Astronomy Principles and Practice Fourth Edition.pdf

340 Modern telescopes and other optical systems
Figure 20.7. A Schmidt telescope or camera.
coverage as wide as 25 ◦ . They are primarily intended to be used as high-speed cameras rather than as
conventional telescopes.
One of the obvious uses of Schmidt telescopes is to allow star surveys of large areas of the sky.
Perhaps the most famous survey undertaken is that of The National Geographic Society—Palomar
Observatory Sky Survey whereby 935 pairs of plates from the blue and red parts of the spectrum have
been obtained covering the whole of the sky north of declination −33 ◦ , each plate having a field
diameter a little larger than 5 ◦ . The Palomar Schmidt camera used in the survey has a corrector plate
aperture of 1·25 m (the telescope is known as the 48-in Schmidt), a main mirror diameter of 1·83 m
(72 in) and the system is used at f/2·44.
A complementary survey of the southern hemisphere skies involves the UK 1·2 mSchmidt
telescope at Siding Spring Observatory W20.6 , Australia (see figure 20.8). With the newer emulsions
now available, this survey records objects one magnitude fainter than the Palomar survey. The
photographs are taken on 14 in square glass plates, each covering a wide field of 6·◦5. On each exposed
plate, there may be a million images. The task of measuring their brightness and position is a colossal
one and special scanning systems have been developed to extract the enormous quantity of information
recorded on such plates (see section 19.2).
The plates are stored at the Royal Observatory Edinburgh, UK, with high quality copies being
made available to astronomers throughout the world. Together with other astronomical plates, they
form an extremely valuable archive.
Another use of the Schmidt camera is for recording the positions of artificial Earth satellites.
Although the basic Schmidt system provides good quality images, they are not perfect. It is
obvious that some chromatic aberration is introduced by the correcting plate. The plate also only has
a correct shape for removing spherical aberration for one particular wavelength and, thus, the residual
spherical aberration is wavelength dependent. Coma is also present in the images away from the centre
of the field.