Space Telescope Science Institute
COS Instrument Handbook
help@stsci.edu
Table of Contents Previous Next Index Print


Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 13: Spectroscopic Reference Material > 13.4 Spectrograph Design Parameters

13.4
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 = a4r4 + a6r6, where z is measured along the vertex normal. The quantities γ, δ, rc, and rd 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
V1 axis to aperture (mm)
α (degrees)
β (degrees)
4.04595
αβ (degrees)
a4
1.45789 109
1.45789 109
1.33939 109
a6
4.85338 1015
4.85338 1015
1.4885 1013
γ (degrees)
71.0
62.5
rc (mm)
4813.92
4363.6
rd (mm)
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 . Given the four reflections used in the NUV channel, wavelengths below 1600 , 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 . 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.
Table 13.13: Design Parameters for the NUV Gratings
α (degrees)
β (degrees)

Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 13: Spectroscopic Reference Material > 13.4 Spectrograph Design Parameters

Table of Contents Previous Next Index Print