Michel Duban

Holographic aspheric gratings printed with aberrant waves

In astrophysics, holographic gratings use, as far as possible, spherical gratings surface and printing laser waves. However, to obtain high spectral resolution, aberrations must be corrected up to the fourth degree, which generally requires aspheric surfaces for the grating or the laser waves. To date, technological progress makes the fabrication of such gratings possible, so we have developed a complete theory of these deformations and have written a computer program which we believe solves any gratings aberration problems.

Third-generation Rowland holographic mounting

Holographic gratings using aspheric blanks and/or aberrated laser recording sources are most often required to achieve high spectral resolution over a large spectral range. We propose a configuration derived from the optimized holographic Rowland mounting, where the grating is recorded by the interference of two aberrated wave fronts diffracted from concave holographic gratings. These two auxiliary gratings are recorded with laser point sources, and the three blanks are spherical, which is well suited to the severe far-UV constraints on shape and polishing. In addition to the correction of astigmatism, coma C(1), and spherical aberration S(1) given by the optimized Rowland mounting, this mounting cancels, at least at one point of the spectrum, coma C(2) and spherical aberrations S(2) and S(3).

Recording method for obtaining high-resolution holographic gratings through use of multimode deformable plane mirrors

We propose a new way of recording in which a spherical blank and a deformable mirror are used to obtain high-resolution holographic gratings. The reflection of one of the two laser recording waves upon this mirror provides the deformations necessary to image correction to as much as seventh-order aberrations inclusively.

Optical simulation for a fixed spherical solar collector

To calculate the absorber dimensions for a fixed spherical solar collector, an optical simulation of the raytracing type is proposed. The physical quantities, which have an effect upon these dimensions, are described as well as the measurement methods. Once the dimensions are determined, the incident flux on the absorber surface can be calculated by the same program in terms of different zenith distances. These calculations can be checked by comparing the calculated flux on the surface of the absorber with the measured flux at different points along the absorber aimed at the full moon instead of at the sun. Through the data obtained from the measurements, fluctuating points of high flux and permanent zones which receive double and triple reflection rays have been studied.

Theory of spherical holographic gratings recorded by use of a multimode deformable mirror

We present the theory of spherical holographic gratings recorded by use of a deformable plane mirror and consider its application to the optimized Rowland Mounting. We illustrate the efficiency of such a mounting by computing two high-resolution gratings (3800 grooves/mm) with f/24 and f/10 apertures.

Illustration of the use of multimode deformable plane mirrors to record high-resolution concave gratings: results for the Cosmic Origins Spectrograph gratings of the Hubble Space Telescope

To illustrate the efficiency of using a deformable plane mirror to record holographic gratings, we have computed the three gratings for the Cosmic Origins Spectrograph. Their working conditions are severe, since they have to correct the residual spherical aberration of the Hubble Space Telescope. Nevertheless, all images obtained are largely diffraction limited with regard to the resolution.

Recording high-dispersion spherical holographic gratings in a modified Rowland mounting by use of a multimode deformable mirror

To reduce the uncorrected higher-order aberrations for holographic gratings requiring an extreme dispersion, we have modified the Rowland mounting by moving the recording laser sources away from the grating. Then, with a multimode deformable plane mirror to record the grating, the correction of all the aberrations up to the fourth order inclusive is found sufficient to obtain a high-quality image. Applied to the FUSE-LYMAN grating, with a groove density of as much as 5740 grooves/mm, for which a resolution of 30,000 was required, this new recording device produces a resolution from 139,000 to 222,000 over the spectral range.

Improved Wadsworth mounting with aspherical holographic grating

For a point source at infinity, the Wadsworth mounting produces an axial image with neither astigmatism nor coma. These properties are the same with a holographic grating printed with two coherent sources Σ1 and Σ2 at infinity. Such a mounting is of great interest for UV spectrography of stars and galaxies because it produces a spectrum using only one reflecting surface (the grating) plus a collimating entrance baffle. To improve the image quality, we consider an aspherical

Theory and computation of three cosmic origin spectrograph aspheric gratings recorded with a multimode deformable mirror.

The theory of three Cosmic Origin Spectrograph holographic gratings recorded with a deformable plane mirror is presented. Their working conditions are severe, since they have to correct the strong spherical aberration and the field astigmatism of the Hubble Space Telescope. Recorded on aspherized substrates, the gratings produce images that are diffraction limited with regard to spectral resolution.

Comparison of grating designs for the Lyman Far-Ultraviolet Spectroscopic Explorer spectrograph.

Various grating designs have been proposed by several investigators for possible use with the Lyman/Far-Ultraviolet Spectroscopic Explorer mission. The image quality, the feasibility, and the efficiency of five designs are compared, each using a distinct type of grating: (1) a grating ruled on a deformed ellipsoidal or toroidal blank, (2) an ellipsoidal grating recorded holographically with two auxiliary spherical mirrors, (3) a spherical holographic grating recorded with two auxiliary spherical holographic gratings, (4) a spherical ruled grating with variable spacing and straight grooves, and (5) a spherical ruled grating with a groove pattern that is determined theoretically (hybrid grating). From a purely theoretical viewpoint, grating (5) provides the finest images, followed by gratings, (3), (1), (4), and (2). In view of the current technological limitations, the order of practical importance is gratings (4), (1), (2), (3), and (5).