Solar Physics Task Group

How to safely observe the Sun

Observing the sun is fun, beautiful and very educational… but it can be dangerous if not done properly.

From The solar TG we encourage everyone to learn and enjoy the wonders of the Sun, but certain precautions must be taken, specially when using any kind of magnification optics (telescope, binoculars, …). In general improvised filters (photographic films, plastics) are discouraged, as they don´t filter out non-visible radiation.  Only during the totality phase of a total solar eclipse it is safe to observe directly naked-eye the Sun.

If you have doubts is better to ask us before you try anything.

Feel free to print and circulate this pdf that Ricardo Reis has prepared, or this other pdf from the European Southern Observatory (ESO). SunTrek has also a dedicated webpage about safe observations.

If you are interested on the detailed explanation, biological process, the myth of Galileo becoming blind due to observe the Sun, and other interesting facts, read this link.

UPDATE [April 2]: B. Ralph Chou, MSc, OD, FAAO sent us some modifications for the medical reasoning. He himself has also made these recomendations. Thank you!

Brief explanation

The eye is an extremely sensitive complex organ. Although the cornea at the front of the eye is very sensitive, the light sensitive retina has no pain receptors. At the same time, the Sun produces, along with the overwhelmingly bright light,  much infrared (heat), both of which can damage the back of  the eye .
When viewing the Sun, the retina of an unprotected eye can be damaged very quickly.  Because there is no pain sensation when the retina is burned, and because the visual effects are delayed by up to 12 hours, the observer is unaware of the injury until long after the damage is done.
The chances of suffering an eye injury are greatly amplified when you are using light focusing devices, such as binoculars and telescopes, which concentrate the light from the Sun, without special protective filters.
Do not use improvised filters. They may block only visible light, but not the invisible infrared radiation (IR) that can also burn the retina.  Sunglasses protect the front of the eye from harmful ultraviolet radiation and block visible glare, but are not designed to protect the retina against IR in the Sun’s image.

How to observe the Sun?

The safest way to observe the Sun is by using some kind of specially designed solar filter.
The cheapest way is to project the image of the Sun on a surface (Not a mirror). Use a pinhole, binoculars, or a telescope, … and project a focused image on the floor or on a cardboard sheet.
There are also safe inexpensive solar films, eclipse glasses,and welding filters (scale numbers 13 and 14 are safe) … If in doubt, check with a knowledgeable professional before using these devices.
Clouds in general are good UV and Infrared filters, so if it is cloudy and the Sun is low over the horizon (so that the light goes through much more of the atmosphere than at mid-day) you can safely observe the Sun, and may be even see sunspots on naked view, like here:

(Again) What NOT to use

The are some urban myths that talk about other tricks that (supposedly) allow you safely observe the Sun. Here is a short list with some of those that DO NOT work.

- Exposed photographic film

- Smoked glass

- Medical x-rays

- Floppy disks

- Thick plastics

- Cds, DVDs

- …