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Gratings aspherized with k = 3/2. Mounting angle 5q = 30°. Displayed: Jobin-Yvon grating references, line density 1/^, grating order K, blazed wavelength Ab, incident angle a, diffraction angle ft at center of field, spectral ranges A,„in, Amax and spectral resolutions R .

Gratings aspherized with k = 3/2. Mounting angle 5q = 30°. Displayed: Jobin-Yvon grating references, line density 1/^, grating order K, blazed wavelength Ab, incident angle a, diffraction angle ft at center of field, spectral ranges A,„in, Amax and spectral resolutions R .

4.4.7 All-Reflective Spectrographs Without Central Obstruction

All-reflective spectrographs without central obstruction can be obtained by use of an off-axis region of the design case shown in Fig. 4.13-D. Thus, if the f-ratio of the starting design is fin, then the unobstructed design should be, say, f/(2.2n). This latter design would show residual aberrations of an f/n spectrograph. Hence a design without central obstruction is only practicable if derived from a central obstruction design where the focal-ratio is not too fast.

For such unobstructed spectrographs, active optics provides off-axis plane-aspheric gratings by replication of an off-axis region of a deformable axisymmetric grating submaster - flat at rest - when stressed by air pressure (up to q = 25 Atm.).

These developments were carried out for the design and construction of the Isard and Osiris spectrographs (Lemaitre and Richardson [38]). Isard is an f/5-f/25 imager-spectrograph dedicated to the 2-meter Bernard Lyot telescope at Pic du Midi observatory for faint object studies. Osiris is an f/5 telescope and f/2.2 spectrograph on board the Odin satellite launched in 2001.

4.4.8 Advantages of Quasi-all-Reflective Spectrographs

Several advantages of the above quasi-all-reflective spectrographs render these systems attractive for moderate brightness and faint object studies.

Apart from the field lens, the all-reflective form of the imaging mode is simply obtained by interchanging the aspheric reflective grating by an aspheric mirror; if necessary, the addition of a slightly cylindric lens before the cryostat window field lens flattener somewhat improves the imaging quality.

All-reflective multi-object spectrographs and imager spectrographs benefit from:

(1) a uniform dispersion law of all spectra over the field of view,

(2) a quasi-constant linear dispersion over the full spectral range,

(3) an almost distortion free instrument in the nebular direction,

(4) a high throughput from the atmospheric cut-off at 320 nm to the far-infrared.

These features are of substantial interest to increase the accuracy in the data reduction process, particularly for instruments dedicated to 3-D spectroscopy.

4.4.9 Diffraction Gratings and Electromagnetic Theoretical Models

The manufacturers of relief-surface diffraction gratings have achieved a high degree of performance in the construction of the facet angle called blaze angle. In a given diffraction octave, this technology controls the diffracted energy and is in close agreement with electromagnetic theoretical models. For such broad-band spectral coverage, reflective gratings are more efficient than transmission gratings. For line-density gratings 1/i < 900mm-1 used in the visible, the absolute reflectance measured at or near the blaze angle reaches the metal theoretical reflectivity; for instance, Jobin-Yvon classically obtains an efficiency of 90% for aluminum (see hereafter).

For relief-surface diffraction gratings having a groove spacing smaller than five wavelengths (i/X < 5), electromagnetic effects of light propagation appear and modify the reflectance of the polarization components with X. Local discontinuities such as the Wood anomalies [89] are well known by experimenters since the beginning of the last century. Preliminary explanations were investigated by Rayleigh [62, 63] and later by Meecham [52], Stroke [78] and Maréchal and Stroke [46]. A new formalism was obtained by Petit and Cadilhac [58] and Pavageau and Bousquet [57].

Theoretical analyses have been investigated and developed by Petit [59, 60], Maystre [48-50], Neviere [54-56], McPhedran [51], that allow predicting the reflectance curves RmX) and important local variations with X. These models allow consideration of many cases of groove profiles such as triangular semi-rectangle echelette, holographic sinusoidal, rectangular lamellar, or low groove density echelle. For example, in the case of transmission gratings such as used in some astronomical instruments with so-called Carpenter gratings or grisms - i.e. a grating glued on a prism which refractive index provides a compensated deviation -, it has been found that the transmitted efficiency could be substantially enhanced by deposition of a metallic thin layer onto the small "side-facet" of the grooves. There is no doubt that manufacturing these gratings would present technological difficulties. The case of reflective gratings allows one to obtain the highest efficiency.

The degree of achievement of the electromagnetic models allows prediction of where the grating manufacturers can obtain progress of some percent on the reflectance of a given grating type. A theoretical review on the efficiencies of reflective gratings in function of the blaze and deviation angles was carried out by Loewen et al. [42, 43]. This survey provides comparisons between semi-rectangle and sinusoidal grooves. Figure 4.18 displays an example of these theoretical results for

Fig. 4.18 First-order efficiency curves for a 8° 38' blaze angle echelette grating, perfectly conducting, used with 45° deviation angle. Solid-line: in the S-polarization plane. Dashed-line: is the P-polarization plane. The light lines give the results for Littrow mount as reference (Loewen, Neviere & Maystre [43])

Fig. 4.18 First-order efficiency curves for a 8° 38' blaze angle echelette grating, perfectly conducting, used with 45° deviation angle. Solid-line: in the S-polarization plane. Dashed-line: is the P-polarization plane. The light lines give the results for Littrow mount as reference (Loewen, Neviere & Maystre [43])

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