Preparing for the 1991 Total Solar Eclipse
Fred Espenak checks on his telescopes during the 1991eclipse in Baja, Mexico.
(click for more photos)
Close-ups of the eclipsed crescent Sun and detailed portraits of the solar corona require the use of a single-lens reflex (SLR) camera. SLRs also feature interchangeable lenses, from extreme wide-angle to high-power telephoto. You can even remove the lens and hook the camera body up to a telescope so that the telescope provides the optics. The modern SLR is an electronic marvel that features auto-focus, programmed auto-exposure, and a built-in motor drive to advance the film. But older SLRs are also fine for eclipse photography. SLRs come in several film-size formats, but this chapter will concentrate on the 35 mm SLR.
No decision affects your eclipse photography more than the choice of a lens. Wide-angle lenses have focal lengths of 35 mm or less, while normal lenses fall within the range of 45 mm to 70 mm. Telephoto (high-magnification) lenses begin at 105 mm and go to 500 mm or 1000 mm. Zoom lenses provide adjustable magnification and new designs cover a wide range of focal lengths. Additional magnification can be obtained with tele-converters. Tele-converters fit between the camera body and the lens to increase the lens's focal length by 1.4 to 2 times, thus increasing magnification.
 The viewfinder of an single lens reflex camera uses a prism and mirror system that allows you to look directly through the same lens that takes the actual photograph. When you press the shutter button, the mirror flips up and the shutter curtain opens, permitting the image to reach the film. An instant later, the shutter closes and the mirror returns to its original position.
 Although zoom lenses usually have smaller apertures and are slower than fixed lenses, they work fine for photographing eclipses if they are of good optical quality.
Image Size Vs. Focal Length
Graphic ©1999 by Fred Espenak
The size of the Sun's image on your film is determined solely by your lens's focal length. For eclipse close-ups, a telephoto lens or telescope with a focal length of 500 mm or greater is recommended. A 500 mm focal length yields a solar image of 4.5 mm on a standard 35 mm negative. Allowing for one solar radius of corona on either side of the Sun, an image of the total eclipse covers about 9 mm on the film. You can calculate the Moon's or Sun's image diameter on your film (in millimeters) for any camera system by dividing the focal length (in millimeters) by 110.
The least expensive 500 mm telephoto lens is a catadioptric telescope. Catadioptric telescopes use a combination of mirrors and lenses to focus the light into your eye or onto film. Their folded light path allows a long focal length to fit within a short portable tube, making small catadioptric telescopes easily portable--ideal for most eclipse photography.
Coupling a 2x tele-converter with a 500 mm lens will produce a 1000 mm focal length, which doubles the Sun's size to 9.1 mm. For the Sun and corona together, their image size increases to 18 mm.
The ideal focal length for telephoto photography of solar eclipses ranges from 500 to 2000 mm, depending on whether you are concentrating on the corona or on the prominences. Again allowing one solar radius of corona on either side of the Sun, you can calculate that lenses with focal lengths longer than about 1400 mm may not contain all the corona and focal lengths longer than 2500 mm may not capture the entire solar disk. The longer the lens, the more expensive and the less stable a telescope/telephoto system will be, which means that you will need a heavy-duty tripod or mount, adding to your expense and weight.
Improvements in lens technology have revived refracting telescopes as long lenses for eclipse photography. The new extra-low-dispersion (or apochromatic) lenses almost eliminate the color halos around images that handicapped older refractors. Apochromatic refractors have wider objective lenses, yet shorter tubes, making them fast cameras. Apos require equatorial mounts and they cost more and are not as portable as catadioptric telescopes, but they are prized by advanced eclipse photographers for their image quality.
 Michael Covington: Astrophotography for the Amateur (Cambridge: Cambridge University Press, 1988), page 59.
 Catadioptric "mirror lenses" are made by all the major camera manufacturers (Canon, Minolta, Nikon, Pentax, etc.), as well as independent lens makers (Phoenix, Sigma, Tamron, Tokina, Vivitar, etc.).
 Manufacturers of apochromatic, fluorite, and extra-low-dispersion refractors include AstroPhysics, Meade, Takahashi, Tele-Vue, and Vixen.
Because of the magnification required for eclipse close-ups and because your platform, the Earth, is rotating on its axis, a sturdy equatorial mount with clock drive is needed to take quality photographs with a telephoto lens or telescope with a focal length longer than about 1200 mm.
A clock drive slowly turns your telescope one rotation per day so that you can track the Sun, Moon, and stars from east to west as the Earth rotates. Without a clock drive, the Sun drifts through your camera's field at the rate of the Sun's apparent diameter every two minutes. The higher the magnification, the smaller your camera's field of view. Without a clock drive, you must frequently redirect your camera to center the Sun or it will drift to the edge or even out of the frame during the eclipse. A clock drive keeps the Sun centered so you can concentrate on watching and photographing totality. A clock drive also permits you to use longer focal lengths and take longer exposures without fear of blurring your photos due to the Earth's rotation.
