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Paper: The Modernized Spectroheliograph at Coimbra
Volume: 368, The Physics of Chromospheric Plasmas
Page: 549
Authors: Klvana, M.; Garcia, A.; Bumba, V.
Abstract: Application of CCD chips in astronomical instruments constructed for the registration of measured data on a photographic emulsion brings some special issues. The problems increase, if we need to digitize the whole solar disk. The spectroheliograph of the Coimbra university has been constructed for registration of the full-disk spectroheliograms on a photographic emulsion. Its construction is mechanically highly precise. Among others, it perfectly compensates the spectral line curvature in such a way that the resulting monochromatic solar disk is without any deformation. The compensation requires an adjustment of the entrance and exit slits, which are both curved.

The fact that during the transition from the photographic to the CCD registration it was necessary to preserve the original function of data registration on a photographic emulsion brings a whole number of specific problems. It was necessary to change the parameters of the optical system, to remove brightness and shape deformations of the solar disk, originating from the recording medium change. In this paper we analyze the mentioned issues and we describe how to treat them. We also present alternative spectroheliograph usages, for example for measuring Doppler velocities.

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