• N C Das

      Articles written in Pramana – Journal of Physics

    • Aberration-corrected catadioptric magnifiers useful for simple microfiche readers

      M Vxxx R K Murty N C Das R P Shukla

      More Details Abstract Fulltext PDF

      The present paper considers the magnifying power and aberration properties of three different catadioptric magnifying systems useful for application as simple hand-held microfiche readers. These are in the form of a plano-convex lens, a meniscus lens and an achromatic doublet lens and all their outer surfaces have been coated with a semi-reflecting film so that they behave like catadioptric elements. These can be designed to give magnification in the range of 15 × to 25 × with sufficient eye relief. Using such magnifiers, it is possible to read microfiche having frame sizes of 16 × 12 mm and 12 × 9 mm. The cemented achromatic doublet coated on the outer surfaces seems to give a very good image for use in microfiche reader.

    • Flat field spectrograph using convex holographic diffraction grating and concave mirror

      N C Das M V R K Murty

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      In this paper we have discussed the aberration properties and the design procedure of a spectrograph which uses a convex holographic grating as the dispersing element and a concave mirror as the focusing element. Both the concave mirror and the convex grating have common axis and they are concentric. The grating is constructed by recording the interference fringes on the convex surface which are formed by two convergent light beams incident from opposite sides of the axis. The illuminating source of the spectrograph is located on a plane perpendicular to the axis and passing through the common centre. It has been found that under certain conditions both positive and negative order spectra are perfectly focused on this plane. Aberration properties of the zero order image as well as the positive and the negative order spectral images have been studied by actual ray tracing. Using this system, design parameters of a medium-sized spectrograph having moderate dispersion and good resolution throughout theuv-visible region of the spectrum have been specified. The performance of the spectrograph has been evaluated by plotting spot diagram.

    • Holographic grating based Ebert spectrograph

      N C Das M V R K Murty

      More Details Abstract Fulltext PDF

      In this paper we discuss the aberration properties and design procedure of in-plane Ebert spectrograph using conventional as well as holographic diffraction gratings. In both cases the gratings are situated at the well-known √3-position so that the spectrum can be recorded on a flat surface. It has been found that the holographic grating system has better resolution than the conventional grating system. The design parameters of a medium sized holographic grating spectrograph in which a concave spherical mirror is mounted in the off-axis configuration have been specified. The performance of the spectrograph has been evaluated by plotting spot diagram.

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