A variety of other science targets are selected. These include optical counterparts of ROSAT sources, stars, and serendipitous targets. These samples are not complete (with the exception of spectrophotometric standards and brown dwarfs), but are assigned to excess fibers left over after the main samples of galaxies, LRG's, and quasars are tiled (§ 4.9).
For typical values of the ratio of optical to X-ray flux of various classes of Galactic and extragalactic sources, there is an excellent match between the depth of SDSS in the optical and that of the ROSAT All Sky Survey (RASS; voges99) in X-rays. Objects detected in SDSS imaging data are positionally matched against X-ray sources from the RASS catalogs. SDSS objects within the RASS positional error circles (commonly 10-20'' radius) are scrutinized further by target selection algorithms tuned to select likely optical counterparts to the X-ray sources. In decreasing priority order, spectra are obtained for SDSS/RASS coincidences that: (a) are also radio sources (flagged ROSAT_A); (b) have SDSS optical colors of AGN/quasars (ROSAT_B); (c) fall in a broad intermediate priority bin that includes bright and/or moderately blue stars and galaxies (ROSAT_C); (d) are otherwise bright enough for follow-up spectroscopy (ROSAT_D). Objects flagged ROSAT_E are not targeted; they fall within the ROSAT error circle, but are either too faint or too bright for our spectrographs. Objects with fiber magnitude brighter than 15 in , , or are not targeted, because of cross-talk between fibers in the spectrograph.
This algorithm targets of order 10-15 objects per plate; over half are AGN or quasars.
A variety of stars are also targeted by their distinctive colors. Specific stellar spectroscopic target categories include blue horizontal branch stars (flagged STAR_BHB), carbon stars, both dwarf and giant ( STAR_CARBON), low-luminosity subdwarfs (STAR_SUB_DWARF), the central stars of planetary nebulae (STAR_PN), cataclysmic variables (CATY_VAR), red and brown dwarfs (STAR_RED_DWARF and STAR_BROWN_DWARF), and hot white dwarfs (STAR_WHITE_DWARF). The planetary nebula and cataclysmic variable categories are essentially completely contained within the quasar target selection region of color-color space, and are much rarer than quasars; only a handful of these objects have been found. The definition of red dwarf was too inclusive in the version of target used, resulting in the observations of a large number of ordinary M dwarfs. The brown dwarfs are very rare (typically one every several plates); unlike the other categories of stars, these are tiled. However, in the EDR data, the algorithm for selecting these objects was modified several times, making the formal definition of a complete sample difficult.
Additionally, of order 30% of the objects targeted as quasars turn out, in fact, to be stars, for the most part outliers from the stellar locus by definition (but occasionally objects in the stellar locus as well, due to early errors in the code). This is of course not a complete sample, but it contains several thousand spectra of uniform data quality of stars of all spectral types.
Finally, we have an open category of targets, referred to as serendipity, whose criteria are subject to change as different regions of parameter space are explored. There are several categories of these in the EDR. These are flagged as SERENDIP_RED, SERENDIP_BLUE, and SERENDIP_DISTANT for objects lying outside the stellar locus in color space (the latter refers to distance from the stellar locus). These overlap somewhat with the quasar and some of the stellar target selection categories. Objects coincident with FIRST radio sources are flagged SERENDIP_FIRST; this matching goes fainter than the equivalent for quasar target selection, and has no restriction on stellarity. Finally, SERENDIP_MANUAL targets are selected by hand, as the name implies. A wide variety of interesting objects have emerged from serendipity spectroscopic targets, ranging from highly unusual categories of white dwarfs to radio-loud broad absorption line quasars menou01.