As a veteran solar observer, I have been asked many times about safe solar observing, and how to go about it. Firstly, let me stress that the Sun is dangerous, and at no time should anyone chance looking at it either with the unaided eye or with any optical aid even when near the horizon. You will be bringing to a focus all of the heat and light onto your eye, and the result will be blindness!
Contrary to popular belief, although Galileo looked at the Sun without any filters through a telescope 400 years ago, he only did so at sunrise and sunset and observed safely throughout his life. Galileo’s collegue, Christopher Scheiner first invented the safe observing method of solar projection and it was this method that Galileo used for most of his solar observing. More details on solar projection are below. Although Galileo became blind at the age of 72, this was not caused by solar observing, but from a combination of cataracts and glaucoma.
A number of strict rules should always be adherred to when observing the Sun.
- When using a solar filter, always place the filter on the sun facing end of the telescope as the focused Sun will melt filters at the eyepiece end in a matter of seconds.
- If stopping down a large aperture telescope, make sure that the stopped down area is not receiving any stray light and blocks the Sun completely.
- Make sure that the filter cannot be blown off by gusts of wind – use some masking tape to secure the filter in place.
- Never leave a telescope pointing at the Sun unattended.
- Always check the solar filter is in place before you look through the eyepiece.
- Never look at the Sun through an unfiltered telescope using eclipse shades or a welder’s filter. And don’t use a camera with a telephoto lens, even if the lens has photographic filters on it that appear to darken the Sun.
- Filters that are not safe, though they’re sometimes recommended in old books, include smoked glass, stacked sunglasses, crossed polarizing filters, and neutral-density camera filters. While these may greatly dim the Sun’s glare, invisible radiation may get through and damage your eyes.
When I started observing back in the late 1950’s, I used a homemade telescope made from cardboard tubes and old spectacle lenses. This was of course useless by today’s standards, but it was enough to show me the craters of the Moon, rings of Saturn etc etc… and of course the detail on the Sun. I had a set-up, which allowed me to project safely the image of the Sun onto a white card about 300mm behind the eyepiece, which gave me a disk about 130mm in diameter.
This was sufficient to show me any detail that was visible such as the sunspots and nearer the limb, the faculae, (bright patches that are visible in the ‘limb darkening’). It was fairly crude but effective! And still is effective if you do not want to go to the expense of proper glass filters, or cannot afford them. Nowadays of course, these projection aids can readily be purchased at very little cost, but why not try to make your own?
When projecting the image of the Sun you should first see a bright circle of light. This is the disc of the Sun! Adjust the distance between the screen and the telescope until the disc is about the size of a small paper plate. The image will probably be blurred; focus the telescope until the circle becomes sharp. Using this method, you can see considerable detail in and around the sunspot groups, and many observers use this method to make very intricate drawings of the spots.
With today’s technology, there are many ways to do safe observing. One of the most popular is by using a full or part aperture solar filter. By part, I mean if you happen to be using something like a Meade® LX90 8-inch, then a small aperture of about three inches is all that is required with the rest stopped down. These can be bought ready made, or you can make your own very simply from a cardboard box with a hole cut for the filter. When using this type of instrument, make sure you have the hole off-centre so as not to be in line with the central obstruction on the corrector plate.
On my Meade® ETX 90, I use a Thousand Oaks® Type 2+ full aperture filter which is completely safe and gives a good contrasty orange coloured disc. I also use the Thousand Oaks® Type 3+ that is a bit brighter and is suitable for photography only, (the image is too bright to the eye!). As well as these, I also use a homemade full aperture filter made from a sheet of Baader® Solar Film. This filter gives a pale blue/white image that is perfect for the observation of contrasty faculae. Also, notice in the image that the finder scope has the cap on!
One should always be aware of the dangers of using those supposedly safe filters that screw into the eyepiece, these can splinter without warning by the heat, the result being an eye full of glass, its better to avoid these completely, and if they happen to come with the telescope, don’t use them! Be careful also if some of the inner parts of the telescope or eyepieces are either plastic or rubber, if the heat of the Sun touches it, it will melt or at worst catch fire!
For the images below, I used an Olympus OM1 camera body coupled to an adaptor, which gives a nice full disc image of the Sun at the Prime Focus of 1250mm. In effect, it is a 1250mm telephoto lens; the only difference being it is mounted on a good camera tripod. The image below left shows the orange colour of the filter. I have always used Kodak E100S and 1/60th of a sec., which gives a good result. The second image is one that has been computer scanned and the colour adjusted to near ‘normal’. These spots were visible to the naked (but protected!) eye. the 3rd image was taken handheld with a digital camera.
Mark Stronge, using a Minolta Dimage7 digital camera with a 40mm plossl eyepiece and a 8” LX90 equipped with a Baader® solar filter, took the image below. Notice how contrasty the image is?
It is also possible to use binoculars for observing the Sun as shown in the diagram to the right, but always be sure to leave one object glass covered with the cap.