In this paper, we present the development and application of a full-aperture backscatter diagnostics system at the Texas Petawatt Laser (TPW) facility. The diagnostic system includes three independent diagnostic stations. With this system, we obtained TPW on-shot focus properties, and high-harmonic spectral emission from solid foils (e.g., Cu and Al) and their Si substrate in an experiment to study laser hole boring, which show the hole-boring mechanism at relativistic intensities. The measured on-target full-power focal spots from ultrathin film targets help determine the optimum target thickness at certain laser contrast parameters for particle acceleration and neutron generation experiment, which is also a relative measurement of shot-toshot intensity fluctuations.