Ertel, Munich 1862

Glass, metal

INAF – Rome Astronomical Observatory

Lorenzo Respighi and Angelo Secchi are indissolubly linked to the invention of the objective prism, a device which allows stellar spectra to be observed at once in the visual area of the telescope. In 1869 Respighi experimented his objective prism, by placing it before the Merz telescope of the Campidoglio Observatory. He must be given the merit of having picked out this research technique from stellar physics. The idea of using an objective prism was in fact first proposed by the German astronomer Joseph von Fraunhofer, who soon abandoned it though, for technical reasons due to the prism characteristics. Respighi realized that the prism used by Franunhofer had too large a refracting angle and he had a rectangular- based prism of only 12° aperture, purpose- built by Ertel in Munich. In the same period Angelo Secchi, as well, had an objective prism built, which he used with the telescopes of the Collegio Romano Observatory, with the Cauchoix refractor (see Rm. 01) and the large Merz equatorial telescope (24 cm aperture) which was destroyed by a fire at the Rome Observatory in July 1958. Secchi and Respighi augmented on who was the inventor of this instrument, as the first observations carried out with the respective prisms, were published almost at the same time. The objective prism has been largely used for astrophysical research throughout the 20th century, especially in conjunction with Schmidt-type telescopes, to make wide field photographs of thousands of stellar spectra. [a.a., m.f., f.p.]

Bibl.: Calisi 1991, p. 41; Chinnici 2000; Altamore 2012.