Orthonormal polynomials have long been used as a convenient tool to describe optic surface errors. Previously, the products of Fourier and Legendre polynomials have been employed to describe the surface errors for grazing incidence optics. We have applied an alternate set of polynomials, Fourier-Fourier polynomials, to describe the surface errors. This new set differs functionally from the Fourier-Legendre set in that each term has a finite bandwidth frequency spectrum. The advantages of this difference with respect to predictions of grazing incidence telescope performance based on measured surface figure errors are discussed.