Seeing Stars - Inverness Courier, Fri 7th May, 2010
Great Ball of Fire
By Antony McEwan - Highlands Astronomical Society
The Sun is our best friend and our worst enemy. Our local star provides us with the light and heat we need to live, and yet bombards us with dangerous radiation that could destroy us if it were not for our atmosphere and magnetosphere. Without our protective shields against the deadly solar radiation Earth would be unable to support our vast range of species.
Much has been written about the composition and activities of the Sun; for example, the Seeing Stars article of July 2007 and June 2005. I intend for this article to focus more closely on what features can be seen on the Sun by amateur observers, and how to go about seeing them.
Safety first. Never look at the Sun through a telescope unless it has a special full-aperture solar filter fitted, or unless it is a telescope that has been specially designed or modified for solar observing. Dark glasses or smoked glass will not suffice. If possible, seek advice from an experienced astronomer before viewing the Sun. More on that later.
The most traditional mainstream way of solar observing is the projection method. A telescope is set up and pointed at the Sun, with an eyepiece in place. Care has to be taken when pointing the telescope not to look through the eyepiece, but once the telescope is properly positioned, the image of the Sun can be projected onto a large sheet of white card or paper held some distance away from the eyepiece. Extreme care has to be taken that no one ever accidentally looks through the eyepiece, and that any finderscopes on the telescope are removed or covered up. This method of observation gives a good, large image that can be brought to sharp focus by varying the distance between the eyepiece and the card. Binoculars can also be used for projection, though the image will be less magnified. To enhance contrast, the image can be projected into a darkened area, such as a large box or the inside of a shed. This helps any visible features to become more obvious.
Using this method, Sunspots should be quite apparent and you should be able to resolve considerable detail in them. The dark, central portions of sunspots are the umbra. These are the coolest part of the sunspot, and there is a brighter portion around them, known as the penumbra. Sunspots form because of interactions between the magnetic field lines of the Sun and the differential in the rotation period. Different latitudes of the Sun rotate at different speeds, resulting in a “winding up” of magnetic field lines, which eventually break through the Sun’s surface to form pairs of spot-like features.
The next step up on the solar observing ladder is to fit your telescope with a full aperture solar filter. NEVER use the small filters that used to come with telescopes, and which were screwed into the eyepiece, as these are simply not safe. A full aperture filter is made of a material that rejects a huge majority (about 99.8%) of the Sun’s energy, allowing the viewer to observe the solar disc through the telescope in complete safety. You can buy sheets of the material and make your own filter cell for it, or buy pre-made ones, all ready to simply slot or screw onto the front of your telescope. The filter has to be very securely attached, so that there is no risk of it slipping off. The advantage of this over solar projection is an increase in the amount of detail that can be seen and a more easy to use observing system. Even using a full aperture solar filter, there are still potential pitfalls, so always play safe and seek advice if you are in any doubt.
A “White-Light filtered” telescope (as this is known) can show Sunspots in considerable detail, and you can increase magnification as far as seeing conditions will allow, letting you zoom right in to distinguish the fine structure. Granulation can also be seen. Granulation is the term used for the tops of convection lozenges. Imagine a pan of porridge bubbling away on the stove and then transfer that image to the Sun’s photosphere. Each convection cell is about 1000 to 2000 km across, and although they are short-lived they are replenished continuously. The effect is subtle, but can be made out with a filtered telescope as small as 80mm or so.
I mentioned the “photosphere” here deliberately, as that is the layer of the Sun that is revealed to us in visible “white light”. It is usually thought of as the ‘surface’ although the Sun doesn’t actually have a true surface. Another feature that can be seen in filtered “white light” observation, are faculae. These are clouds of solar material that are associated with Sunspots, and are more easily seen when they are close to the Sun’s limb, due to the limb darkening effect. The central portion of the Sun’s disc will appear brightly lit through a filtered telescope, but the limb, or outer edge, will appear slightly darker, with this effect being known as limb darkening. The Gas Giant Jupiter also shows it to a noticeable degree.
Now, to appreciate the white-light view even more, the next step is to invest in a Solar (or Herschel) Wedge. This is a clever device that plugs into the eyepiece end of a refractor telescope and diverts 99% of the incoming energy away from the viewer’s eye, either into a heat-sink or out through a vent. The remaining energy has to be tamed slightly for visual comfort using a polarising filter, but the views through this set-up are a good step up from those through the traditional white-light full aperture filter. Granulation, in particular, is more obvious and the structure within Sunspots is easier to make out. These devices are particularly popular with photographers too, yielding spectacular results.
So far I’ve only touched on what can be seen in the photosphere. Next month I’d like to explore the chromosphere with you, and explain some of the exciting features that can be seen in that particular layer of the Sun’s structure. If you are at all interested in observing the Sun, please keep an eye open for Highland Astronomical Society’s forthcoming Solar Sessions, which will take place at weekends through the summer. They will be announced on our website and in the Seeing Stars articles. For any further advice on observing the Sun safely, please contact us.