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(5.87)

Submasters for grating aspherization are generally made of high-strength metal alloys. The maximum stress must be lower than the elasticity limit Oeo.2%- Up to f/1, a convenient determination of the thickness is easily obtained with stainless steel Fe87Cr13 alloy. After a quenching and prestressing cycle for linear stressstrain increase, the submaster is polished flat. Although uniform loads up to 15 bar have been used, more typical values are in the range 4-5 bar. A typical design of an active submaster requires a thick boundary ring strengthened by the enclosing plate (Fig. 5.18).

For instance, a submaster design generating corrected gratings for Lamost f/1.5 second generation low-resolution spectrographs, by Zhu and Lemaitre [31] (see Sect. 4.4.11), also includes the spherical aberration corrections of a field lens flat-tener and spherical collimator. The 246 mm built-in plate is a quenched stainless steel Fe87Cr13 (E = 201 GPa, v = 0.305). For continuity reasons, the junction at

Fig. 5.18 Typical design of a metal active optics submaster generating aspherized gratings by inner uniform load q and replication. The thickness of the built-in plate decreases from center to edge. (Left): On-axis replications. (Right): Off-axis replications for telescope beams without central obstruction

Fig. 5.18 Typical design of a metal active optics submaster generating aspherized gratings by inner uniform load q and replication. The thickness of the built-in plate decreases from center to edge. (Left): On-axis replications. (Right): Off-axis replications for telescope beams without central obstruction the built-in radius ro is realized by a quarter torus of radius 8 mm tangent to the ring inner side. The aspherization load - air pressure - is q = 5 bar and the maximum radial stress at the surface of the submaster does not exceed or = 92 MPa (Table 5.6).

Aspherized gratings are generally obtained on fused silica substrates or vitro ce-ram substrates such as Schott-Zerodur. Because of extremely low tensions existing after resin polymerization, the replication process can be carried out equally with circular, square, rectangular, or other perimeter shape substrates. Also, the process can use holed substrates for on-axis input of the telescope beams. This was the case of the Uvpf spectrograph at f/1.22 of Cfht [1, 19-21], (cf. Table 4.3). The moderate focal-ratio spectrographs Marly 1, Marly 2, and Carelec have been built with aspherized gratings [22, 23] for Haute-Provence and Purple Mountain observatories (see Table 4.4).

Table 5.6 Design of vase-form submasters generating the aspherized gratings of LAMOST low-resolution spectrographs, R = 2,000, red and blue channels at f/1.5. From Table 4.5 in Sect. 4.4.11,

the flexure is Zsub =

0 0

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