The spectroscopic survey aims to observe complete samples of three categories of astronomical objects (Table 26):
The following sections describe the algorithms used to select targets for spectroscopic observation. The quantity primTarget records the results of target selection for each object by using the target bit masks in Table 27. An object can be targeted by more than one algorithm. The quasar and galaxy targets are marked as ``tiled'' in the table to indicate that we intend to obtain a uniform and complete sample. The TARGET_STAR_BROWN_DWARF and TARGET_HOT_STD targets are also tiled, as described below.
The data included in the EDR are commissioning data, and were used, among other things, to refine the algorithms discussed here. Table 20 lists the versions of the target pipeline that were used to select which objects to observe spectroscopically. The exact algorithms used are not uniform throughout all the data released, and Table 29 summarizes the important differences. The results of target selection are recorded in the quantity PrimTarget, which appears in two classes. The class SpecObj records the results of target we used to select objects for spectroscopic observation (v2.2a, v2.5, and v2.7), while the class PhotoObj records the results of target run on the final photometric catalog (target v2.13.8).
The galaxy magnitude limit has changed slightly, as has the criterion for selecting low-surface brightness galaxies, and a number of changes have been made to quasar target selection. Since the pipeline software versions, the photometric calibration, and target selection criteria change, there will be slight mis-matches between the value of PrimTarget for the same object recorded in class SpecObj and PhotoObj.
The quasar selection algorithm is substantially different in target v2.13.8. A complete description of these improvements is beyond the scope of this paper.