Astronomy Principles and Practice Fourth Edition.pdf

348 Modern telescopes and other optical systems
Figure 20.16. A simple coelostat.
20.9 The coronagraph
Until the invention of the coronagraph, the Sun’s corona could only be seen on the occasion of a total
eclipse of the Sun. The brightness of the corona is generally much less than the brightness of the
daytime sky which surrounds the Sun. However, by choosing a good observing site at a height in the
mountains, the amount of scattered light seen round the Sun is reduced and the chances of observing
the corona are greatly improved.
An ordinary telescope gives rise to scattering in the optical system and this would again prevent
the corona being seen. A specially designed instrument, called a coronagraph, has been developed
which provides an obscurator for the solar disc and keeps any scattered light down to a minimum. The
essential parts of a coronagraph are depicted in figure 20.19.
The objective is a single-element lens, specially chosen for its freedom from blemishes such as
air-bubbles and scratches which give rise to scattering. A metal cone is fixed to the field lens so that
the Sun’s disc is obscured and its light reflected out of the optical path. A diaphragm with a small
central obscurator is placed prior to the camera objective to eliminate light which is diffracted by the
coronagraph objective. Although the camera lens can provide correction for the chromatic aberration
which is produced by the single element objective, the obscurator, in any one position, is effective
only for a narrow band of wavelengths. This is not too much of a disadvantage as narrow-band filters
are frequently applied in coronal measurements. Use of the instrument over a range of wavelengths is
achieved by adjusting the position of the obscurator.
With the aid of the coronagraph, a much longer observation time is made available than at an