Observing and Photographing Lunar Eclipses

Lunar eclipses are leisurely events and a pleasure to watch and photograph.

"Like a distant peach hanging in the sky!" That's how Akira Fujii described the July 2000 eclipse - a nighttime spectacle that was the most beautiful lunar eclipse the veteran astrophotographer had ever seen.

A lunar eclipse occurs when the Earth lies between the Sun and the Moon, so that Earth's shadow darkens the Moon. This can only happen at full Moon. The dark, central shadow is the umbra, while the lighter shadow that surrounds the umbra is the penumbra. The penumbral shadow is difficult to detect; for most observers a lunar eclipse really starts when the umbra first touches the lunar surface. Totality occurs when the Moon is completely immersed in the Earth's umbral shadow.
Lunar eclipses get their colorful red-orange hues from sunlight that is filtered and bent by the Earth's atmosphere before it reaches the Moon. Bright and colorful eclipses occur when our planet's upper atmosphere is transparent. Major volcanic eruptions that spew dust and aerosols skyward often result in darker lunar eclipses.

The Moon's brightness varies greatly from eclipse to eclipse, which would tell a lot about the state of the Earth's upper atmosphere if only we understood it better. To help in compiling statistics of this kind, many observers rank each eclipse they see on the five-point Danjon Scale article.
An even simpler technique was advocated 80 years ago by Harvard astronomer Willard Fisher, who graded midtotality into three classes depending on the equipment needed to see details on the Moon's surface: (1) the naked eye, (2) a 50-millimeter finderscope or binoculars at 7x, or (3) a 150-mm (6-inch) telescope at about 20x. Fisher reasoned that the Moon's surface brightness is unaltered by the aperture, but the visibility of low-contrast features like lunar maria depends greatly on the image scale. The darker the eclipse, the greater the aperture (and power) needed.

Still another useful gauge is the Moon's stellar magnitude at mideclipse. People who wear thick eyeglasses can simply take them off, turning the Moon and bright stars or planets into blobs of equal size for easier comparison. Looking through the wrong end of binoculars also helps.

Photographing a Lunar Eclipse

The best advice for photographing a lunar eclipse is to take lots of pictures at many different exposures and throw most of them away. You never know just how bright your subject is going to be.

Most importantly, you'll need a telescope or telephoto lens that enlarges the Moon to a good size. For a typical lunar eclipse the size of the Moon's image on film equals the focal length divided by 110, with the result in the same units as the focal length. Thus an 8-inch f/10 telescope, with a focal length of 80 inches (2,030 millimeters), will produce a Moon image 0.73 inch (18.5 millimeters) across, easily fitting on a frame of 35-mm film, which measures 24 by 36 mm.

But an ordinary point-and-shoot camera with a lens of about 40-mm focal length will produce an image of the Moon barely 0.4 mm across, probably not large enough to record even as much detail as you can see with your unaided eye. If you have a point-and-shoot camera with a zoom lens, zoom the lens out as far as you can to get the highest possible magnification.
The minimum focal length for getting a good-looking Moon is about 300 mm. With a 500-mm lens and a 2-second exposure you can probably get away with a camera on a fixed tripod, but longer exposures or focal lengths will require a tracking mount to prevent blurring due to the turning of the Earth. A simple, do-it-yourself tracking camera mount is described on page 32 of the May 1996 issue of Sky & Telescope.

The full range of possible eclipse images - from sequences showing how the event unfolds to close-ups of the shadow's edge on the Moon's surface - calls on a rather large bag of photographic tricks. Compared to recording a normal full Moon with a given camera setup, a lunar eclipse requires exposures perhaps 4 to 1,000 times longer - plus everything in between! Much depends on the stage of the eclipse you're trying to record, the darkness of the shadow, and the pictorial effect you are after.

The shortest exposures are useful only during the partial stages near the beginning and end of the eclipse, and they'll probably bring out little more than the smooth gradation in the shading of the outer shadow, or penumbra. That's also what you'll get if you simply let a camera's built-in metering system take control of the exposure. To record the well-defined edge of the inner shadow, or umbra, which is so striking to a visual observer, you'll have to override the system and expose several times longer. Typically that means a ¼-second exposure at f/8 with ISO 400 film, "burning out" the Moon's bright limb but getting sharp detail near the umbra's edge.

As for totality itself, there is no safe bet - the brightness of the fully eclipsed Moon is too unpredictable. Because film is cheap compared to the rarity of a total lunar eclipse, you can hardly go wrong trying every camera setting you've got. There's plenty of time to experiment during this leisurely event.

Eclipse Photography in the Digital Age

The ancient Greek philosopher Heraclitus once said, "There is nothing permanent except change." Nowhere is that more evident than on the rapidly evolving digital-imaging front. One of the latest technological innovations to hit the consumer market in recent years is the digital camera. Compact, lightweight, and loaded with features, digital cameras are fast gaining popularity among point-and-shoot photographers as well as advanced amateurs and professionals. But unless it's a professional model, the camera is unlikely to have a lens with a focal length large enough to provide a satisfying lunar image. The solution is a combination of a digital camera and a telescope.

Photographic film still surpasses digital cameras in terms of resolution and color accuracy. And, unlike dedicated astronomical CCD cameras that use cooled chips, the CCDs in consumer cameras generate objectionable "noise" during long exposures. As such, their useful exposure times are limited to a few seconds at most. This is fine for bright subjects such as the Moon and planets, but too short for all but the brightest stars. And don't even think about galaxies or nebulae. Digital cameras generally need a computer to manipulate and print images, but digital-imaging technology is evolving rapidly, and already there are printers that work independently of a computer.

