A brief history of the OSN

Observatory of Sierra Nevada



In 1976 the recently created IAA used the installations and instruments of Cartuja, in Mohon del Trigo, also known as the Observatorio del Mohón del Trigo (OMT) belonging to the Company of Jesus. The observatory was situated in the Sierra Nevada (Granada) at an altitude of 2605 meters. The Jesuit University of Georgetown (USA) donated a Cassegrain reflecting telescope, KYOTO, of 32 centimeters to the OMT. The principle measuring instrument was a photometer based on the IP21 photo-multiplier with a manual filter-wheel containing Johnson UBV filters with an electrometer amplifier giving analogue records.

The principle line of investigation by the IAA in the OMT was limited to the photometric study of various variable starts. During these years the OMT was used intensively, frequently receiving invited astronomers and institutions. In particular a long collaboration was established with the Royal Greenwich Observatory (RGO) of United Kingdom involving a frequent interchange of personnel.

The birth of the OSN

From this collaboration, an agreement was reached in 1977 between the Science & Engineering Research Council (SERC) of United Kingdom and the Consejo Superior de Investigaciones Científicas (CSIC) for the RGO and the IAA install and operate jointly a 75 cm telescope in the Sierra Nevada.The IAA asked the CSIC to construct its own observatory, it not being possible to modify the OMT because it was not its property.

During the planning for the new observatory another agreement was reached by the CSIC with the Centre Nationale pour la Recerche Scientifique (CNRS) of France. Under this agreement the Nice Observatory would transfer a 60-cm telescope, a dome of 5 meters and a modern photometer for joint use in the Sierra Nevada.

Thus the new observatory had expanded to two domes. Several potential sites were studied for the location of the new observatory and selecting finally the Loma de Dilar in the Sierra Nevada. At an altitude of 2896 meters it had supporting infrastructure and the most suitable observing conditions.

The building

The building

The original concept, very simply, intended two domes of equal height joined by a connecting passage that would allow a future optical interconnection between the two telescopes.

The building plan view is rectangular nave of 10  by 20 meters floor dimension and with the mayor axis aligned east to west. 

There are two floors and a semi-basement. The two domes, of 5 and 8 metres, are mounted on circular walls situated on the extreme east and west of the roof. There is a living space below the level of the domes with the essentials for long stays in the high mountain.

Among the architectural peculiarities of the building are the pillars of the telescopes. These are very heavy in anticipation of future instrumentation of greater dimensions (it is presently thought possible to be able to accommodate telescopes up to 1m aperture). Both pillars are off-set to the south to accommodate equatorial mountings, ruling out the use of azimutal type or other mountings.

The work was completed in October 1981.

The telescopes

The british telescope, called  Stevenson Telescope in honour of first owner who donated the telescope to the RGO, was originally a reflecting Cassegrain type.

The telescope was modified in the workshops of the RGO by adding a third flat mirror to obtain two foci and mounting tracking instruments in the tube. The original mount of the fork mount type was adapted to the new latitude and the mount extended to accept larger instruments. It was transported from the RGO to the Sierra Nevada and was installed in the 8 m western dome of the OSN.

The Observatory of Nice Cassegrain reflecting telescope, with an aperture of 60cm and mounted on an equatorial table, was despatched from France. It was equipped with stepper motors and encoders for both axis and a sophisticated control console based on microprocessors.

In December of 1981 both telescopes were installed beneath their respective domes.

The instruments

The 60 cm french telescope arrived containing an integrating photometer with a Geneva system of filtering and giving analogue data outputs.The photometer was based on a 16 dynode Lallemand high gain tube cooled by closed circuit Carnot cycle cooling.

The Stevenson telescope was equipped with a photometer known as the 'Peoples Photometer' which was totally motorised and automated by the IAA. Also the telescope was provided with a photon counter and a data acquisition system which were designed and constructed by the IAA. The photo-multiplier used was EMI-6865-A that was selected for it's wide spectral response and low noise. This was cooled in a criostat by Peltier cooling. The UBV Johnson and uvby Stromgren filter systems were available for this photometer.

Shortly after the Strömgren photometer was installed in the second focus. This photometer was constructed in the observatory of the Copenhague University with the collaboration of IAA.

Strömgren Photometer

The photometer has six channels, four of them belong to Strömgren uvby narrow-band system. The data acquisition is simultaneous in the four channels thanks to a diffraction grating and four photo-multiplier tubes along with their associated electronic chains of preamplifiers, discriminators and counters, all designed and constructed within IAA. Alternatively the photometer can operate in the Hß mode using a beam splitter, two interference filters of wide and narrow bandwidth respectively and two photo-multipliers tubes with following electronic stages.

In addition to the instrumentation, a computer for automatic control and interactive real time data recording and processing was added.

The total automation of the Stevenson telescope was achieved in 1986.

The years of service

Telescopes provide service during 8 years. The optimisation and maintenance of the Stevenson telescope was performed by the IAA and at the same time as substituting new systems of control for the old ones. The Geneva photometer and the 60 cm telescope were improved and maintained by the Nice Observatory. The IAA installed a data acquisition system to allow digital data analysis by a computer.

Of the time allocated for observations over the years 1981-1989, the British used 70% of their time, the French used 80% and the Spanish used 90%. There were also invited observers from other institutions. The sky quality over the Sierra Nevada for this period was estimated at 60% suitable for photometric observation. 

