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Astron. Astrophys. 339, L89-L93 (1998)


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Letter to the Editor

A general method of holographic grating recording with a null-powered multimode deformable mirror

The case of the Cosmic Origins Spectrograph for HST 2002

Gérard R. Lemaître 1 and Michel Duban 2

1 Observatoire de Marseille, Université de Provence, 2 place Le Verrier, F-13248 Marseille CX4, France
(e-mail: lemaitre@observatoire.cnrs-mrs.fr)
2 Laboratoire d'Astronomie Spatiale du CNRS, B.P. 8, F-13376 Marseille CX12, France

Received 12 August 1998 / Accepted 25 September 1998

Abstract

A new general method for recording high resolution holographic gratings by using a null-powered multimode deformable mirror is described. Up to now, the principle of this active method has been applied only to make gratings correcting a single aberration mode. The extension of the method to gratings correcting several aberration modes is proposed. An analogy between Clebsch modes (Clebsch 1861, Saint Venant and Flamant 1881) in elasticity, and Zernike modes in optics, allows generation and coaddition of aspherical surfaces up to a high order in aberration compensation. To illustrate the efficiency of the method in a difficult case, the three gratings for the Cosmic Origin Spectrograph have been computed. Their working conditions take into account the compensation of the HST residual spherical aberration. The image quality obtained are nearly diffraction limited in the direction of the dispersion and also in the other direction for the third, less dispersive, grating. The three grating substrates are aspherical but retain rotational symmetry , thus being easily achievable by an active "vase form" and by spherical polishing. Only three optical modes have to be coadded onto the multimode deformable mirror. We propose to call such an active mirror form a "multimode mirror" or a "Clebsch-Zernike mirror". This general method will greatly simplify grating manufacturers' holographic recording mountings.

Key words: holographic gratings – grating recording – high resolution – active optics – elasticity – multimode mirrors – Clebsch-Zernike mirrors – aberration compensators

Send offprint requests to: G.R. Lemaitre

© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998

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