Investigation of substrates and mounting techniques for the High Energy Focusing Telescope (HEFT)

The High Energy Focusing Telescope (HEFT) is a balloon-borne system for obtaining arcminute imagery in the 20-100 keV energy band. The hard X-ray optics are baselined to use thin epoxy-replicated aluminum foil substrates coated with graded-d multilayers, and we show some results on X-ray performance of prototype foil substrates. We also propose an extremely promising alternative substrate - thermally-formed glass. The advantages of thermally-formed glass substrates, their fabrication and preliminary metrology on sample pieces are discussed. If ultimately feasible, the thermally-formed glass is a better substrate due to its superior hard X-ray reflectivity and scattering properties in comparison to similarly coated epoxy-replicated aluminum foil. We also discuss some preliminary work on the HEFT mirror mounting concept and the associated angular resolution error budget.