Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

13.3 COS Optical Elements

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13.3.1 FUV Gratings

Table 13.2 provides the dimensions of the gratings used in the FUV channel. 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 and they are coated with MgF₂. G140L has grooves with a laminar profile and is also MgF₂ coated.

Note that the surface of the optic is a sphere of the quoted radius, but with a deviation of z = a₄r⁴ + a₆r⁶, where z is measured along the vertex normal. The quantities , , 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 13.2: FUV Optical Design Parameters.

Dimension	G130M	G160M	G140L
secondary mirror vertex to aperture (z, mm)	6414.4
V1 axis to aperture (mm)	90.49
aperture to grating (mm)	1626.57
(degrees)	20.1	20.1	7.40745
(degrees)	8.6466	8.6466	-4.04595
- (degrees)	11.4534
grating to detector (mm)	1541.25
detector normal vs. central ray (degrees)	9.04664
groove density (lines mm -1)	3800	3093.3	480
radius (mm)	1652	1652	1613.87
a4	1.45789 × 10-9	1.45789 × 10-9	1.33939 × 10-9
a6	-4.85338 × 10-15	-4.85338 × 10-15	1.4885 × 10-13
(degrees)	-71.0	-62.5	10.0
d (degrees)	65.3512	38.5004	24.0722
rc (mm)	-4813.92	-4363.6	3674.09
rd (mm)	5238.29	4180.27	3305.19
recording wavelength (Å)	4880

13.3.2 NUV Gratings

Several of the NUV optics and gratings are coated with MgF₂ on Al (see Table 13.3), both to maintain high reflectivity and to suppress FUV light. The NUV MAMA detector 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-, are very 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 with G230L to appear on the NUV MAMA, especially in the long-wavelength stripe.

Table 13.3: NUV Grating Parameters.

Dimension	G185M	G225M	G285M	G230L
groove density (mm-1)	4800	4800	4000	500
useful range (Å)	1670 - 2100	2000 - 2500	2500 - 3200	1700 - 3200
(degrees)	27.24	33.621	35.707	5.565
(degrees)	25.85	32.23	34.32	1.088
peak efficiency wavelength (Å)	1850 ± 100	2250 ± 100	2850 ± 100	2300 ± 100
coating	Al + MgF2	Al only	Al only	Al + MgF2

13.3.3 Mirrors

NCM1 is a flat, fused silica mirror 40 mm in diameter, coated with aluminum and MgF₂.

"MIRRORA" and MIRRORB" refer to the same optical element used in different ways; see Section 6.2.