Many modern telescopes are equipped with 12-volt battery-driven clock drives. A battery-driven clock drive frees you from worrying about AC access, voltages, frequencies, and plug sizes used in other countries. If your telescope uses DC power, bring two fresh 6-volt lantern batteries and wire them together in series. They will run your telescope for several hours.
For your telescope to follow the Sun throughout the eclipse, the axis of the equatorial mount must be aligned with the celestial pole at the eclipse site. This task can be accomplished by arriving at your eclipse site the night before the eclipse and aligning on Polaris in the northern hemisphere or Sigma Octanis in the southern hemisphere. If the eclipse is south of the equator, make sure your clock drive has a reversible motor for use in the southern hemisphere.
More likely, however, you will have to set up your telescope just hours before the eclipse, with no opportunity for nighttime polar alignment. In this case, you will need a bubble angle finder and a good-quality magnetic compass. Bubble angle finders let you measure angles with respect to the horizontal and are available in most hardware stores.
First, level your equatorial mount and tripod using the bubble angle finder. Use your compass to align the azimuth of your polar axis on north. Since magnetic north can differ from celestial north by many degrees, you need to know the offset, or magnetic deviation, for your observing site and make that correction in azimuth. The National Geophysical Data Center web site provides the magnetic deviation for the longitude and latitude of your observing site (www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl). You can also get this information from the nearest airport or from aviation or topographic charts.
Next, adjust the polar axis of the mount to the latitude of the observing site using your bubble angle finder. The polar axis should be set at an angle from the horizontal equal to your geographic latitude.
Unless you have a clock drive, you must use relatively short exposures because the rotation of the Earth causes the Sun to appear to drift westward 1/2° every two minutes. For your image to be in sharp focus, here is a formula for the longest exposure allowable with no clock drive:
Exposure (seconds) = 340 / focal length (millimeters) For longer exposures, you need a clock drive to compensate for the Earth's rotation.
 If your clock drive requires AC, check on a source of electricity at your observing site or use a drive corrector with a built-in converter that changes 12-volt DC to 60-cycle, 110-volt AC. Many countries use 220-volt AC electricity. If you want to plug into their power, bring along an AC converter, available in many hardware stores.
 This quick-and-dirty polar alignment should be good enough for the eclipse. If you need a better alignment, you might try a technique called drift polar alignment (eclipse.gsfc.nasa.gov/align.html).
 Michael Covington: Astrophotography for the Amateur (Cambridge: Cambridge University Press, 1988), page 95.
Sequence of the Total Solar Eclipse of 1970 March 7
60mm Tasco refractor (f.l.= 910mm, f/15, ISO 160).
(click to see more photos)
Flimsy tripods are the main reason that eclipse photographs come out fuzzy and blurred. Small portable tripods that are so nice for airline travel are not study enough to hold your camera and heavy telephoto lens steady for clear, sharp eclipse pictures. Because you will be touching your camera to adjust exposures, your tripoly 30 Partial Solar Eclipses quickly.
For greatest stability and to minimize vibrations, don't extend the tripod's legs more than half way out and do not extend the center column. Try adjusting the height so that you can easily reach the camera controls while kneeling on the ground or sitting on a chair. Test your setup by tapping on the camera or tripod leg while viewing the Sun through your camera. Vibrations should be small and should damp out quickly.
You can further decrease vibrations to your camera by suspending some weight under the tripod. Put rocks or sand in a sack or in plastic zip-lock bags and hang them from the center of the tripod using string or duct tape.
Before you ever travel to an eclipse, make sure that your lens on its tripod can be pointed at the Sun's predicted altitude for the eclipse and verify that all controls work smoothly. You don't want to discover that your equipment becomes unstable when you try to view the crucial portion of the sky on eclipse day.
 Bogen/Manfrotto, Gitzo, and Slik all make heavy-duty tripods.
You cannot take crisp, high-quality close-ups of a total solar eclipse without a cable release specifically designed for your camera. Cable releases can fail unexpectedly, so have a spare.
A right angle finder is a little prism/eyepiece device that attaches to your SLR's viewfinder. It allows you to look through the viewfinder when your head is above the camera instead of behind it. Your camera will be pointed upward during the eclipse. Without a right angle finder, you might have to get down on your hands and knees or maybe lie your back to look through your camera. A right angle finder allows you to comfortably center and focus the Sun's image. You will also be facing the top of your camera so you can see all the controls during totality.
For more information, see:
Totality - Eclipses of the SunSecond Edition
by Mark Littmann, Ken Willcox and Fred Espenak
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Contact Espenak (at MrEclipse) for more information.
All photographs, text and web pages are © Copyright 2007 by Fred Espenak, unless otherwise noted. All rights reserved. They may not be reproduced, published, copied or transmitted in any form, including electronically on the Internet or WWW, without written permission of the author. The photos have been digitally watermarked.
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Last revised: 2008 Jan 22