Astronomy Principles and Practice Fourth Edition.pdf

428 Practical projects
24.5 Solar disc phenomena: practical exercises
Occasionally, when there happens to be a large sunspot on the Sun’s disc and the Sun is seen through a
thick haze, it is possible to see the spot with the unaided eye. However, for a more serious study of the
solar surface phenomena, optical equipment is required and, in the first instance, a basic telescope is
essential. Useful work may be done with telescopes of 50 mm diameter or larger. Both a refractor and
a reflector may be used although it is generally accepted that thermal distortions of the optics when the
instrument is directed to the Sun are less severe for a refractor so giving better images than a reflector.
24.5.1 Visual observations
Danger to Eyesight
It cannot be emphasized too strongly that the Sun should
NEVER
be viewed using instruments without special precautions.
Ordinary telescope eyepieces sometimes carry a screw thread to allow absorbing filters to be
attached to them. This system is not suitable for use with the Sun as either the filter may not be
sufficiently absorbing or the solar heat may cause it to splinter suddenly.
Direct telescopic views of the Sun should only be attempted with special equipment designed for
the purpose. An old-fashioned device is the solar diagonal whereby the converging beam from the
telescope is split into two very unequal components. Only about 10% of the collected light is used for
viewing through an eyepiece, which also requires the addition of an absorbing filter; and the remaining
90%, which is in the second beam, is passed out of the system. An alternative is to fix an absorbing
filter securely over the telescope collection aperture. These are commercially available in the form of
coated mylar sheets stretched on a ring which fits over the end of the telescope tube.
24.5.2 Recording sunspots by drawing
A more convenient way of looking at the features on the solar disc is by the projection method. If
smooth white paper is held at a distance behind the eyepiece of a telescope which is directed to the
Sun, a large image may be focused on to the paper, this being very suitable for viewing by groups
of people. The best eyepiece to use is one of low power, preferably a Huyghens type in which the
component lenses are not cemented together. (Cemented lenses are prone to damage by the heat from
the Sun.)
By making a frame which can be attached to the telescope so that the paper may be clipped to a
board, it is then an easy matter to draw the positions and details of the solar markings. The contrast of
the projected solar image may be improved by also fitting a shadow board around the telescope tube.
A schematic projection frame is depicted in figure 24.19.
The projection technique also allows considerable detail to be seen and drawn. For example,
it should be noticed that the solar disc is not uniformly bright and that it fades towards its edge
(limb darkening). Sunspots and sunspot groups should be seen with structure—the spots showing a
strong core (umbra) surrounded by a less black region (penumbra) which may show structure such as
filaments radiating outwards. Small bright patches (faculae) may be evident particularly near to the
spots or towards the edge of the Sun where the limb-darkening effect produces a background with
lower illumination. The detection of faculae may be made easier by gently shaking the frame which
holds the paper.
When drawing the solar markings by the projection method, it is important to provide axes to
which specific points may be referred and the north–south, east–west lines may be generated for this
purpose. As convention, the north, south, east and west limbs of the Sun are defined according to the