Observing Solar Eclipses with Filters

The safest way to view the Sun is certainly by projection, but having a direct protected view of the eclipse stages is more enjoyable. This can be done with approved safe solar filters which provide a beautiful orange image of the Sun against a black background.

Please follow only the methods described in this section. These methods of observing solar eclipses should be performed only under the supervision of an astronomy specialist who belongs to a reputable astronomical institution, planetarium or astronomical society.

A solar filter is an optical piece specially designed to reduce the glare of the Sun to a safe level for viewing and to block the harmful solar ultraviolet and infrared radiations.

There are several types of solar filters that allow the observer to view the eclipse either with his/her eyes or through a telescope or binoculars.

A safe solar filter must reduce the amount of incident sunlight to only a tiny fraction of a percent (about 0.001%) and cut off the harmful ultraviolet and infrared radiations coming from the Sun. A filter has these optical characteristics due to a metal coating of aluminum, chromium or silver.

The best type of solar filters for amateur astronomers is termed to be of optical density 5. Some telescope and camera dealers also offer lower density solar filters which are for solar photography only (optical density 4); these filters are completely unsafe for visual solar eclipse observations.

Experienced amateur astronomers use these filters to photograph solar eclipses with the proper equipment. These photographic solar filters are not recommended to be used to photograph a solar eclipse with digital cameras, videos or the astronomical CCD cameras.

Direct Protected Observations of Solar Eclipses

A solar filter may be mounted in a rectangular frame to be held in hand and it is lightweight, and wide enough for the observer to view the Sun with both eyes.

Through most solar filters, the Sun appears as a beautiful orange disk seen against a dark background. (The blue color of the sky turns into black due to the extreme reduction of the solar white light.) Some other types of solar filters transmit a bluish white image of the Sun.

A solar filter may be manufactured to be worn on the head like eye glasses. This is a very famous type of solar filters that is widely known as "eclipse glasses", "eclipse shades" or "eclipse spectacles".

Eclipse glasses are cheap and have been used widely to observe recent solar phenomena such as the 11 August 1999 total solar eclipse and the 8 June 2004 transit of Venus.

Before the 11 August 1999 total solar eclipse, the European observers were advised to purchase only the eclipse glasses bearing the CE mark; they are approved and safe for solar eclipse observations.

The PSC has provided safe solar filters and eclipse glasses for the 29 March total solar eclipse observations.

Viewing Solar Eclipses through Telescopes with Solar Filters

Some solar filters are designed to be adapted to the front of a telescope. They provide a filtered telescopic viewing of solar eclipses. These filters serve to protect both the observer's eyes and the optical instrument.

Viewing solar eclipses through a telescope has the great advantage of showing such interesting solar features as sunspots and faculae and reveals the jagged limb of the dark New Moon as it gradually conceals the solar disk.

While observing, be sure the filter is firmly affixed to the telescope's front so that it will not fall, slip or blow up in a gust of wind.

Caution: Do not place the filter between your eyes and the telescope's eyepiece! In this position the filter may shatter due to the focused solar heat!


Caution: There is a very dangerous type of solar filter that is rarely sold with small telescopes by dishonest dealers! It is called the "solar eyepiece" or "eyepiece Sun filter". Being placed near the focus of the telescope during the observations, the "eyepiece Sun filter" receives intensified light and heat coming from the Sun. Thus, the "solar eyepiece may crack due to the heat of the Sun, allowing the sunlight focused by the telescope to reach the observer's eye. It may also slip clearing an unprotected view of the Sun.


Observing Solar Eclipses with a Welder's Glass

There is also an alternative safe solar filter! Number 14 welder glass is safe for observing solar eclipses. It is cheap and can be purchased from any welding supply store. It is about 20 cm across, so it is wide enough to observe the eclipse with both eyes.

Caution: Never place the welder's glass between your eyes and the eyepiece of a telescope as it may shatter due to the intensified heat of the Sun coming through the telescope. In general, the welder's glass is not safe for observing solar eclipses with telescopes or binoculars.

Important Notes

Purchase the solar filter you prefer from a reputable dealer. You should also consult an expert; you may get the right consultation by contacting renowned planetariums or astronomical observatories.

Any type of solar filters must be handled with care since any scratches or cracks on the filter will allow sunlight to pass through the filter and harm the observer's eyes. Before observing the eclipse, check the filter and its frame for any cracks or scratches.

Please be advised that even if you are using a safe solar filter or eclipse glasses, do not stare at the Sun for long periods. Glance at the Sun for about 4-5 seconds, and then give your eyes a break by looking away from the Sun. The Moon moves so slowly that the eclipse stages do not change rapidly and you do not need to watch for long periods.

Unsafe Solar Filters

Some substances may reduce the glare of the Sun to some extent, but they have other serious drawbacks that make them extremely unsafe for viewing the Sun. Such materials include:

  • Sunglasses of any type
  • Smoked glass
  • Compact disks (CD-Rom)
  • Overexposed color films
  • Overexposed black and white films
  • Medical X-ray films
  • Polarizing filters