The imaging silicon pixel array (ISPA) tube is an opto-electronic camera with single photon detection capability at room temperature and ∼1 MHz detection rates. To further improve the ISPA spatial resolution, its radiation entrance window is made of scintillating YAlO3(Ce), which serves at the same time as its gamma detector. The photocathode is coated at its vacuum surface and the escaping photoelectrons are guided by a parallel electric field (proximity focusing) toward a pixelized silicon anode. With this new window configuration, we achieved 0.25 mm ISPA spatial resolution with 0.35 mm collimator holes. This agrees with 0.22 mm expected spatial resolution, deduced from the evaluated hit-number of photoelectrons on the anode pixels and the photoelectron cluster residuals.