The defects of the Steavenson telescope

Although the optical quality of the Stevenson telescope was good it was not the best but was considered sufficient for photometric operations.The developments introduced and the intensive use of the Stromgren photometer gave excellent results for differential photometry for a large number of observers. Nevertheless, thanks to shorter integration times, dark photometric nights and the optimization of the data reduction, the use of a smaller field of view (FOV 8") for the photometer was demanding more frequently. In doing so, a cyclic error was discovered in the sidereal tracking of the telescope. Since this time the quality of the observations was in doubt. The Stevenson telescope had achieved as much as it could and the pressure was for something better.

Thus the agreements with CNRS and SERC (previously SRC) were terminated and the 60cm telescope returned to France. The Stevenson telescope was given to IAA, who, in turn, donated it to the city of Granada who installed it in the Science Park located in the city.

The agreement with the Nanking Astronomical Instruments Factory (NAIF)

Various firms and professional telescope constructors wereconsulted over a new telescope. It was established that a suitable diameter would be 1m.

The equatorial mounting was a requirement dictated by the original construction of the OSN building. The offers received exceeded the finances available to the IAA even with the allocated support of the CSIC.

The IAA was deliberating over the budget necessary with the CSIC when the IAA had a visit from Professor Hu Ninsheng, director of the Nanking Astronomical Instruments Factory and a member of the Chinese Academy of Sciences.

Professor Hu and confirmed that it was possible to install a telescope up to 1.5m in the 8m cupola and a telescope of 90cm in the 5m cupola.

For commercial and political reasons Professor Hu was able to offer to supply both of the telescopes under economical conditions that were agreeable to the IAA and CSIC.

There followed the initial specifications and pre-design to confirm the findings of NAIF. A study was made of the implications of changes necessary to the building within the limitations established.

The support of the CSIC was obtained and a collaboration established between the CSIC and the Chinese Academy of Sciences with the IAA and NAIF to construct the two telescopes and associated control consoles.

The agreement was signed in Shanghai during November 1987. The optical and mechanical design was completed in the following six months. The IAA started the design of the control consoles in January 1988.

The construction of the telescopes started in June 1988 and finished in May 1991. The telescopes arrived at the OSN on the 25th of August 1991. 

The New Telescopes

Telescopio de 0.9 metros

Fig. OSN 0.9 m Telescope

The general characteristics established for the new telescopes from the beginning were to be:

Aperture: 1.5 and 0.9 m.
F number: f/8 for both telescopes.
Optical Design: Ritchey-Chretien.
Mount: Fork mount.
Foci: Double instrument carriers, enabled by switching of the tertiary mirror, for each telescope.


Eventually, to reach the desired optical specification quality (for photometry), the initial idea of a short focus, f:2.2, was abandoned. A thicker primary mirror close to 1/5 of the diameter was used and the primary focus relaxed to f:2.6. 

This led to a weight increase of the primary (1800kg and 450kg for the 1.5m and 0.9m respectively). This had consequences for the system of mechanical support.

NAIF preferred to abandon the existing solution for the axis bearing that would have proved expensive for this weight and opt for a pressurised oil bearing normally used in large telescopes.

Although this solution was excellent from the point of view of friction and precision it led to another overall weight increase.

The resulting telescopes were very robust but required the pillars of the OSN to be modified. For the cupola of the 90cm telescope a further modification was necessary to accommodate a larger cupola of 6.5m, an ASH DOME. 

Instalation and calibration

The mounting of the telescopes in the OSN commenced on the 2nd of September 1991 with the help of 12 technicians from NAIF and 6 from IAA.

From September to the middle of November was taken up mounting the telescopes and optics. At the end of November and beginning of December the first calibrations were made using the ANTARES instrument given by the European Southern Observatory (ESO).

After a slack period due to the winter, during which the installation of the control console took place, adjustments of the mechanics and optics was resumed for the summer of 1992.

From July to October, optical alignment and adjustment was performed with the help of Professor Hu Ninsheng. New calibrations were performed with ANTARES and a Schack-Hartmann device designed and made by Professor Hu.

The results were presented to the IAA in November of 1992 and considered to be satisfactory. The reception of the telescopes was signed.

On the 7th of October 1993 the new installations of the OSN were inaugurated by the Minister of Education and Science. 

Next Stage: The first generation of instruments

1994 saw a new phase in the operation of the OSN, the installation of the scientific instruments. The Stromgren photometer (described previously), the CCD cameras and the new optical fibre spectrograph ALBIREO occupied the foci for most of this year and the following year.

The Strömgren photometer with a more secure fixing is no longer accessible by the astronomers during an observation. An intensified camera is used to centre the object by remote control in case of errors as with the acquisition, reading and control.

The CCD cameras are in an adapter at the focal plane and incorporate auto-tracking to aid centralisation of the object.

The optical fibre spectrograph ALBIREO developed and being automated by the Observatory of Paris-Meudon (France) for remote control.

In 1990 the IAA acquired from the Astronomical Observatory of the University of Copenhagen a spectrograph and camera for faint objects, ALFOSC (Alhambra Faint Object Spectrograph and Camera).

ALFOSC has 10 slits covering 3.6 minutes by between 0.5 and 10 seconds of arc. There is a filter wheel with a capacity for 11 filters. The prisms used allow a coverage between 850 and 60 Å/mm and a spectral range between 3.500 and 10.000 Å.

The camera contains a Thompson CCD sensor of 1024x1024 pixels and is sensitive to the blue end of the spectrum.

The IAA has the task of planning and supervising the schedules for the telescopes and will form a Committee for the Allocation of Observation Time when the first instrument is operational.

The two new telescopes are open for the use of all the astronomical community, subject to the selection of proposals on their scientific quality and technical viability as assessed by a committee of astronomers.