Quasars representative of the populous faint end of the luminosity function are frustratingly dim with m \sim 24 at intermediate redshift ; moreover groundbased surveys for such faint QSOs suffer substantial morphological contamination by compact galaxies having similar colors .
In order to establish a more reliable ultrafaint QSO sample , we used the APO 3.5-m telescope to take deep groundbased U -band CCD images in fields previously imaged in V,I with WFPC2/HST .
Our approach hence combines multicolor photometry with the 0.1 ^ { \prime \prime } spatial resolution of HST , to establish a morphological and multicolor survey for QSOs extending about 2 magnitudes fainter than most extant groundbased surveys .
We present results for the “ Groth-Westphal Strip ” , in which we identify 10 high likelihood UV-excess candidates having stellar or stellar-nucleus+galaxy morphology in WFPC2 .
For m _ { 606 } < 24.0 ( roughly B \lesssim 24.5 ) the surface density of such QSO candidates is 420 ^ { +180 } _ { -130 } deg ^ { -2 } , or a surface density of 290 ^ { +160 } _ { -110 } deg ^ { -2 } with an additional V - I cut that may further exclude compact emission line galaxies .
Even pending confirming spectroscopy , the observed surface density of QSO candidates is already low enough to yield interesting comparisons : our measures agree extremely well with the predictions of several recent luminosity function models .