Solar Filter

Viewing and photographing the Sun can be a very rewarding endeavor, but it is one that requires a great deal of caution. The surface brightness of the Sun at Earth's distance is equal to an eye-blowing 1.5 million candlepower per square inch. That much light will very quickly destroy the retina of the eye if you even glimpse at it for any length of time. Imagine that light amplified many times by the size of a telescope objective. To safely view the Sun, you must have a properly designed solar filter. There are two ways in which this type of filter can be added to the telescope. You can use either an aperture filter, which fits over the front end to the telescope, eliminating more than 99% of the Sun's surface brightness and total light, or an eyepiece filter. This is a filter that screws into the open end of the ocular barrel. The only type of filter that you should consider using is the aperture filter. The reason why is because this filter is not required to absorb sunlight that has been amplified by the telescope. An eyepiece filter must absorb the sunlight amplified many times by the telescope objective. This can cause the filter to become extremely hot and under that kind of heat the glass element may crack or shatter while you're looking through it leading to disaster.

Aperture filters come in different types. The simplest type is a full-aperture filter that filters light passing through the full width of the objective. Most filters utilize a neutral density glass that renders the Sun in its natural yellow color. Because the Sun is the brightest object in the sky, a large aperture is not necessary to produce

Figure 2.5. Mylar solar filter. Photograph by author.

a bright, crisp image. Celestron offers a 3-inch off-axis filter that fits over the 8-inch objective of the C8. This reduces the total amount of light reaching the eyepiece providing a more comfortable view. A disadvantage of this is that it will limit the amount of magnification you can use. Another type of filter that many people use now is a Mylar filter. This type of filter uses two sheets of Mylar each of which is aluminized on a single side. The two Mylar sheets are laid over each other with the aluminum on the inside, protecting it from scratches. These types of filters produce sharp images of the Sun at a fraction of the cost of their glass counterparts. If you're considering a Mylar filter, don't be turned off by your first look at one. The aluminized Mylar looks like lightly crumpled aluminum foil leaving one to wonder how this could ever transmit a sharp image. Believe me, it does. Mylar filters do have one disadvantage. The aluminized Mylar strongly absorbs the red end of the spectrum rendering images of the Sun that are nearly powder blue in color. When I work with solar images, I always convert them to grayscale anyway so this does not bother me all that much. The Sun has been burning with the same color for all of human history and that value is very precisely known. The exact color that you see in the scope has no scientific importance at all, but if the aesthetic value of a yellow Sun is important to you then you should avoid a Mylar filter. Solar filters run in price from about $80 and up in today's market.

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