Written by User 1

Last Updated: July 30, 2012, 01:03 pm (UTC)
Originally created on June 25, 2012

The sun is the brightest and most massive object in the solar system. Its brightness makes it easily accessible as it is unnecessary to use a large telescope. Yet, this extreme brightness also creates dangers which require special precautions to be taken to ensure everyone's safety including that of any equipment being used.

First, an important warning that has been repeated perhaps millions of times: NEVER LOOK AT THE SUN WITHOUT A PROPER FILTER! Brief glances may not do much harm by themselves but over the course of a lifetime, the effects of intense UV exposure can lead to serious eye conditions. This warning may sometimes be safely ignored when viewed through thick haze such as at sunset. When in doubt, however, heed the warning.

Cameras often withstand the intense radiation better but pointing one at the sun for long periods of time may still cause damage to the electronics, especially if it is connected to an unfiltered telescope.

With that in mind, it is possible to view and photograph the sun safely. A few simple steps can make the difference between setting a fire and enjoying the view of sunspots.

With a Solar Filter

The most common way to view the sun is with a solar filter. These are sold by various companies such as Baader Planetarium and Thousand Oaks Optical. The filter must rated at ND5 or higher indicating that they transmit 1 / 10^5 (0.001%) of the incident light. Filters rated ND3.8 (or similar) are also available for photography but they should not be used visually.

These filters may be used directly by holding them up to the sun or by placing them over the front of the telescope. If placed over a telescope, it must cover the entire aperture so that no stray light can enter the tube. This makes open-tube Newtonian telescopes unsuitable. Also ensure that no pinholes or scratches are visible before installing by holding the filter up to the sun. If any are visible, cover them up with a felt-tipped black marker. If numerous pinholes and/or scratches are visible and cannot be adequately covered, the filter should be replaced or used solely for direct observation without a telescope.

Solar filters are made either with glass or film. Glass filters are optically flat glass panels coated on both sides with a thin layer of metal. These tend to be expensive compared to their film counterparts due to cost of glass and expense needed to make them optically flat. Poor quality glass filters may even show a double image from internal reflections. Film filters consist of a thin, polymer sheet coated on both sides with metal. These are available either in whole sheets or as prefabricated filters. Very inexpensive and safe solar filters can be made by buying sheets of film and constructing filter holders from household materials such as cardboard.

During solar eclipses, vendors frequently sell eclipse glasses. These are essentially film filters mounted in wearable glasses. These often cost just a few dollars and so are a low-cost way to view the sun.

Through Projection

An even simpler way to see the sun is to project an image of it onto a white surface such as a piece of paper. This works with small refractors and reflectors of up to 3" in aperture. While possible with larger apertures, there is a danger of damaging the telescope. As a result, it should never be done with a catadioptric telescope as the heat buildup inside the tube can cause irreversible damage.

To project the sun, point the telescope at the sun without looking into it. This is best done by watching the shadow of the telescope while moving it until the shadow is minimized. Then, hold a paper (or any other white surface) several inches in front of the eyepiece. A white blob should be projected onto the paper. Focus the image until this blob becomes a distinct circle -- this is the projected image of the sun. It may help to shade the image and/or create a fixture to hold the paper.

Visible Features

The most obvious features on the sun, 95% of the time, are sunspots. These are patches of black cores (the umbra) surrounded by areas of gray (the penumbra). These actively evolve and can change substantially within a 24 hour period. Sunspots are formed by bubbles of magnetic field and are cooler than the surrounding surface. With good atmospheric conditions and a decently powerful telescope (at least 4"), the surface (photosphere) is seen to be made of tiny bubbles of gas called granules, each being the size of the Earth. These change rapidly and granules may form or disappear in just a few minutes.

Sometimes, extremely bright solar flares may be visible. These are massive explosions on the sun which occur within a few minutes and appear as a brilliant white flash. These explosions toss off large amounts of gas in Coronal Mass Ejections which, if Earth-directed, can cause geomagnetic storming often resulting in colorful auroras.

Most flares, however, are too dim to see without a special hydrogen-alpha filter or telescope. These isolate the light from the chromosphere, the layer just above the photosphere, by blocking all wavelengths of light that are not at 656.3 nm. At this particular wavelength, light from the photosphere is blocked -- only that of the normally dim chromosphere is visible. As solar flares are events in the chromosphere, a hydrogen-alpha filter permits them to be visible even when they are too dim to be visible otherwise.

Hydrogen-alpha filters also show many other features in the chromosphere not visible in white-light (ie. no H-alpha filter). The edge of the sun will no longer be a flat (ie. circular) but will instead show tendrils of suspended gas called spicules. Sometimes, there will be loops of gas called prominences. When such prominences occur over the disk, they are seen as dark filaments. Sunspots, a photosphere feature, are no longer visible. In their place, bright areas of plague show the intricate magnetic structure above them. These filters are sold by various companies including Coronado (part of Meade), Thousand Oaks Optical and Daystar. They tend to be very expensive and often run into thousands of dollars for the better units.

Planetary Transits

Before the advent of General Relativity, some scientists believed that an undiscovered inner planet, named "Vulcan," was perturbing the orbit of Mercury much as Neptune perturbed the orbit of Uranus. To find it, astronomers carefully watched the face of the sun for a transit of an unknown planet. Though this unknown planet was never found, transits in front of the sun by Mercury and Venus do occur and can be interesting to watch.

Transits of Venus occur in a 243-year cycle -- 8 years separated by alternating periods of 105.5 and 121.5 years. Renowned astronomer Edmund Halley of Halley's Comet fame wrote of a method to compute the distance to the sun using these transits. As a result, major expeditions to foreign lands were made to observe the events in 1761, 1769, 1874 and 1882. With the space age, satellites were able to measure that distance to within a few meters thus obsoleting this technique. As a result, the recent transits of 2004 and 2012 were not as scientifically important with research focused on the atmosphere of Venus and exoplanet search techniques. The next Transit of Venus will occur of December 10-11, 2117 and will favor southeast Asia and Australia. Transits of Venus are visible without any optical aid as a small dot being about 1 arcminute in diameter.

Transits of Mercury are more common with ~13 transits per century. As Mercury is farther from the Earth and smaller than Venus, its disk is much smaller -- usually 10 arcseconds -- and thus requires a telescope to view. These occur in either May or November. The next Transit of Mercury occurs on May 9, 2016 and will favor those around the Atlantic basin.

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