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The Schmidt Camera
This instrument was built in 1952 by Grubb-Parsons of Newcastle-upon-Tyne
and replaced an older telescope in the existing dome, which had been
made by T. Cooke & Sons Ltd. of London & York at the time of the move
of the Solar Physics Observatory from South Kensington to Cambridge.
It is a `Classical Schmidt' - the simplest and most efficient form of
the ingenious wide-field camera invented in 1930 by Bernhard Schmidt
of Hamburg Observatory. Light from the sky falls upon a 61 cm
(24-inch) mirror with a spherical reflecting surface, at the bottom of
the tube. It is reflected to a focus in the centre of the tube and
half-way up it, 163 cm (64 inches) from the primary mirror. At the
focus a photographic plate P 15 cm (6 inches) in diameter, which must
be bent to fit a curved surface, records the star images in an area of
sky 5 degrees in diameter. (The full Moon is half a degree in
diameter.)
Without any further optical element the star images would be of poor
quality owing to ``spherical aberration'': light falling near the edge
of the mirror would come to a focus too close to it, and light falling
near the centre of the mirror would be focused a little too far away.
Schmidt's invention was to place at the centre of curvature of the
primary mirror, near the top of the tube, a weak meniscus lens (in
this case 43 cm (17 inches) in diameter) with one aspheric optical
surface: this makes the light which passes through it near the edge
diverge slightly, lengthening the focus of the outer parts of the
mirror, and makes the light passing through near the centre converge,
shortening the focus of the centre of the mirror. This optical
combination of lens and mirror forms a fast, efficient camera giving
sharp star images of uniform quality over the full 5 degrees field. It
is an ideal sky-surveying instrument; by contrast the
36-inch (91.4 cm)
telescope, with its paraboloidal mirror of 4.1 metres (162 inches)
focal length (f/4.5), has a field of view only 7.2 arc minutes in
diameter with images smaller than 2 arc seconds.
The auxiliary 15 cm (6 inch) telescope is for guiding. The exposure
time is usually of the order of 10 minutes, and during this time the
image can wander about on the photographic plate mainly because of
irregularities in the refraction in the earth's atmosphere. These are
corrected by maintaining a star image at the intersection of the
cross-wires in the guiding telescope.
The Palomar and U.K. 48-inch (1.22 metres) Schmidt cameras which were
used to make the all-sky surveys (now kept in the
Cambridge
Astronomical Survey Unit)
have apertures nearly three times as large
as our telescope, but focal lengths (and tubes) only twice as
long. Only one Schmidt camera (the 53-inch (1.35 metres) at
Tautenburg, Germany) has ever been built larger than these two. The
reason is that, if a Schmidt camera is simply scaled up, its image
size is also scaled up, and as Bernhard Schmidt himself predicted, the
48-inch Schmidts are close to the practical limit. The main image
defect arises because the thin lens can correct the ``spherical
aberration'' of the mirror exactly in only one colour of light,
(usually blueish green), red light is under-corrected, and blue or
ultra-violet light is over-corrected. To minimise the length of the
tube, and so the size and cost of the dome, the 48-inch Schmidts have
been made with an aperture of f/2.5 (ours is f/3.7) and the
``spherical aberration'' of the mirror is then 3.2 times as large as
in our camera. Three of the largest Schmidt cameras have been fitted
with ``achromatic'' lenses which reduce the residual colour errors,
but astronomers now use very fine-grain emulsions, and wish to observe
a wide range of colours of light, so these large Schmidts are still at
the practical limit of size.
Fortunately, a new design of wide-field telescope, using three mirrors
and no lenses, has been developed;
a prototype can be seen nearby.
2002 October
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