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Fig. 5.19 He-Ne interferograms of aspherized reflective gratings in zero-order with respect to a plane. Zerodur substrates. (Left): One of the gratings of f/3.18 MARLY spectrographs 5 = 30°, 1,200 i/mm, 84x84 mm area, 66.04x76.25 mm elliptical clear aperture. (Right): One of the gratings off/2.5 CARELEC spectrograph 5 = 30°, 1,200 i/mm, 102x128 mm area, 100x115mm elliptical clear aperture (Coll. LOOM-Horiba Jobin Yvon Corp.)

Fig. 5.19 He-Ne interferograms of aspherized reflective gratings in zero-order with respect to a plane. Zerodur substrates. (Left): One of the gratings of f/3.18 MARLY spectrographs 5 = 30°, 1,200 i/mm, 84x84 mm area, 66.04x76.25 mm elliptical clear aperture. (Right): One of the gratings off/2.5 CARELEC spectrograph 5 = 30°, 1,200 i/mm, 102x128 mm area, 100x115mm elliptical clear aperture (Coll. LOOM-Horiba Jobin Yvon Corp.)

The optical tests of the final aspherized grating replicas were carried out from He-Ne interferograms with respect to a plane (Fig. 5.19). All the submasters were made in quenched Fe87 Cr13 alloy as follows.

- Marly spectrographs: ro = 40.44 mm built-in radius, t(0) = 5.45 mm, q =

- Carelec spectrographs: ro = 65.70 mm built-in radius, t(0) = 6.83 mm, q =

Off-axis designed spectrographs avoid the central obstruction of usual collimated telescope beams towards the grating and make the detector access to the focal plane easier. These designs result from a sideways section of the previous designs. Thus, grating replicas can be aspherized by off-axis print of an axisymmetric submaster. In order to obtain the same focal ratios than for on-axis designs, the amount of the submaster flexure must be at least 2-2.5-times larger which could lead to difficulties for the construction of fast off-axis spectrographs. Off-axis reflective imager-spectrographs have been developed and built with Isard for faint object studies at the 2-m Bernard Lyot Telescope at Pic-du Midi, and with Osiris/Odin for a survey in space (Lemaitre and Richardson [24]). Off-axis aspherics - mirror and gratings - were obtained by a double replication process with quenched Fe87Cr13 alloy submasters, first with a flat optical surface when unstressed, and then stressed (Fig. 5.20).

Fig. 5.20 He-Ne interferograms with respect to a plane of off-axis aspherics generated from double replication of an axisymmetric submaster for ISARD imager and spectrograph at Pic du Midi 2-m telescope. (Left): Aspherized mirror. (Right): Aspherized gratings 75 and 150i/mm in zero-order (Coll. LOOM - Horiba Jobin Yvon Corp. [24])

Fig. 5.20 He-Ne interferograms with respect to a plane of off-axis aspherics generated from double replication of an axisymmetric submaster for ISARD imager and spectrograph at Pic du Midi 2-m telescope. (Left): Aspherized mirror. (Right): Aspherized gratings 75 and 150i/mm in zero-order (Coll. LOOM - Horiba Jobin Yvon Corp. [24])

5.4.4 Axisymmetric Gratings with k = 0 and Circular Simply Supported Submasters

Aspherized reflective gratings with a geometrical profile k = 0 - corresponding to M = 0 - are less performing than with k = 3/2 because, as we have seen, Astm 5 is not balanced in the field with a small amount of Astm 3 (cf. Sect. 4.4.5). However, these pure r4 profiles can be of interest for single object spectrographs using single array detectors where long rectangular pixels are much larger in the y direction than in the x direction of dispersion.

Starting from a plane master grating, the replication conditions are the same as in (5.75) except that M = 0. The aspherization of the grating is generated from replication of a tulip-form deformable submaster. With spectrograph mountings working in normal diffraction ft = 0 at the center of the spectral range, axisymmetric submasters provide straightforward solutions. The aspherization is achieved by a central force under perimeter reaction. The thickness distribution corresponds to the case VTD2 in Sect. 3.3.2. For this configuration, the thickness t of the simply supported submasters is, from (3.30a),

0 0

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