographic gratings.
- New, fully-depleted CCDs on the red side, with much better red response, as well as blue CCDs with improved blue response.
- New fibers (1000 rather than 640 per plate), with smaller holes (2″ rather than 3″).
The blue cameras cover at least 3600-6350 Angstroms. The blue limit is set to include the wavelength-calibration Cd I arc line at 3610.51 Angstroms, although the throughput requirements for the Lyman-alpha forest analysis begin at 3700 Angstroms. The red cameras cover at least 5650-10000 Angstroms. The red limit is set by detection of the Mg b 5175 A and the Na D 5893 A absorption features redshifted to z = 0.7.
The main scientific goals of BOSS could be achieved using a broad range of spectroscopic resolution. The adopted design has a resolution running from R = 1560 at 3700 Angstroms to R= 2270 at 6000 Angstroms (blue channel), and from R = 1850 at 6000 Angstroms to R = 2650 at 9000 Angstroms (reds channel). The higher resolution and the extended red wavelength coverage is necessary to capture the Lyman-alpha break and the strongest Mg lines redshifted to about 9300 Angstroms.
Main Parameter Summary
- Number of spectrographs
- Spectral resolution
- 1560-2270 in the blue channel, 1850-2650 in the red channel
- Wavelength coverage
- 3600-10,400 Angstroms
- Fiber diameter
- 2 arcsec
- 4Kx4K fully-depleted LBNL CCDs with 15 micron pixels for the red side, blue-sensitive 4Kx4K e2V CCDs with 15 micron pixels for the blue side
- Collimator coating reflectivity
- >95% from 420 nm to 1000 nm