OBSERVING THE SUN
by Brian Halls
The following is a guide for the astronomy newcomer on how to observe the Sun. Always seek advise from
someone if you are not sure how to safely observe the Sun.
Many people seem to forget (or are unaware) that our Sun is a star, just like all those bright points of light in the
night sky. Compared to those stars however our Sun is small and quite insignificant but to us it is the most
important star in the sky as without it there would be no life on Earth.
Unlike looking at the stars which are far away we have to be careful how to observe the nearby Sun as the
radiation emanating from it which gives us life on our world can cause serious eye damage if we were to look at
the Sun directly through any form of optical aid – binoculars, telescope, direct camera viewfinder or even naked
eye.
The safest way for using a telescope to observe the Sun is via a system called projection – the telescope is
pointed towards the Sun and an image is projected onto a piece of white card held behind the eyepiece; if the
card is enclosed in a box that excludes stray light a more detailed image appears (see below).
If the image is projected onto a card with a circle and grid, the positions of the sunspots and any other solar
features can be plotted and measured if required. This method dates back almost to the start of the invention of
the telescope way back in 1609.
Sunspots are areas of magnetic activity – magnetic fields within the Sun break through its extremely hot surface
– in the region of about 5500° C. The magnetic field has the effect of 'cooling' the surrounding area by about
2000° and these cool areas appear dark – sunspots!
Because of what they are, sunspots are not permanent objects – they grow, shrink and change their shapes.
The Sun also rotates on its axis in about 28 days so a sunspot that might appear at its eastern edge will
disappear at the other edge about 14 days later, that is if it lasts that long. It is not uncommon to see a small
sunspot appear, grow into a large group of several spots and then decay to a single sunspot once again, all in
the space of a few days. It is also not unknown for really large sunspot groups to last weeks or even months as
they rotate around the Sun.
What sort of telescope is best for observing the Sun?
Generally it is the refractor type of instrument – the telescope with a lens at one end, and an eyepiece at the
other. Care should be taken however – budget priced telescopes for example – generally have plastic
components in them and these will melt if exposed to the type of heat that will flow through the solar observing
telescope. If in doubt about what sort of telescope to use, have a chat with someone who has a bit more
experience and they will be only too happy to help.
For someone who regularly observes the Sun as often as weather and other commitments allow there are other
methods of observing our nearest star. Special solar filters that fit over the lens that both dim the solar image
and, really importantly, BLOCK the blinding radiation from getting to the eye are one method.
These come in two types – metallised glass coated with chromium and nickle layers, which are expensive but
long lasting or the metallised plastic foil that comes in sheets that can be cut to size and fitted into a holder.
These will need to be replaced every few years as dirt or small holes in the foil occur. Always read and heed the
instructions that come with the filters.
Glass solar filter with a holder
Solar foil also in a holder; the wrinkles do not effect image quality
Both kinds of filter require careful handling and storage when not in use. The glass type filter produces an
orange cast to the solar image while the foil produces a whitish-blue which oddly is a more natural looking
appearance to the solar image.
There is another kind of solar filter – actually a kind of eyepiece – that is employed. This is called a 'solar-wedge'
and is manufactured to extremely high degrees of accuracy. These allow sunlight to get into the optical path but
diverts the solar heat into a 'trap' or heat-sink. These are expensive and can not be made at home!
What will you see when observing the Sun using the above methods? The bright photosphere is the most
obvious feature. Astronomers tend to refer to this as the solar surface but actually it is actually the densest part
of the solar atmosphere where sunspots and faculae – bright cloud like features that are seen around the edges
of the Sun – exist.
As previously mentioned, the spots will be seen to evolve and decay as time progresses. Sunspot numbers
appear to increase and decrease over a period of about 11 years. These cycles have a pattern to them. New
sunspot bearing areas appear at high latitudes north and south and over a period of the cycle they will slowly
make their way to the solar equator. About four years into the cycle sunspot numbers increase and the Sun can
appear to be very spotty and this is solar maximum. Once maximum has occurred sunspot numbers begin to
decline and over the following six years or so sunspot groups reach the solar equator and begin to die out. In the
meantime sunspots of the new cycle will be making their appearance at high latitudes and the cycle starts
again.
For the real solar observing enthusiast yet another method of observing the Sun is also available. The methods
described, projection, aperture filters, let us observe the Sun in what is called white-light. We can also observe
the Sun at different light frequencies – the most common is hydrogen-alpha. By allowing us to observe the Sun
at specific areas of the solar spectrum we can observe things that we might only witness, fleetingly, at solar
eclipses.
The hydrogen-alpha (or h-alpha) telescope blocks out all the colours that white-light is made of until we see the
656.28 wavelength of the red part of the spectrum. Observing at these frequencies we can observe that
otherwise hidden world of the Sun called the chromosphere – a layer of solar atmosphere that lays above the
more visible photosphere.
We can see bright areas of hot hydrogen called plages that exist around sunspots, flares, filaments and
prominences.
A dark filament thousands of kilometres long, snakes behind a
sunspot (false colour view)
A filament arcs out from the Sun – filaments seen like this are
called prominences
An h-alpha telescope is another piece of precision
built equipment but once the Sun is observed using
one, an observer can be hooked for life. Solar
radiation is blocked by the special in-built filters and
safe solar observing can be done.
These kind of telescopes do not come cheap however
and are only of use to the regular solar observer.
The arrow points to a small backward C shaped flare over a
sunspot group; a number of filaments and bright plages are visible
With the proliferation and ease of modern digital
photographic techniques (relatively inexpensive digital
single lens reflex cameras, mobile telephones, tablets
etc) the urge to capture images of solar behaviour is
one that can now be easily fulfilled by an observer.
I hope these very brief notes on the basics of solar
observing are of use. Just remember – if in doubt
DO'NT – there will always be someone there to help.
Joining an astronomy group is a good way to find other
like minded people who enjoy the pleasure of looking
at the night (and occasionally) the day time sky. Help
and advise is on hand for anyone starting out in
astronomy.
Clear skies!
Brian Halls
A white light view of a large sunspot group, May 2015