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30 January 2004

Solar Astronomy: A winter warm-up

by Paul Curtin

On June 8, 2004, Venus will cross the face of the Sun for many Earth-based observers. If for no other reason this rare event should get amateur scientists observing the Sun in anticipation. Transit observations will be the subject of a later article. For now let us concentrate on the basics.

Using proper equipment the amateur can study a number of solar phenomena. Standard programs involve documenting the number, size, structure and location of sunspots. One can also observe limb darkening and granulation. See Gupta for information on reporting systematic observations. More sophisticated equipment now commercially available allows the study of prominences and other objects visible only through Hydrogen Alpha filters.

CAUTION: Regardless of the method employed, extreme care must be exercised when observing the Sun. Whether observing in groups or alone, strict safety protocols must be observed including the cardinal rule that telescopes must never be left unattended when pointed at the sun.

Observers often begin by projecting the solar image through the eyepiece onto a white surface. A white screen can be hand held or mounted on a tripod for short sessions. For more serious work the screen should be attached to a boom mounted on the telescope tube. Remove or cover the finder scope. This is an excellent method for school groups and public viewing.

The ideal telescope for this is the small refractor with a long focal length. Reflectors tend to develop heat problems very quickly, the larger the aperture the faster. Verify that your telescope does not have plastic parts in the optical path as these may be damaged. Remember that unfiltered sunlight concentrated by an optical device will cause serious damage to the eye. Warnings should abound when demonstrating these techniques to others.

There are a number of white light filters available these days. These avoid the high temperatures and potential for damage associated with projection. The filters cover the objective and reflect the heat, harmful wavelengths and most of the white light from the optical path. With the filter on you can observe in comfort, using the eyepieces and accessories you would use on the moon or planets. Large reflectors can use a small offset filter to avoid diffraction from the secondary.

Make sure you get a filter for your finder scope as well or remove it for projection. Check with the manufacturer of your telescope to see if they make a filter for your model. If not there are many companies that produce filters of different sizes. Mylar filters can be purchased pre-mounted or in sheets for homemade mountings.

Regardless of what filter you decide to use be sure that it cannot come off unexpectedly due to wind or telescope movement. If in doubt, over engineer! Velcro strips make good, lightweight anchors. Make certain the tube element to which the filter is attached is also secure: many dew caps, especially on refractors, are quite loose and should also be reinforced. The storage of solar filters is also of prime importance.

In addition to the precautions necessary for any optical element it is imperative, for safety reasons, that no damage or flaws, however slight, be present on the filter surface. It is best to store filters seated snugly on a dummy telescope end within a sealed container. Check with the manufacturer regarding the assessment and repair of filter problems.

Observers should become comfortable with setting up this equipment, running safety checklists, locating the Sun in the telescope (something of a challenge in the beginning!) and making observations. Transits and eclipses are quick at best and merciless to the unprepared. Get the drill down now so you can enjoy the big event.

Readers who would like to discuss observing techniques or filters are welcome to contact the author at pcurtin@legalaidbuffalo.org.

Bibliography

Beck, R. (et al.). Solar Astronomy Handbook. Richmond, VA: Willman-Bell, 1995.

Gupta, R. (ed.) Observers Handbook 2004. Toronto: University of Toronto Press, 2003.

Kepple, G. R., Sanner, G. W. The Night Sky Observers Guide. Richmond, VA: Willman-Bell, 1999.