Photographing Lunar and
Solar Eclipses

by Ralph Paramor

Any attempt to view or photograph the sun with a telescope or camera will require the use of an effective solar filter and this must fully cover the objective of the instrument. Modern solar filters consist of glass or mylar sheets on which is deposited a reflective metal coating. They are available in frames which fit over telescope objectives or screw onto camera lenses. Some filters are designed for both viewing and photography while others are suitable for photography only. Users should double check with their suppliers that the filters they buy or borrow will properly protect their eyesight. Other types of filters are suspect and definitely not recommended for instrument work. Obviously the telescope or camera must not, under any circumstances, be pointed in the direction of the sun without filters being fitted. On a telescope the finder must be covered or, preferably, removed altogether. The only camera type recommended is that with a through-the-lens viewfinder. A solar filter fitted on the lens of such a camera will provide eye protection.

Photographing with a telescope with tracking capability is straightforward, providing the user has the attachments to fit a 35mm camera to it. A rough polar alignment is all that is required. Finding the sun may require a little patience, and a shade to shield the user's eyes would be helpful. Focusing on the edge of the moon is fairly easy, particularly if the camera has a fine focusing screen, and refocusing between shots will certainly produce at least some crisp shots. A cable release for shutter operation is essential, and shutter vibration can be reduced further if the camera has a mirror which can be locked in the up position. As a rough guide an exposure of 1/60 sec for an f10 instrument with a solar filter using ISO 100 film at prime focus (or its equivalent with other parameters) is near the mark and a bracket of exposures around that will produce a satisfying result.

Using a 35 mm camera with its own lens is not quite so simple. Assuming there is no telescope available to carry it, it must be mounted on a firm tripod which has clamps to enable the camera to be moved to find the target and for incremental adjustments over time. A screen on the camera or tripod should be set up to shade the eyes while searching and focusing, though a right angle viewer would obviously be desirable. A standard 50mm lens will produce a solar (or lunar) image of only 0.5mm on a 35mm negative. It may be possible to enlarge this to as much as 7mm, but that is hardly satisfactory. A 400mm lens will produce an image size of 3.6mm. with possible enlargement to about 55mm - the longer the focal length the better. Correct focusing is critical in telephoto lenses; once again, a fine focusing screen is desirable with refocusing between shots. Anti-vibration measures as previously mentioned should be taken.

Lunar eclipse 17 September 1997. There are not, of course, the same hazards in this type of photography as in solar work. In this eclipse the lateness of the hour is the difficulty; the moon starts to enter the penumbra at 00.11am and clears it at 5.22am. This is a total eclipse and the moon will be completely within the umbra from 2.15am until 3.17am (all WAST). Because the moon is full at eclipse time, it could be painfully bright to observe it through an instrument until it is well into the penumbra. A light reduction filter could be useful up to this time, but can be removed as the eclipse progresses and replaced later if required. It is worth noting that, while an eclipse of the sun is visible in varying degrees of disc coverage depending on geographical distance from its centreline, in a lunar eclipse the entry of the moon into the earth's shadow can be seen from all locations which would normally be in moonlight at that time.

Both the sun and moon subtend about 0.5 degrees at the earth so previous comments on photographic image size apply. The following yardsticks for exposure are recommended to an f10 instrument using 100 ISO film with no filter:

• a. full moon - 1/250 sec.
• b. moon in penumbra - 1/30 sec.
• c. partial (umbra and penumbra together) - 10 secs.
• d. total (completely in umbra) - from 90 secs to 5 mins (+).

The reason for the wide range in d. is that the light being reflected from the moon, even in totality, varies enormously with differing atmospheric conditions on earth. Sometimes the moon can be seen with the naked eye while in the umbra while at other times it is difficult to see even with a telescope. Readers who attempt this shot should use colour film as the moon can glow in a rich red to copper colour. They should also use a tracking device set, if possible, at the lunar rate of movement.

A fixed camera which permits multiple exposures on a single frame can be used for both types of eclipses to produce interesting shots showing the sequence of development of the eclipse. Anyone trying this would have to be very confident of exposure times and, at least for the moon, would have to change exposures between shots. The beginning and end of shooting would have to carefully planned so that the sequence fits onto the frame and the camera is properly oriented. The timing between exposures should be selected using the rates of apparent movement; for the sun and the shadow cone of the earth it is 1 degree in 4 minutes westward while the moon's westward movement is less than this by 1 degree eastwards in 2 hours. This is a fairly ambitious project and a rehearsal using the uneclipsed sun or moon is recommended to those considering it.

In conclusion it should be stated that this article merely offers basic notes on eclipse photography, a subject on which highly competent practitioners have written copiously. Much of this material is available in our library. It is hoped, however, that some of our members who have expressed an interest in astrophotography are able to "have a go". Those who do should keep a record of their endeavours for future eclipses - lunar events are fairly frequent and there is another solar eclipse visible in WA in February 1999.

ABOUT THE AUTHOR

Ralph Paramor's interest in astronomy started when he was required to complete a unit which included astronomical observations in surveying as part of an engineering course. His interest remained dormant until he joined ASWA about 16 years ago, and since that time he has made several presentations at Society meetings and written various articles for The Sidereal Times. He is interested in all aspects of astronomy but is currently wrestling with the art of astrophotography.