Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1983
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983msngr..33...21b&link_type=abstract
The Messenger, No.33, P.21, 1983
Astronomy and Astrophysics
Astrophysics
9
Scientific paper
Surveying in extended field objects which are recognizable only by their spectral leatures would be an almost impossible task without instruments having simultaneously a wide field and some spectral discrimination capabilities. Monochromatic imaging with colour or interference lilters offers such a means, which has been widely used lor searching lor H11 regions and planetary nebulae in the Galaxy and in external galaxies. An alternative method is to use objective prisms or transmission gratings which supply for each object in the field 0f the telescope a spectrum, usually recorded on a photographie plate. However, the difficulties involved in manufacturing very large gratings or prisms, limit this method to telescope diameters 01 about 1 metre, the approximate size 0f the largest eXisting Schmidt telescopes. Fortunately, there is a variant 01 this set-up adapted to larger telescopes, in which a prism, or a grating, or a combination 0f both is inserted in the converging lightbeam at a short distance 0f the local plane. These devices, Usually litted on the prime focus adapter, are called GRISMs or GRENSes. A GRISM combines a transmission grating and a prism with opposed dispersion to compensate the aberrations (coma, astigmatism and field curvature) produced by the Wating in the convergent beam, and is associated with a wideheld corrector. A GRENS has a grating grooved on one lace 01 the last lens of a wide field corrector, and also has minimal aberrations. Both are blazed such that most of the light is concentrated in the first order. The GRISM technique has already been widely used to search for quasars through their emission lines, mainly at the Cerro Tololo Inter-American Observatory (Hoag and Smith, 1977, Astrophysical Journal 217,362; Osmer, 1982, Astrophys. J. 253, 28), and at La Silla for the detection of carbon and M stars in nearby galaxies (Westerlund, The Messenger No. 19, December 1979, p. 7).
Breysacher Jacques
Lequeux James
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