BASIC OBSERVING SERIES

by Greg Burnett (gburnett)

Solar Observing

[This article was first published in Star Stuff, the newsletter of the Ford Amateur Astronomy Club, in May, 1993.]

The most important watchword for solar observing is safety. This is perhaps the only area of amateur astronomy where there is real danger and potential for serious injury. The necessary precautions are simple, but extremely important. They can be summed up in a single statement: Never observe the sun without proper filtration. This applies to naked eye viewing in addition to telescopic observation. The importance of caution cannot be overemphasized. Even an accidental glance through an unfiltered telescope pointed at the sun will cause immediate, permanent eye damage. You are literally playing with fire; 5000-degree fire!

Excluding specialized telescopes like coronagraphs, there are basically two methods for safely observing the sun: projection and front aperture filtration. Both of these methods are adaptable to most amateur telescopes. Projection involves arranging a projection screen of some sort, a stiff piece of white cardboard for example, at a suitable distance behind the eyepiece. The solar image is then focussed on the screen through the eyepiece. Commercially made mounting brackets are available for this purpose, or they can be homemade. The projection method has the advantage of being relatively inexpensive. It has the disadvantage that fine detail is often difficult to see in a projected image. Also, the full energy from the sun passes through the telescope optics. This can cause heating, particularly in the eyepiece, where cemented lens elements could be damaged. For this reason it is best to use a simple eyepiece for solar projection, like a Huygens or Ramsden type.

Front aperture filtration is accomplished by covering the main objective aperture of the telescope with a filter specially made for solar observing. Such filters may be made of glass or Mylar plastic. Both types rely on a thin coating of metal, usually an alloy of aluminum, to provide the proper filtration. These filters are designed to eliminate all of the ultra-violet and infra-red radiation from the sun, and to admit only about a thousandth of a percent of the visible light. An advantage of front filtration is that the operation of the telescope is essentially unaffected. That is, different eyepieces, diagonals, etc. can still be used as usual. Also, the full energy of the sun is kept out of the telescope. The disadvantages are that front filters, particularly glass ones, are somewhat expensive, and they require a certain amount of care.

Another style of solar filter deserves mention because it is not considered safe. Solar filers that attach to the eyepiece are often supplied with "department store" telescopes. These small filters absorb all of the solar energy collected by the telescope objective. They become very hot in just a few minutes and have been known to fracture if over-heated. This could have disastrous consequences for an observer using the telescope at the moment of failure. For a few dollars more, it is better to be safe than sorry.

Space does not allow a more in-depth analysis of the equipment available for solar observing. Those interested in pursuing the subject further would be well advised to obtain one of many books containing detailed information on the subject. Check the book lists in ASTRONOMY or SKY & TELESCOPE magazines.

So, once suitable equipment has been arranged, what observing opportunities does the sun offer to the amateur astronomer? Clearly, a solar eclipse is the most dramatic phenomenon associated with the sun. And indeed, except during the few minutes of totality, eclipse observing requires essentially the same equipment as regular solar studies. However, solar eclipses are relatively rare events, and there is quite a number of interesting aspects of solar observing that are present nearly all of the time.

The most obvious features on the sun's surface are sunspots. Sunspots vary in number, size, and location with the eleven year solar cycle, but at least a few are almost always visible. Amateurs with the patience for long term projects can take sunspot counts and compare their counts with the professionals'. Counting sunspots is something of an art; what constitutes a single fragmented sunspot as distinct from a group of multiple spots is often somewhat subjective. Observers with a flair for drawing can record the infinitely varied appearance of sunspots at high magnification, and can note their changes over time. Sunspots sometimes change noticeably over periods of several hours. Often more detail can be captured in a drawing than in a photograph, because the human eye is not as easily confounded by atmospheric seeing conditions as is photographic film. During the day seeing is usually worse than at night. The best solar images will be seen in the late morning, after the sun has risen to a moderate height, but before most atmospheric turbulence has time to develop.

In addition to counting, measurements of the latitude of sunspots can be used to construct the so-called "butterfly diagram" as the average spot latitude changes over the 11-year cycle. A graphic plot of spot latitudes over the course of a cycle yields a plot resembling the wings of a butterfly.

During periods of high solar activity, solar flares sometimes erupt on the sun's surface, usually in or very near to large sunspots. These flares produce surges of ultra-violet radiation and high-energy particles that are responsible for shortwave radio blackouts, power surges, and aurorae. Visual detection of flares are useful for predicting these effects (particularly aurorae), which occur after a delay of several hours following the flare event itself. [Since this article was written, it has been discovered that aurorae are more directly associated with "coronal holes" rather than flares -- G.B.]

Also visible on the sun are "phlages" or "flocculae", lighter markings that are usually associated with sunspots. New spot groups can sometimes be predicted by observing the formation of flocculae. "Granulation" can be seen over the entire surface of the sun. This pattern, caused by convective movement of the photosphere, can be seen to change in a matter of minutes. Sometimes bright "pores" are seen among the granulations. Pores are often the precursors of sunspots.

The sun is, so far, the only star we are able to study in real detail. For the professional astronomer, the sun is a laboratory for studies in the UV, IR, radio, and X-ray portions of the electromagnetic spectrum. For the amateur, the sun can be a source of many satisfying observations easily within the reach of modest equipment. A number of books are available that contain much more information than I am able to provide here. And, as always, don't neglect your fellow astronomy club members as sources of information and experience.