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.