Diagnostic tools for understanding the edge plasma behavior in fusion devices are essential. The main focus of the present work is to present the infra-red (IR) diagnostics installed on Tokamak Energy’s spherical tokamak (ST40) and the IR thermographic inversion tool, Functional Analysis of Heat Flux (FAHF). FAHF is designed for multi-2D thermographic inversions within the divertor tiles using the finite difference method and an explicit time stepping scheme. ST40’s re-entrant endoscope allows the acquisition of IR data with the highest available effective spatial resolution. With these data, FAHF calculates the plasma perpendicular heat flux density on the divertor—a crucial quantity for edge plasma analysis. Although FAHF demonstrates significant sensitivity to user-selected settings, precise heat flux values are recoverable by ensuring a sufficiently high resolution. Implications for the optimal resolution of both the code and the IR diagnostic system are discussed. FAHF’s simplifications are shown to give an error within 10% with respect to COMSOL Multiphysics® simulations. Finally, by means of comparison with Langmuir probe heat flux data, the accuracy of the FAHF heat fluxes is estimated to be satisfactory. As such, FAHF is proven to be a precise and accurate tool for IR thermographic inversions in ST40.