Selection constraints on high redshift quasar searches in the VISTA   kilo-degree infrared galaxy survey
  The European Southern Observatory's (ESO) Visible and Infrared Survey Telescope for Astronomy (VISTA) is a 4-m class survey telescope for wide-field near-infrared imaging. VISTA is currently running a suite of six public surveys, which will shortly deliver their first Europe wide public data releases to ESO. The VISTA Kilo-degree Infrared Galaxy Survey (VIKING) forms a natural intermediate between current wide shallow, and deeper more concentrated surveys, by targeting two patches totalling 1500 sq.deg in the northern and southern hemispheres with measured 5-sigma limiting depths of Z ~ 22.4, Y ~ 21.4, J ~ 20.9, H ~ 19.9 and Ks ~19.3 (Vega). This architecture forms an ideal working parameter space for the discovery of a significant sample of 6.5 <= z <= 7.5 quasars. In the first data release priority has been placed on small areas encompassing a number of fields well sampled at many wavelengths, thereby optimising science gains and synergy whilst ensuring a timely release of the first products. For rare object searches e.g. high-z quasars, this policy is not ideal since photometric selection strategies generally evolve considerably with the acquisition of data. Without a reasonably representative data set sampling many directions on the sky it is not clear how a rare object search can be conducted in a highly complete and efficient manner.   In this paper, we alleviate this problem by supplementing initial data with a realistic model of the spatial, luminosity and colour distributions of sources known to heavily contaminate photometric quasar selection spaces, namely dwarf stars of spectral type M, L and T. We use this model along with a subset of available data to investigate contamination of quasar selection space by cool stars and galaxies and lay down a set of benchmark selection constraints that limit contamination to reasonable levels whilst maintaining high completeness... 
J. R. Findlay, W. J. Sutherland, B. P. Venemans, C. Reyle, A. C.   Robin, D. G. Bonfield, V. A. Bruce and M. J. Jarvis
astro-ph.CO