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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