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Due to the lack of highly transparent materials monochromators for the vacuum ultraviolet spectral range are therefore normally equipped with just one optical element, namely a ruled spherical grating. Further, three additionnal properties of such type of mounting would be useful. First would be a single grating motion, an attractive possibility being a simple rotation of the grating. Second, fixed entrance and exit slits would simplify the mechanical aspects of many experimentals set up, particularly when using experimental chambers for optical testing in this spectral range. A last feature would be to use the grating either at normal incidence to avoid an excess amount of astigmatism in the spectral range 4 000 to 500 A either at large angle of incidence, attempting to extend the short wavelenght limit, through the increased reflectivity associated with the total reflection phenomenon. The first order focusing properties of ruled concuve gratings, derived from Fermat's principle, are then well known and various mountings working in the gaussian image plane are currently used in this spectral range. If the well known Seya Namioka monochromator fulfil the two first conditions, the angle 2 Θ between the incident and diffracted beam is a constant equal to 70° 30'. This angle is chosen to minimize the deviations from the rowland conditions and therefere this mounting satisfy approximatively to the condition T + T' = 0 (eq. 3), T and T' being respectively the equation for the object and image tangential focus. In this mounting as in the Rowland circle or Wadsworth mounting the high order terms in the equations of Fermat's principle as well as the possibility of balancing different aberration terms has been ignored. It is well known, however, from lens design studies that a great improvement of an optical element can be achived, by using the phase balancing method and by a proper consideration of the focussing properties in both geometrical and diffraction theories. The principle of this method is to replace the spherical reference sphere centered about the gaussian image by a new reference sphere centred about the brightest part of a line at the best plane of focus. This method involves, then, a change of focus in order to decrease the amount of aberration (eq. 7) and consequently ignores all mathematical solutions of the first order focussing condition. If the wavefront aberration over the pupil is small with respect to the wavelength λ, the diffraction pattern is only slightly modified. Then by using the strehl criterion we obtain first the value of the parameters characterizing the change of focus and second the optimum ruled width W0 for a given height L0 of the ruled area for which the residual aberrations 01 (W2/R2) can be neglected. A generalized focussing condition in which the main aberration terms (except for astigmatism) are balanced by the defocussing term is consequently derived from the Fermat's principle (eq. 4). For large phase-errors we have defined a quality factor Q (eq. 8) characterizing the r. m. s. value of the width of the image pattern which is connected to the slope of the wavefront deviation. Such image evaluation implying an association of ray density and energy flux, the averaging in the image plane uses the geometrical intensity distribution in the image plane as a weighting factor. Using the same procedure as above we have obtained for each value of W0 and L0 the limiting resolution, a second expression for the generalized focussing condition (eq. 5, table 1). Generallly the tilt of defocus coefficients and then the type of correction are different for the physical optics and the geometrical optics limit (fig. 1 to 5). Using an iterative process the equation 5, has been solved for two particular wavelengths λi and λf and for an one meter 1 831,8 lines/mm concave grating. Figure 6 shows in function of θ, the variations of the object and image distance and leads to the design of a two entrance beams monochromator. Indeed to one image distance r' = Re' correspond two θ values θ1 = 10° and θ2 = 49° 48' 45" at which occur a perfect focus. For other λ values the instrumental defocusing (eq. 15) limit the useful spectral range and the practical resolution (fig. 7) which can be obtain. Indeed this defocussing must remain inferior to a tolerated value (equivalent to the deep of the focus of a lens) arising from the quality factor (eq. 16-16'). The A. S. M. 100/2E mounting made by creusot Loire Instrumentation using a single rotation of the grating gives in a fixed direction diffracted beams of spectral width of 3 to 5 A from 256 to 4 000 A. This mounting has been used in particular to test the efficiency of various holographic gratings and some results are summarized on figures 10 to 13.

On montre theoriquement que le coefficient de reflexion diffuse est la somme de deux termes dont l'un caracterise uniquement la direction de reflexion speculaire, et dont l'autre fait intervenir la pente des rugosites. Lorsque la hauteur de ces dernieres est inferieure a la longueur d'onde, l'expression de l'energie renvoyee comporte un terme proportionne la 1/Λ4, qui tendrait a faire croire que pour les courtes longueurs d'ondes l'energie diffusee est tres importante. En fait on demontre que si σ/Λ≫ 1, le second terme ne fait intervenir, dans le cas de distribution ‘normale’ des irregularites, que la valeur moyenne de leur pente. La premiere partie de ces experiences comporte letude de surfaces metalliques et les resultats obtenus permettent de souligner l'importance de ce facteur.

Une serie de mesures de diffusion ont ete effectuees a 1608 A sur des echantillons varies dans le but d'obtenir des materiaux ou des depots susceptibles de posseder des proprietes photometriques regies par la loi de Lambert. Plusieurs echantillons depolis de verre et de metaux differents ont ete etudies ainsi que des depots de ‘velvet coating’ noir et blanc, et de salicylate de sodium. Un detecteur a surface delimitee decrit un arc de cercle centre sur l'echantillon, et l'energie detectee est enregistree dans le cas de differents angles d'incidence. Le profil angulaire de l'energie reflechie de facon diffuse est donne pour des echantillons types. Les resultats obtenus montrent qu'un depot de salicylate sur de la magnesie peut jouer le role d'un diffuseur parfait dans l'ultraviolet lointain.

The purpose of this communication is to give quantitative values for the gain in luminosity which arises from optical corrections in the case of any far ultraviolet mounting. Using the Strehl criterion applied in the case of a line image given by a rectangular pupil, we have expressed the luminosity as a function of the optimum value of the resolving power. The results show the high luminosity of corrected symmetrical mounting.

The purpose of this communication is to design the optimum optical parameters of far ultraviolet corrected symmetrical monochromators using concave gratings.

A review of the principal features of reflected intensity distributions in the vacuum ultraviolet is given. The results can be explained in terms of the characteristic function of the distribution of the slope of the irregularities, when the rms height σ of the irregularities is very large relative to the wavelength λ.

On presente une etude experimentale de la distribution spatiale du rayonnement de fluorescence diffuse dans 4π sr par des depots de salicylate de sodium de diverses epaisseurs. Les resultats obtenus sont independants de la longueur d'onde du rayonnement primaire dans l'ultraviolet lointain; la distribution angulaire, differente suivant la face de l'echantillon observee, depend peu de l'angle d'incidence du faisceau primaire, mais varie avec l'epaisseur. Les couches epaisses constituent des diffuseurs parfaits.

The purpose of this communication is to design the optimum optical parameters of a corrected Seya-Namioka mounting, by using the Strehl criterion.

The problem of image evaluation in vuv optical mountings of ruled concave gratings has been treated by means of the method of phase-error balancing. In the classical diffraction limit, the tolerances on the aberrations of normal-incidence mounts are evaluated by means of a modified Strehl criterion. For large phase errors, a quality factor that implies an association of ray density with energy flux, leads to determination of the tolerated magnitude of large aberrations. In addition, we give the related generalized focusing conditions that include shifts, off the gaussian image plane, as functions of the amounts of aberrations.