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Jacobus Kapteyn Telescope

General Information

The Jacobus Kapteyn Telescope (JKT) has a parabolic primary mirror of diameter 1.0 m with two interchangeable secondaries. It is equatorially mounted, on a cross-axis mount, which allows operation east or west of the pier. Normally it is east of the pier. There is a choice of two secondary mirrors. The f/8.06 Harmer-Wynne system uses a spherical secondary and a doublet corrector to give a field of 90 arcmin diameter for photographic astrometry over a wide field. The other secondary is a hyperboloid, which gives a conventional f/15 Cassegrain focus. The JKT normally operates in f/15 mode.

The optical telescope assembly weighs 13,615 kg, and including the mounting (and no instruments) 39,915 kg.

In the early 1960s there was a flourishing school of photographic astrometry at the RGO. It was found that very  accurate proper motions could be determined by comparing plates taken 50 or more years apart with the same telescope. Too strict a reliance on old telescopes was not a policy which could be pursued indefinitely and thoughts turned to the design of a dedicated astrograph incorporating the latest technology, especially in optical design.

The 1973 Scientific Case for the Northern Hemisphere Observatory called for three telescopes, the smallest to be a 1 m similar to the Boller and Chivens telescope of that size on Siding Spring Mountain. This telescope has f/8 and f/18 secondaries interchangeable with a flip top arrangement. The f/8 arrangement is a Ritchey-Chrétien which requires an elliptical primary. Thus the f/18 is not a true Cassegrain which requires a parabolic primary. The Ritchey-Chrétien is free from coma by design and the field is limited by astigmatism which amounts to an arc second 20 arc minutes from the axis. Moreover the optimal focal plane is curved concave to the secondary with a radius of 135 cm.

While this design is not the ideal astrograph it was quickly realized by those who had been urging the case for a new astrograph that the differences in specification were small enough to be negotiable. Nevertheless it was clear that there was no ideal astrograph and that each parameter of the final design must be a compromise between trade-offs in different directions.

The first parameter to establish was the effective focal length. f/8 had been chosen for the Siding Spring telescope because this matched a seeing disc of one arc second to the granularity of the Estman 103a0 emulsion. Since that time finer grain emulsions such as the Kodak III range have been introduced but by a happy accident the seeing on La Palma has proved better than anticipated so the 8 m focal length does not introduce a marked mis-match. If shorter focal lengths are used then the detector (photographic plate) for stellar images is undersampled but speed is gained on extended objects because each unit area of plate receives more photons. Unhappily this is also true of the brightness of the night sky which limits the exposure during which the emulsion can register fewer photons so that the signal/noise suffers. Because of its granularity the photographic plate can only store so much information per unit area, with fine grain emulsions able to store much more. As the focal length of the telescope is increased the seeing discs are spread over a greater area of emulsion which can contain more information if the exposure is long enough to gather it. This argument implies that the limiting magnitude of a telescope is purely a function of its focal length; the only limitation is the prohibitive length of the exposures.

Having decided on an overall focal lengthof 8 m we are still free to fix the focal length of the primary. A short focal length to the primary implies a shorter tube so that the telescope can be covered by a smaller dome. As the dome can cost a large fraction of the total cost of the project, and this cost rises sharply with size, this is a powerful consideration. Likewise, it is easier to construct the telescope tube with the requisite stiffness when it is comparatively short. The strongest argument for a longer focal length arises indirectly from the astrographic requirement that the field must be flat. With a curved focal plane the plate must be bent in two directions during exposure which results in internal stresses. These stresses are relieved when the plate is removed from the plate-holder for processing and ultimately for measurement. To reduce measurements it is necessary to assume that both the glass plate and its thin coating of emulsion behave purely elastically under stress. This was felt to be a dangerous assumption in the most precise astrometry so it was specified that the field should be flat. Now Petzval's theorem implies a direct connection between the field curvature and the focal length of the primary in a two-mirror system so that a flat-field requires a primary focal length of 4.6 m, a metre longer than the Siding Spring telescope. The astrographic requirement was felt to be the overriding consideration and the focal length of the primary was fixed at 4.6 m in spite of the contrary arguments rehearsed above. The only other argument in favour of such a long focal length was the reduced amount of ceramic which has to be ground away from the primary and the relative ease of polishing and figuring.

With the focal lengths of the two mirrors fully specified the astronomer is still free to decide on the field size. Although he wants the biggest field possible, the larger the field the more difficult it is to design a system with acceptably small aberrations. For precise astrometry symmetrical aberrations like astigmatism are less dangerous than asymmetric ones like coma. Because the photographic plate is a non-linear detector the apparent centre of an asymmetric image varies with the length of exposure and this is quite unacceptable in an astrograph. Equally unacceptable is for the apparent separation of two stars to be a function of their colours. With these requirements in mind astronomers at RGO embarked on a series of conversations with Prof C. G. Wynne FRS and C. F. W. Harmer to design a telescope with a field in excess of one degree and acceptably small, symmetric images whose positions were independent of colour. The outcome was a field diameter of 1.5 degrees with symmetrical images nearly all smaller than 0.5 arc seconds over the wavelength range 365-852 nm. The parabolic primary could provide a conventional Cassegrain with a hyperbolic secondary but in the wide-field mode uses a sperical secondary, totally insensitive to disalignments about its centre of curvature. The wide-field correction is provided by an afocaldoublet in the middle of the tube, with all surfaces spherical for ease of fabrication. Both components are of the same glass type so that there are no chromatic effects. For a 1 m telescope the corrector lens is 326 mm in diameter which implies that this design cannot be extrapolated to very large telescopes. 

The design of the telescope was fixed by 1977 well in advance of the SERC decision to support the NHO project or the firm decision to site the observatory on La Palma. The order to build the telescope came from the Department of Trade and Industry, inspired by the then government of Great Britain in an effort to relieve unemployment in Newcastle-upon-Tyne. The firm of Sir Howard Grubb Parsons were asked for a conventional 1 m telescope suitable for sale to a wide range of customers. The RGO produced a User Specification for the instrument but the engineering specification was produced by Grubb Parsons. Their intention was to provide a conventional analogue control system which would operate by itself or which could easily be interfaced with a computer, whichever the customer wished. Thus the RGO had little control over the detailed design of the telescope and the adaptation of the design to computer control caused much more trouble than either party had expected.

The telescope was erected on the ground floor of the Isaac Newton Telescope building at Herstmonceux in 1982 and interfaced to its computer. It was shipped to La Palma in the summer of 1983; its base plate was the landing pad for a Royal Navy Sea Harrier which put down on the cargo ship Alraigo after losing its aircraft carrier. The telescope was built into the dome on La Palma in October 1983 and the building handed over in January 1984. The telescope control system was installed in February and March and the first plate taken in March 23/24 1984. The first scheduled astronomers used the telescope on May 29.

It was an initial user requirement that the telescope should have an uninterrupted horizon in all directions so that whenever a supernova, bright comet or other interesting transient object appeared, at least one of the LPO telescopes should always be able to observe it. For this reason the JKT was sited to the south of the INT and 22 metres above it close to the Caldera lip. A limiting condition was the ruling by the National Park Authority (ICONA) that all the telescopes must be below the skyline when viewed from Cumbrecita on the opposite Caldera Wall so as not to destroy the beauty of the national park. The ability to view the horizon in all directions is aided by the telescope's cross axis mounting design which allows any star in the sky to be observed from above the intersection of the axes.

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Last modified: 10 April 2012