An Efficient Targeting Strategy for Multiobject Spectrograph Surveys: the Sloan Digital Sky Survey “Tiling” Algorithm

The Sloan Digital Sky Survey (SDSS) will observe around 10^6 spectra fromtargets distributed over an area of about 10,000 square degrees, using amulti-object fiber spectrograph which can simultaneously observe 640 objects ina circular field-of-view (referred to as a ``tile'') 1.49 degrees in radius. Notwo fibers can be placed closer than 55'' during the same observation; multipletargets closer than this distance are said to ``collide.'' We present here amethod of allocating fibers to desired targets given a set of tile centerswhich includes the effects of collisions and which is nearly optimallyefficient and uniform. Because of large-scale structure in the galaxydistribution (which form the bulk of the SDSS targets), a naive covering thesky with equally-spaced tiles does not yield uniform sampling. Thus, we presenta heuristic for perturbing the centers of the tiles from the equally-spaceddistribution which provides more uniform completeness. For the SDSS sample, wecan attain a sampling rate greater than 92% for all targets, and greater than99% for the set of targets which do not collide with each other, with anefficiency greater than 90% (defined as the fraction of available fibersassigned to targets).Comment: 22 pages, including 8 figures, submitted to A