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The Sloan Digital Sky Survey is carried out by a purpose-built 2.5m telescope at the Apache Point Observatory (APO); New Mexico. It has been designed to obtain deep (g ≈ 23) photo- metric observations using a large mosaic CCD camera made up of 30 2048×2048 (13×13′) photometric CCDs (Lupton et al., 2001) capable of quasi-simultaneous imaging in five broad optical bands, ugriz, centred at effective wavelengths of 3500, 4800, 6250, 7700 and 9100Å respectively (Gunn et al., 1998; Fukugita et al., 1996).

SDSS is also designed to take spectroscopic observations of> 106 objects in 104deg2 of sky at |b| > 30◦, to a limiting magnitude of g ≃ 20. This is achieved using 3◦ diameter spectrographic plates which hold 640 fibres leading to two 320-fibre-fed spectrographs (each with two 2048×2048 CCDs) with a range of 3900−6200Å for the blue beam and 5800−9200Å for the red beam with a resolving power of 1800, or 167kms−1 (York et al., 2000).

The main purpose of the SDSS is to accrue photometric and spectroscopic data on galax- ies and QSOs. First the acquired data is reduced by a photometric pipeline (Lupton et al., 2001), which separates stars from galaxies, using their surface brightness distribution and computes magnitudes in the five ugriz colours. Objects are chosen for spectroscopic observations by a number of different selection algorithms (Stoughton et al., 2002). These algorithms cover colour loci of quasars (Richards et al., 2002), cool stars, white dwarfs, white dwarfs plus cool dwarf binaries. CVs are found serendipitously as their colours overlap with those of hot stars, quasars, WDs and M stars, depending on how much the accretion disc, or accretion column, contributes to the optical light over that of the underlying WD and late-type secondary star. Once spectra of target objects have been acquired, the data are wavelength- and flux- calibrated, and atmospheric disturbances are corrected using sdF stars. The resulting spectra are classified as stars, galaxies, or quasars, and redshifts are determined.

During the earliest stages of SDSS, a number of photometric selection criteria were at- tempted to try and maximise the chances of finding CVs (Szkody et al., 2002b). It was found that CVs easily separate from main-sequence stars in u−g colour space but as previously men- tioned overlap with quasars (Fan, 1999) and white dwarfs. The simultaneous red and blue colour selection criteria u−g < 0.45, g−r < 0.7, r−i > 0.3, i−z > 0.4 were used to select CVs with little disc contribution, however, the problem was that this was successful in identifying mostly non-interacting WD plus M dwarf binaries and only a few CVs. It was then decided by Szkody et al. that the most efficient way to detect CVs was to use the fact that since spectro- scopic fibres were used to target candidate quasars and their colours overlapped with CVs (see Figure 2.3), a pipeline could be used to search through spectra of these candidate quasars to find CVs. Figure 2.3 shows the exclusion regions of the SDSS quasar target selection (light-blue shaded boxes), which was designed to limit contamination of the quasar search by white dwarfs, A-stars and white dwarf main sequence binaries (WDMS) (Richards et al., 2002). The black

Figure 2.3: The panels show the location of stars and white dwarfs (dark grey), quasars (light grey) from SDSS in u−g/g−r, g−r/r−i and r−i/i−z colour-colour diagrams. The black dots are CVs. Shown as

light-blue shaded boxes are the exclusions regions of the SDSS quasar target selection, designed to limit contamination by white dwarfs, A-stars and white dwarf main sequence binaries (WDMS).

dots represents all known CV identified by SDSS, 213 so far. On inspection, one may wonder as to why there are so many CVs within the exclusion regions. There are two reasons for this: Firstly, the implementation of the exclusion algorithms did not come into effect until after the first SDSS Data Release (DR1; July 2001) which resulted in the identification of the first CVs published in (Szkody et al., 2002b). Second, in addition to the exclusion boxes there are also two inclusion regions which overlap with the colour-cuts of the exclusion regions: (1) “mid-z”, used to select 2.5<z<3 quasars whose colours cross the stellar locus in SDSS colour space, and (2) “UVX”, used to duplicate the selection of z.2.2 UV-excess quasars. As a result, spectroscopic follow-up of some of these targets has led to the discovery of a number of CVs. Further details of these inclusion and exclusion boxes, and spectroscopic follow-up processes are discussed in (Richards et al., 2002).

CVs are formally identified in the SDSS spectra through an algorithm that selects ob- jects with Balmer and helium emission/absorption lines at zero redshift (which typically implies ongoing accretion), and the resulting objects are then classified by eye. According to Szkody et al. (2007a) it is estimated that more than 90% of CVs that exist in the SDSS data base are found with a few missed through mis-identification or low S/N of spectra.

The features of SDSS mean that it is the most comprehensive method of CV detection to date. Its broad colour selection range gives it serious advantages over previous blue-only surveys like Palomar Green and Edinburgh Cape. Its deep magnitude limit will allow for the detection of intrinsically faint systems, with lower accretion rates out to many hundreds of parsecs above the Galactic plane, sampling several scale heights of the CV population.

A series of papers on CVs have been published annually; Szkody et al. 2002b, 2003a, 2004, 2005, 2006, 2007a, henceforth PSI–PSVI. At the time of writing, 213 CVs have been identified through SDSS, 177 of which are new discoveries (Szkody et al., 2007a). Of the sys- tems for which orbital periods have been determined, approximately 70% have periods under 2hours, 10% are in the period gap (mostly Polars), and 20% are long-period systems above the period gap.