Abstract. The National Research Institute of Astronomy & Geophysics in Egypt operates the Kottamia Observatory in the desert some 100 km from Cairo. The 74-inch telescope at Kottamia is the largest optical/infrared telescope in North Africa and the Middle East. The Carl Zeiss company was recently contracted to modernize the optical system of this telescope. New Zerodur mirrors were manufactured by the Schott Glassworks in Mainz and the primary and secondary mirror cells were modified by Zeiss. Numerous modifications and adaptations were required to ensure proper integration of the new and old systems on this telescope. Equipped with new optics, the telescope is poised to embark on numerous astronomical research projects of a local, regional and international nature.
Sommaire. L'Institut National de Recherche en Astronomie et en Géophysique Egyptien (NRIAG) exploite l'Observatoire de Kottomia situé en plein désert à quelques 100 km du Caire en direction de Suez. Le télescope de 1,93 cm de diamètre de Kottomia est le plus grand télescope optique et infrarouge d'Afrique du Nord et du Moyen-Orient. Récemment, la Société Carl Zeiss a été chargée de moderniser le système optique de ce télescope. De nouveaux miroirs en Zerodur ont été fabriqués par la Société Schott Glassworks de Mayence et les barillets des miroirs primaire et secondaire ont été modifiés par la Société Zeiss. De nombreuses modifications et adaptations ont été nécessaires pour s'assurer d'un assemblage correct des nouvelles et anciennes parties de ce télescope. Equipé d'une optique nouvelle, le télescope est fin prêt pour être engagé dans de nombreux projets de recherche en astronomie aux niveaux local, régional et international.
In 1905, Sir Reynolds, an amateur astronomer at that time and later treasurer of the Royal Astronomical Society in London, presented Helwan Observatory with a 30-inch reflecting telescope.
Thanks to the clear sky of Helwan and the astronomers' skills, Helwan Observatory soon grew in importance to become one of the leading centres at that time. Observations were essentially photographic, and several hundred photographic plates were exposed over a period of 50 years of nebulae, comets, the eighth satellite of Jupiter, and Pluto. When it became evident that the instruments' capabilities did not meet the requirements of new tasks, the astronomers in charge recommended to the the authorities the purchase of a larger telescope equipped with spectrographs. The Egyptian government signed a contract with Messrs. Grubb Parsons of Newcastle, UK for a 74-inch (1.9-m) telescope in the same year (1948) that the giant 200-inch telescope on Mount Palomar, California was erected. The Kottamia telescope (Fig.1) is similar to the 1.9-m telescopes at Mount Stromlo in Australia and at the Observatoire de Haute Provence in France. It is equipped with both a Cassegrain and a Coudé spectrograph. To house the new equipment, a new observatory was built in the desert at Kottamia, some 100 km from Cairo. The dome is shown on the front cover of this publication.
The quality target specified by the Egyptian astronomers was a wavefront aberration of less than one-twelfth of the 633 nm measurement wavelength, i.e. the mean deviation from the ideal shape of the optical surface must not on average exceed 52 nm. To ensure that this quality could be maintained after the installation of the mirror in the telescope, the primary mirror cell had to be modified accordingly.
The mechanical modifications had to ensure that major design parameters of the telescope tube, such as the spacing of the mirror vertices of the existing position of the focal plane, remained unchanged. It was essential that the modification of the mirror support system did not affect the overall balance of weights and moments of force in the telescope, as optimum balancing of the instrument is of primary importance for tracking in the sky.
During the subsequent stages of manufacturing and testing of the mirrors, NRIAG scientists visited Carl Zeiss in Jena several times and were always satisfied with the results obtained at the individual stages. The performance and results of all tests were verified by NRIAG representatives and by an independent expert commissioned by NRIAG, and were meticulously documented in a test report. Figuring of the primary mirror comprised two stages: first, the mirror surface was machined to a spherical shape, providing optimum conditions for the subsequent aspherization process. This was followed by mechanical and interferometric testing. In the second step, the mirror's aspheric shape was produced as specified by the optical designers. To enable the necessary testing, the use of an additional lens system was indispensable to compensate for the optical power of the aspheric surface.
The mirror was the provided with a reflective aluminium coating and a protective coating. The convex secondary mirror was manufactured at the same time.
The 1.88-m telescope in the open dome.
The maximum width of the opening is 5 metres.
The old mirror cell had been provided with thermal insulation which, when removed, revealed a surprise: the amount of work involved in the modification proved to be much greater than expected. The central base plate inside the cell had to be completely removed and replaced by a new, ribbed structure. This was the only way of ensuring a stable basis for the 18 new axial supporting systems (Fig.2). All mirror supporting systems have been designed as purely mechanical, maintenance-free weight-lever systems. Three assemblies mounted on the edge of the mirror cell and acting on the mirror's circumference fix the mirror in the radial position. The design of these assemblies guarantees compensation for the different thermal expansion behaviours.
The mirror cell. The 18 supporting points ensuring stable seating
of the mirror are clearly visible in the cell.
The secondary mirror cell was also modified to compensate for the differences in expansion behaviour between the steel cell and the ceramic glass of the mirror.
All these operations once again confirmed the experience that it is usually easier, in technical terms, to manufacture an entirely new instrument than to integrate state-of-the-art features into an instrument which is several decades old. This started with the use of imperial threads necessitated by the interfacing with the telescope, continued with a material analysis of the reused cell components and extended to the production of adapter components to match the vertex focal length of the optical system to the existing instrumentation.
After the alignment of the mirror system and rebalancing of the telescope, the installation process was completed on July 5, 1997. An initial test showed that the systems were working properly. The acceptance test of the optical system was performed in the autumn to verify to NRIAG the alignment status of the telescope in the presence of the highest political representatives of the Republic of Egypt.
Inquiries have already been received from astronomers abroad who are interested in using the telescope. With its new optical system, the upgraded telescope will participate in local, regional and international projects dealing with the study of variable stars, star clusters, interstellar matter, stellar structures, stellar atmosphere, comets, meteors, minor planets, celestial mechanics, stellar dynamics and other fields of astronomy.
Figure 3: The installation of the new primary mirror in its modified old cell was an exciting challenge.
This article originally appeared in the Carl Zeiss magazine Innovation, No. 3, October 1997, pp. 18-21. We thank Carl Zeiss for permission to reprint it with adaptations for African Skies/Cieux Africains. The assistance of Ms Gudrun Vogel of Carl Zeiss is gratefully acknowledged.