A Warning

Before describing how to observe and image the Sun, I must give the standard warning. The Sun is the only astronomical object that could cause harm to an observer. If any observing aid, binocular or telescope is used to directly observe the Sun without the use of suitable solar filters, the retina can be irreparably damaged, leaving a blind spot or worse. It is not so much the visible light as the infrared radiation that is the problem. So if a filter is used, perhaps to observe a partial solar eclipse, it is vital that it be opaque to infrared. Any filter used to observe the Sun must be specifically designed for this use and it must be one that will totally block the infrared emission whilst bringing the visible light down to safe levels.

When the Sun is observed (and very great care is taken to use appropriate filters!) it appears to have a sharp edge but there is, of course, no actual surface. We are, in fact, just seeing down through the solar atmosphere to a depth where the gas becomes what is called ‘optically thick’. This deepest visible layer of the atmosphere is called the ‘photosphere’ (as this is where the photons that we see originate) and is about 500 km thick (Figure 12.1). The effective temperature of the photosphere is ~5,800 K. The convective transport of energy from below gives rise to a mottling of the surface − solar granulations that are about 1,000 km across. Each granulation cell lasts about 5–10 minutes as hot gas, having risen from below the surface, radiates energy away, cools and sinks down again.