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Science, Maths & Technology

Observing at Home

Updated Thursday, 19th March 2015

Dr. Lucie Green has more safety advice when looking at the sun

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On 20th March 2015 there will be a total eclipse of the sun by the moon. Most of the world will not see the full eclipse, however residents of Europe will enjoy a partial eclipse in the morning. There are several ways that eclipses and transits can be viewed from home, many using items often found in the house. If you can, then I suggest that you try to practice in advance of the day itself, so you can make sure you have all the things you need set up and ready to go.

So, how can you view an eclipse or transit safely? Obviously you could go to one of the many events being held throughout the UK where you can benefit from the expertise of others, but if you can’t get to one of these then there are a few ways that you can view the eclipses and transits in safety.

Don’t forget – the sun can cause serious damage to our eyes, so never look directly at the Sun with the naked eye, through binoculars or a telescope. You can read more about safety here. Don't forget to supervise children at all times.

Using a Solar Filter
There are filters available, two in particular, which reflect most of the Sun’s light and heat and allow safe solar viewing. The first is aluminised Mylar. However, it is advised that 2 layers of aluminised Mylar are used as this material can become easily damaged and could have unseen pinholes in. The second is the Baader filter which is coated with plastic and less likely to become damaged.

These filters can be held in front of the eyes (to completely cover the eyes) in order to view the Sun. Place the filter over both eyes before looking at the Sun and do not remove it until you have looked away.

Eclipse Viewers
Eclipse viewers are specially made viewers that are worn like glasses and are made from a solar filter. Always make sure that they are ‘CE’ approved and designed for solar viewing. The filter can become scratched and scuffed or could develop holes so be alert for any damage and do not use them if you are in any doubt.

Projection Methods: Pinhole Camera
Making a pinhole camera is a simple way to view the Sun, although the image projected will be small and it will be quite difficult to make out transits, but it will work well for eclipses. There are many ways to make a pinhole camera but the basic principles are the same.

In order to make a good clean pinhole, a 2” square hole should be made in an A4 piece of card and tinfoil taped over it. You can then use a pin to make a clean hole in the tinfoil. The pinhole needs to be directed toward the Sun with a second piece of card held behind it. You must be careful when trying to direct the pinhole towards the Sun – don’t forget that you mustn’t look directly at it with the naked eye.

The sunlight will come through the pinhole and form an image of the Sun on the second piece of card behind. The card will need to be moved back and forth slightly until the image of the Sun is clearest.

The quality of the image of the Sun when using a pinhole camera depends on the size of the hole and the projection distance. For example using a 1mm size hole will need a distance of about 3 metres between the pinhole and the card. This means that the resulting image will be very dim, so you’ll need to shield it from the Sun and surrounding light in order to see it.

This method could be adapted by putting the pinhole between closed curtains and letting the solar image come into a dark room, either falling on a wall or on a screen a couple of metres away.

Another method is to use a large tube, for example one found in a roll of carpet, and make a pinhole camera from it. On one end of the tube tape over your card which has a tinfoil section in the middle with the pinhole in. On the other end of the tube place, and tape down, a piece of tracing paper or baking paper. The image of the Sun will be projected down the tube and will fall onto the paper. Because the paper is translucent the projected image of the Sun can be seen through it.

The image of the Sun made with a pinhole camera will be inverted, so transits may appear to be moving across the Sun in a different place to where they actually are.

Projection Methods: Binoculars
Binoculars are a good way to project an image of the Sun onto a piece of card. If you do you use a projection through binoculars, you must cover one half with an opaque screen (such as taping the lens caps on). Otherwise you will end up with overlapping images of the Sun! Never look through the binoculars at the Sun.

Binoculars will need to be supported, either on a tripod, or on an adjustable mount such as an indoor TV aerial. Point them toward the Sun and hold a piece of card behind. Watch the shadow cast onto the card and move the aerial and binoculars until the shadow stops being oval and becomes circular. At this point the binoculars should be lined up toward the Sun. Move the card back and forth until an image of the Sun comes into focus on it (around 0.5 – 1m). Do not look through the binoculars to align them.

The image of the Sun will be faint and a shield could be made and attached to the front of the binoculars to provide shade. Using binoculars will produce an image of the Sun which is the correct way up.

Never look through the binoculars directly at the Sun. Children should be supervised at all times when projecting an image of the Sun using this method.

Projection Methods: Telescope
A small telescope (aperture up to 3”) could also be used to project an image of the Sun. Hold a piece of card behind the telescope and move the telescope until the shadow stops being oval and becomes circular in order to align it with the Sun. Do not look through the telescope to align it. Then move the card back and forth until an image of the Sun comes onto focus.

Always remember to cover up the finderscope with an opaque screen otherwise an image of the Sun will also be projected through this and could cause skin burns if part of the body is held in the way.

Never look through the telescope directly at the Sun. Children should be supervised at all times when projecting an image of the Sun using this method.

Telephoto Lens and Webcam
A household with a keen photographer will no doubt have a telephoto lens and this can also be used for viewing the transit when coupled with a webcam. If you happen to have compatible equipment then you can connect the two together. It is extremely important to reduce the amount of sunlight entering the lens by using a ‘CE’ approved solar filter such as Baader filter. This needs to be taped securely to the front of the lens so that it is completely covered.

The lens should then be mounted on a tripod and angled roughly toward the Sun. Accurate alignment can be achieved by watching the shadow cast by the lens and moving it until it stops being oval and becomes circular. Never look through the lens at the Sun directly.

The advantage of this viewing method is that it the webcam can be connected to a computer so that the observations can be recorded.

Telescope and dedicated Solar Filter
In contrast with other areas of astronomy, there is no problem with lack of light when observing the Sun. So, a large telescope is not needed and an aperture of 3-4” or smaller is suitable. The sunlight will need to be reduced and this should always be done using a filter at the front of the telescope and not at the eyepiece. Otherwise the light will be focused onto the filter and could cause it to melt or break – definitely not what you want when you have your eye up against it.

The required reduction in sunlight can be achieved by using a dedicated solar filter purchased from a specialist astronomy shop, or by using Baader filter or Mylar secured firmly over the full entrance aperture. Always remember to cover up the finderscope as it will project an image which will burn if it falls on to skin and check the filter very carefully for any damage or scratches. If in doubt, don’t use it.

Align the telescope by circularising the shadow of the Sun, and then make the sun central by looking through the eyepiece once the filter is in place. Don't forget that when you use this method you will see an inverted image.

If you are unsure how to use the filter to observe the Sun then contact the manufacturer of the filter or your local astronomy society.

On the Internet
Many solar observatories will be viewing eclipses and transits from locations where the weather is less cloudy than in the UK! Solar satellites which are above the Earth’s atmosphere and away from the weather will also be making observations. Many are also broadcasting over the internet.

For more information please view this leaflet created by the Royal Astronomical Society.




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