How to observe the Solar Eclipse?

All observers, whether teachers, students, amateur or professional astronomers, must remember:

NEVER EVER LOOK THROUGH AN UNSHIELDED TELESCOPE AIMED AT THE SUN OR THE PARTIALLY ECLIPSED SUN !

If (with or without telescope) you stare at the Sun without a safe filter permanent eye damage can occur within less than a second. The tremendous heating power of sunlight is easily demonstrated using a magnifying lens (a burning glass). The only time it is completely safe to look at the Sun without filter is during the totality.

Below we describe a few, easy and safe methods available by which you can observe a solar eclipse without any risk.
Additional information about safe eclipse-observing by eclipse-specialists:
Eye Safety During Solar Eclipses and Eye Safety And Solar Eclipses



A simple and safe way to view the Sun

Some of the very first cameras contained no lenses but were only equipped with a pinhole. Strictly speaking, this camera is nothing more than a black box with one hole in the front.

Light travels in straight lines and therefore rays from the Sun will hit the back of the box and create a perfect image of the Sun's disc.

It is possible to use this black box as a general camera, not only for photographing the Sun but also for taking ordinary photographs. These black box cameras are easy to construct, but often require long exposure times due to the small aperture.

Camera Obscura

The pinhole camera was even used by French artists painting miniatures during the 18th century. Could you imagine spending a whole day painting miniature copies inside a warm black box like this?

Roger Bacon Originally the Pin Hole Camera was invented by an English monk, Roger Bacon (1220-1292); Roger Bacon's research was considered by his contemporaries as being witchcraft and he spent half his life in prison.

For our project, however, this camera is safe, legal and simple!

The Sun's image inside our box will undergo all the phases of the eclipse, just like the real Sun. This method is safe and may even be carried out by rather young children in groups.

Here are a few hints:

  1. The box must be at least 1 metre long in order to get a reasonably large image. The size of the Sun's image will be around 1 percent of the boxlength. A 1 metre tube may thus give a solar disc size of about 1 cm.
  2. The light hole needs to be small in order to get a sharp image. The optimum size for a one metre box is about 1 mm.

Solar image in a pinhole camera. During the EAAE Conference at ESO in November 1994, the Spanish Teachers presented nice results which were obtained with this Black Box Method. As a dark box, they had used huge cardboard tubes. Such 4 metre long tubes may be picked up, often for free, at storehouses selling carpets.

Do you know that this pinhole effect may also be seen in nature? A leafy forest may absorb almost all the sunlight, only leaving a few rays to pass through occasional holes. The effect visible on the ground will be like this:

If you want to see more details, click on the image, but beware that it is quite large (JPG, 135 kB) and the transfer may take some time.

So during the August 11 eclipse you may observe eclipsed solar discs scattered all around the forest floor.

During the month of August, these mini-Suns may have an elliptical shape. The Sun's disc is of course still circular, and the elliptical shape is only an effect of the projection. If sunlight came from straight above ('Zenith'), the solar images projected on the ground would all look perfectly circular. But in August, the Sun never reaches Zenith in Europe, only in countries south of the Earth's equator.

However, here is what an Astronomy On Line Group wrote from Toulouse, during the October 1996 eclipse :
"What a wonderful weather for this eclipse! There were plenty of people in the park where the old Observatory stands. We could see very well the luminous crescents on the ground through the leaves on the trees - and because of the strong wind, they were sweeping the ground, back and forth."

Click for an American photo of this leafy forest phenomena .


Detailed suggestions how to observe the eclipse

There are several methods which may be used to observe the eclipse, in addition to the one described above.



Method 1: The "Grinning Cat - Mirror Method"

A scientist at the NASA Ames Research Centre (California, USA), Joe Jordan, once wrote these comments:
`One of the best ways to observe a partial solar eclipse is nice and simple: just take a mirror (like a standard square pocket mirror) and reflect the Sun's image onto a wall at least several meters away. No poking holes or cutting and pasting cardboard, etc.! You'll notice that the reflection looks inverted relative to the appearance of the eclipsed Sun in the sky.'

The Sun projected with an 'eclipse'-mirror. Left: an image of the eclipsed Sun as seen with an 'eclipse-mirror'. Observation of the 1996 eclipse by the Vienna Kuffner Group, Austria.

This mirror method is actually a very interesting variation of Francis Bacon's 700-year old Pin Hole Camera. Instead of letting the sunlight pass through a small hole, this method reflects the light back into, for example, a northern, shady classroom.
Besides being totally safe, this method works well when long distances are used.

During the October 1996 Eclipse, this "Grinning Cat Mirror"-method became very popular. A small dentist's mirror, placed about 10 metres away, results in a solar image which is approximately 10 cm in diameter. A larger mirror may still be used, if you cover unused parts with tape or paper.


Method 2: The Handojo Solar Eclipse Projector

A novel, easy to build, safe eclipse projector described by
Andrianto Handojo in Applied Optics, Oct 15, 1989, p 4293
Roger W. Sinnott, Sky & Telescope, July 1991, p 79-85.

