With the transit of Venus on 8th June and with many people keen to observe the event there is a need for some advice on observing the Sun safely. As emphasised many times by astronomers it can be very dangerous to the eyes if the Sun is looked at with any optical instrument and permanent blindness may result. Looking at the Sun with the unaided and unprotected eyes for prolonged periods can also cause damage to the eyes. The Sun can however be observed safely in a number of ways.
One of the simplest methods of observing the Sun safely is to project the image of the Sun, from a telescope on to a screen. By doing this the sunlight concentrated by the telescope does not directly enter the eye. Attaching a piece of card to a board and placing the board about 200 to 300mm behind the eyepiece can be used to do this quite simply. The finder on the telescope must be kept covered all the time to prevent stray light damaging the eye. The telescope can be aligned on to the Sun as follows.
First fit a low power eyepiece into the telescope, a 20 to 25mm will do. Point the telescope roughly in the direction of the Sun without looking through it or the finder. Look at the shadow of the telescope on the card. Move the telescope until the shadow of the tube appears round. At this point there should be a flash of light from the eyepiece on to the card. Carefully adjust the position of the telescope until the image is centralised. Lock the axes of the mounting and adjust the focus until the image is clear. The best way to judge correct focus is to look at the edge of the Sun and adjust the focus until it is sharp.
If a telescope is mounted on a stand it might be worth making a frame to hold the screen and attach it to the telescope. In this way the card will stay in line with the eyepiece as the telescope is moved to track the Sun.
If you do not have a telescope a pair of binoculars can be used in the same way. Make sure only one side of the binocular is used and the lens cover is in place over the other lens. Place the binocular in a suitable position where it can be angled correctly to point at the Sun. A low wall or a chair could be used. Use some books, a brick or a box, placed under the front of the binocular to raise it up to point at the Sun. Place a piece of card behind the eyepiece about 200mm away. Adjust the alignment of the binocular until the image of the Sun appears on the card. Adjust the focus until the image is sharp.
The Solar cycle of sunspots is entering a low phase at the moment therefore it is likely that few or even no sunspot will be visible. If there are any spots the will appear grey or ‘fuzzy’ on the projected image. Venus by contrast will appear black and sharp between 6:30 and 12:30 on the morning of 8th June. Venus will track across the top or bottom of the image of the Sun depending on the telescope used. Binoculars will show Venus moving across the bottom quarter of the Sun.
It will be worth practising the technique on a sunny day before the event so that no time will be lost on the day if the setting up goes wrong.
Lee Mcdonald, our guest speaker in May, is the only local astronomer in the Newbury area to have published a book on astronomy, at least in recent times. His book on ‘How to observe the Sun safely’ is highly acclaimed and is in the shops at the moment at the price of £13.65 or there is a copy in our library. .