The building  that contains this instrument is rather unusual: it comprises a masonry and a carriage on rails which, when  moved out,  lets appear  a bent  refractor. The mechanics was built by Gautier. The objective  of  40cm  diameter and  9 m focal distance,  and the mirrors were cut and polished by the Henry brothers.     It is a typically French instrument developped  by Maurice Loewy.   It is not astonishing  that Raphaël Bischoffsheim who funded the first  coudé equatorial  in Paris made another sample in Nice.     The usual equatorial  required a continual displacement of the observer, to follow the movements of the eyepiece, in incomfortable and tiring positions.  Moreover they required the operation of a heavy cupola.    Loewy had the idea to apply to this instrument the principle of the "bent  refractor" used by the German astronomers  for small meridian instruments, that gets rid of these two drawbacks.     The optical beam of the  refractor is bent at  right angle, using a mirror at the top, which forms an angle of 45°  with the incidental rays and the reflected rays, and another mirror inclined of 45° in front of the objective. The mirror-objective unit turns around the  optical axis  and  can  look successively at all the points of an horary circle. The movement around the polar axis allows  to point all the horary circles.    The eyepiece remains fixed whatever the  direction of the sky is observed .  A camera installed  at the focus refractor made this  coudé equatorial  an appropriate instrument to the quest of  comets,  which was the aim of its installation in Nice.

Seven   coudé equatorials  were built between 1887 and 1893, by Gautier and the Henry brothers in Algiers, Besançon, Lyons, Paris (2), Nice and Vienna, in Austria. This last, was gifted to the Observatory of Vienna  by the baron Albert de Rothschild  to pay tribute to  Loewy   who began his career in this establishment.

  The coudé equatorial of Nice is the largest one still  in use. It was used  until 1940  for  the  search  of comets,  small planets and  the determination of their positions . In 1969, it was restored and  dedicated to   solar observations,  in particular for the study of the solar granulation and the preparation of the first experiments of  helio-seismology.
 

Raymonde BARTHALOT
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 References:

TH. WEIMER,  Un instrument en voie de disparition : l'équatorial coudé ,  Journal for the History of Astronomy, vol.13 (1982).
A. DANJON, A. COUDER, Lunettes et télescopes, Blanchard, Paris, 1935, nouvelle édition en 1979.
B.  H. PERROTIN, Annales de l'Observatoire de Nice, (1899), 51-64.
Rapports annuels sur l'état de l'Observatoire de Paris, 1897-1940, Imprimerie Nationale, Paris.