13.4 Spectrograph Design Parameters

13.4.1 FUV Channel

Table 13.12 presents design parameters of the FUV spectrograph and gratings. The FUV gratings are concave and have holographically generated grooves to provide dispersion and correct for astigmatism. The gratings have aspherical surfaces to correct for HST's spherical aberration. The FUV "M" gratings have been ion etched to produce triangular groove profiles for better efficiency. The G140L grating has grooves with a laminar profile. All FUV gratings are coated with MgF₂ over aluminum.

The surface of the optic is a sphere of the quoted radius, but with a deviation of Δz = a₄r ⁴ + a₆r ⁶, where z  is measured along the vertex normal. The quantities γ, δ, r_c, and r_d are the standard positions of the recording sources as defined in Noda, Namioka, and Seya (1974, J. Opt. Soc. Amer., 64, 1031).

Table 13.12: Design Parameters for the FUV Spectrograph and Gratings.

13.4.2 NUV Gratings

Table 13.13 presents design parameters of the NUV gratings. The NUV gratings are flat and were not constructed holographically. The NUV MAMA has low but measurable sensitivity at FUV wavelengths, and with some gratings second-order light could contaminate the spectrum. To minimize this effect, the coated optics are optimized for wavelengths above 1600 Å. Given the four reflections used in the NUV channel, wavelengths below 1600 Å, including geocoronal Lyman α, are effectively eliminated. In addition, gratings G230L and G285M have order-blocking filters mounted directly on them to block the second-order spectra below 1700 Å. Even with these filters, it is possible for second-order light to appear on the NUV MAMA when G230L is used, especially in the long-wavelength stripe (see Section 5.1.3).

Table 13.13: Design Parameters for the NUV Gratings.