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 \mathrm { deg } ^ { 2 } in the northern and southern hemispheres with measured 5 \sigma limiting depths of { Z } \simeq 22.4 , { Y } \simeq 21.4 , { J } \simeq 20.9 , { H } \simeq 19.9 ~ { } % \mathrm { and } ~ { } { Ks } \simeq 19.3 ( Vega ) .
This architecture forms an ideal working parameter space for the discovery of a significant sample of 6.5 \leq \mathrm { z } \leq 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 as a function of both magnitude and redshift .
We review recent follow-up imaging of the first VIKING high-z quasar candidates and find that the results lend considerable support for the choice of selection constraints .
The methods outlined here are also applicable to rare object searches in a number of other ongoing and forthcoming projects .