This solar eclipse projector is an improvement compared to the grinning cat mirror method.

A positive lens, of say f1 = 0.5 meter focal length should by a first order calculation cancel out when combined with a negative lense of the same value,
f2 = 0.5 meter.

However, this does not happen, second order corrections give the following formula:

feffective = f1 f2 / ( f1 + f2 - d)

where d is the distance between these 2 lenses.

This lens combination has been applied successfully during several recent eclipses.

Exercise

Eyeglass stores often sell lenses with the inverse indication "dioptries". 1 dioptri means a lens of f = 1 meter, 2 dioptries equals a lens of f = 0.5 meters etc.

Buy a lens with +2 dioptries (positive lens, f = 0.5 meters) and a lens with
-2 dioptries (negative lens, f = -0.5 meter).
Perform simple experiments in your lab, check the properties of both positive and negative lenses.

Now combine these two lenses, applying the formula above and show that if the distance between both lenses is 1 cm (d = 0.01 meters):
feffective = 25 meters!

This is thus a magnificant method, achieving large focal length telescopes for a small budget. (Sky & Telescope: typically 40 Dollars).

Adding a simple mirror, you may get an easy to build eclipse projector:

Roger W. Sinnot wrote in Sky & Telescope:
"When focused for a throw of 50 feet (15 meters) the device produces an (solar) image almost 6 Inches (15 centimeters) across. In a (Northern) classroom, with this projector set up students can follow the eclipse's progress on the opposite wall. They can take turns operating the feed mirror to compensate for the Earths rotation.

Additional technical advice: it may be necessary to reduce the effective opening of these lenses in order to avoid what astronomers call "abberation".


Method 3: Using Binoculars (or telescopes) like a slide projector

This binocular method will also give remarkable results. You can see in the figure how it works. Cover one of the two lenses if you don't want double images!

Be careful because the eyepiece of the telescope or binoculars will be heated by the intense radiation of the Sun. Not every eyepiece can withstand this heat (especially not when the lenses in the eyepiece are cemented). What you can do is: make a diaphragm of no more than 5 cm diameter and place it in front of the lens of your telescope to reduce the incoming light.

Once again: Do remember and explain to your friends that they must never aim an unshielded scope directly at the Sun! and don't leave the telescope unattended!


Method 4: Watching the Sun through Negative Film, Welding Glasses and Filters

This method is often described in books and magazines, but is still dangerous for several reasons. Therefore read on carefully!

Many years ago, film plates used silver nitrate. An over-exposed film plate would thus provide an effective metallic shield, similar to the shields used by astronauts in space in front of their helmets.
Nowadays, however, most plastic films are made of other materials and do not give any protection against solar infrared rays.

Therefor we absolutely advise against the use of:
  • over-exposed filmnegatives (black and white as well as color)
  • X-Ray photosheets
  • floppy disks
  • CD's
  • a piece of glass blackened by soot from a candle

Do not look at the Sun through any filter unless you know it to be safe! Safe filters are special 'Eclipse Viewers' with aluminised Mylar or Black Polymer. Inquire at your local Astronomy Club, Planetarium or Public Observatory.

In Sky & Telescope (July 1991, page 80), you will find these comments: One filter entirely safe for direct solar viewing is a shade 14 rectangular welder's glass. It transmits only 1 part in 370,000 of the incident light and meets strict tolerances on transmission of invisible infrared and ultraviolet as well. This costs only a few dollars, but be sure you get the right item.

If you have a good school telescope, be sure to buy a proper optical solar filter. Mounted correctly in front of a telescope this will give a most impressive picture.

Also beware that not only your main telescope, but also any additional finderscope should be covered. Experience shows this is very crucial: looking through an unshielded finder is as dangerous as looking through the telescope! A safe way to point your telescope at the Sun is to look at the telescope's shadow.

Plastic Mylar solar filters are cheapest (for 10 Euro you can build your own safe filter), but seem to be extremely fragile and some only transmit a strange, blue colour.

Glass filters are available too, with prices around 30 Euro and upwards, depending on the diameter of your telescope lens. These transmit nice colours and beautiful photographs are possible. Handle these filters with care, avoid too many scratches.
With a good filter mounted at your telescope (under calm atmosphere and good seeing conditions) you may even be able to see and identify lunar mountains by their silhouettes. Try to identify possible candidates in advance or after the eclipse. What are the height of these mountains? A Lunar Atlas may help, e.g. the Lunar Prospector Maps.

For details of where to buy, take a look and compare prices in astronomical magazines such as Astronomy or Sky & Telescope

Warning: Never use ordinary sun glasses in front of your telescope. Some of these cheap filter glasses have 100% transmission in the invisible infrared, again exposing your eye to lifelong damage.

Some companies sell solar filters to thread into eyepieces (they are often sold with cheap telescopes). Avoid these products! One of our students once mounted such an eyepiece solar filter in his telescope. He pointed his instrument directly at the Sun. Only 30 seconds of sunlight were enough to crack this filter into pieces due to overheating! A bad and very dangerous investment!