Astronomy Principles and Practice Fourth Edition.pdf

The coelostat 347
be viewed simultaneously with the field which can be seen via the two reflections given by I and the
lowerhalfofH .
To permit solar observations, filters of various density are available to cover both the I and H
mirrors.
It is obvious that when the mirror faces of I and H are parallel, I and the transparent half of H
provide identical views of the horizon. By adjusting the lever which carries I, the two images can
be made to overlap, thus allowing the fiducial point or index point to be determined on the graduated
scale.
When the altitude of a celestial object is being determined, the lever carrying I is adjusted until
the object is made to ‘appear’ on the horizon. Since a reflection by a mirror alters the direction of travel
of a beam of light by twice the angle of incidence, an angle read off the graduated scale corresponds
to twice the apparent altitude of the object. This is allowed for in the graduation of the scale, each 10 ◦
of rotation being registered as 5 ◦ . Before an accurate figure of the true altitude is available, corrections
need to be made for any error in the index position, for the observer’s height above the horizon and for
atmospheric refraction. The uncertainty in any one determination is likely to lie in the range between
5 and 10 seconds of arc.
20.8 The coelostat
The coelostat is a special collector system, usually applied to solar observations, providing an image
which is fixed in space. Such a system allows heavy subsidiary analysing equipment to be used. Most
coelostat systems are placed at the top of a tower with the analysing equipment below the level of
the ground where it is more easily stabilized against temperature fluctuations. The positioning of the
coelostat at a height above the ground is optimum in obtaining the best seeing conditions and the
collected radiation’s path to the analysing equipment can be made virtually free from disturbing air
currents by enclosing it in a thermally insulated tube.
The collecting system consists of two circular flat mirrors as illustrated in figure 20.16. The first
mirror is set in a cradle so that the polar axis is parallel to and passes through the centre of the front
reflecting face. The second plane mirror reflects the beam into some imaging device, such as a longfocus
objective, although, in some systems, reflection optics are used throughout. Rotation of the first
mirror about the polar axis, at a rate equal to half the rotational speed of the Earth, allows celestial
objects to be followed.
The range of declinations is achieved by adjustment of the relative positions of the two mirrors.
As in the case of a solar tower, the image-forming optics are in a fixed position and, consequently,
the movement of the second mirror of the coelostat can only allow adjustment by tilt. The first mirror
and its drive are placed on a carriage which can be positioned along the north–south line. For each
declination setting, the position of this carriage and the tilt at the second mirror need to be adjusted.
In order to overcome the problem of the second mirror blocking-off some parts of the sky from
the view of the first mirror, two sets of runners are usually provided for the carriage which holds the
first mirror. These runners lie on either side of the meridian which passes through the second mirror.
For solar observations, it is easy to visualize that the runners on the east side would be chosen to hold
the carriage for pre-transit (morning) observations and that the carriage would be moved to the runners
on the west side for post-transit (afternoon) observations.
There are also two other collector systems which are sometimes used to provide a solar image.
These are the heliostat and the siderostat. The heliostat has the disadvantage that, although the centre
of the solar image is kept in a fixed position, the image rotates slowly. A siderostat is basically a singlemirrored
system but the driving mechanism required to achieve a stationary image is complicated.
The McMath solar telescope, which is a heliostat design, is shown in figure 20.17. The mirror has
a diameter of 1·5 m and the system provides a solar image 80 cm across (see figure 20.18).