1. A heliostat reflectivity measurement system comprising: a control processing unit comprising a processor and addressable memory; the control processing unit configured to: receive an image from a imaging device having a field of view; receive a reading of total energy absorbed by a thermopile having a field of view within the field of view of the imaging device; determine a total number of pixels in the received image containing part of an image of the solar disc; and determine a reflectivity estimate based on the determined total number of pixels in the received image containing part of the image of the solar disc and the received reading of total energy absorbed by the thermopile.