So what are the advantages of going digital? Well, for one thing, you get instant results - no more waiting for your pictures to arrive from the photo lab. You keep only what you want and delete those you don't need. This saves on the price of film and processing in the long run. Depending on the camera's memory capacity, it can hold far more images than any roll of film.

Since the camera's output is already digital, the images are ready for computer processing with programs such as Adobe Photoshop, which can enhance image quality by "stacking" individual exposures or create a wide-field mosaic by stitching together separate frames. The images are Internet-ready - you can send them by e-mail or share them on the Web. They are also easier to sort and organize and take up less physical storage space than photo albums or shoeboxes.

Digital cameras are versatile - they can be easily used to digitize prints and 35-millimeter slides by recopying them. They can also be coupled with binoculars or a spotting scope for wildlife photography, especially for shooting birds. Last, the price of high-resolution cameras is constantly dropping as new models are introduced. You can now get a camera with 3.3 million pixels (3.3 megapixels) for less than $800. All these features make digital cameras very attractive for astro imaging.

Since most consumer-level digital cameras don't have removable lenses, the only way to take images through a telescope is with the afocal method, whereby the camera's lens is aimed directly into the telescope's eyepiece. You can hold the camera by hand, mount it on a separate tripod, make or buy a bracket, or use an adaptor to attach the camera directly to the eyepiece. Unless you're old enough to have learned astrophotography from the Sam Brown booklets (above) that Edmund Scientific sold for less than a dollar during the 1960s, chances are you haven't had much experience with afocal photography and may not even be familiar with the term. Nevertheless, every clear night amateurs with digital cameras are rediscovering this technique, which was happily forgotten by more than a generation of astrophotographers. Now, as then, the reason for choosing the afocal method of mating a camera to a telescope is the same - some cameras have nonremovable lenses. Unlike then, however, the results amateurs are achieving now are consistently spectacular.

To the uninitiated, nothing could be more logical than afocal photography: if you look through a telescope's eyepiece, why wouldn't you point a camera into it? And before single-lens reflex cameras became popular in the 1960s, that's what most of us did when we wanted pictures of the Moon and, for the very adventurous among us, planets. The problem was, we were shooting almost blindly. A host of subtle optical issues made it difficult to know how a picture was focused and framed until after the film was developed. Today these issues are all moot - you can see exactly how the image looks on a digital camera's viewing screen before pressing the shutter button. (In computer lingo it's called WYSIWYG - "what you see is what you get" - and it alone has lifted much of the curse from afocal photography.) And, of course, there's instant feedback when the shutter closes. Don't like the result? Just delete the image; there's no wasting film (or money).

Another pleasant aspect of afocal work in the digital age is that we can ignore all the laborious measuring and calculating that made the process so intimidating in the past.
Like the familiar eyepiece-projection technique, the afocal method dramatically increases the image size but at the expense of substantially slower telescope f/ratio, or "photographic speed." You may also get some vignetting and image distortion. Vignetting is a darkening around the edge of an image. It happens when the camera is held too far from the eyepiece or when the apparent angular field of the camera lens exceeds that of the eyepiece. This, however, is usually not a problem when you are photographing planets or other small objects that are surrounded by dark sky. To reduce vignetting, set the camera as close and centered to the eyepiece as possible. Also, choose an eyepiece with ample eye relief - some short-focus eyepieces have such limited eye relief that a camera cannot get close enough to image through them. Find the best camera position by experimentation.

Because it narrows the camera's angular field, zooming in can minimize, or even eliminate, vignetting (but avoid using "digital zoom," which sacrifices the CCD's resolution). Zooming also increases magnification, which often makes focusing easier.

Image distortion can be caused by a combination of effects in the eyepiece and camera optics. The central portion of the image may be in focus while the outer areas are not, severely restricting the usable field. Be sure you keep the camera's image plane centered on and perpendicular to the telescope's optical axis. Also, keep the eyepiece and camera lenses free from smudges or dust particles, which can degrade the image quality and contrast.

Mounting the camera on a separate tripod can reduce telescope vibration and eliminate the need to rebalance the scope. With such an arrangement it helps to orient the eyepiece so that it moves directly toward or away from the camera as the telescope follows its subject across the sky. Use a black cloth or cardboard mask to shield the eyepiece and camera from stray light.
Although not essential, a telescope motor drive is convenient to have since it keeps the subject centered on the camera frame as you focus and compose the shot. For those scopes without a drive, you can still take good pictures provided you keep the exposures short. Position your subject at the edge of your frame opposite the direction of drift, then wait for the object to glide near the frame's center before opening the shutter.

As with film astrophotography, a good-quality finder is helpful for aiming the telescope. For imaging at high magnifications, a guidescope with an illuminated-reticle eyepiece that is precisely aligned to the main scope can facilitate centering small targets such as planets.

Taking the Shot

Unlike conventional single-lens reflex (SLR) cameras, there is no mirror slap in digital cameras that could cause vibration when you are taking exposures, but you can still cause shaking when touching the camera to release the shutter. To prevent this, use the camera's self-timer or wireless remote release, if it has one. Otherwise, you have to press the shutter yourself (very gently!) without shaking the setup. Some people have fashioned homemade brackets to hold the end of a cable release over the shutter button.

Many camera models have no manual override for exposure (you cannot control the camera's shutter speed and aperture), so you have to rely on the autoexposure feature. This function works best with large, bright, uniformly illuminated subjects such as close-ups of the Moon. But the camera's light meter may either overexpose or underexpose the image of the crescent Moon or planets, so you need to manually correct using the camera's exposure-compensation capabilities (usually +2 to -2 stops). Remember to bracket your exposures. Preview the results on the LCD screen, and save the sharpest and best exposed. Don't be afraid to experiment with exposure compensation - try one or two stops brighter or darker than normal, and note the best camera settings.
For the planets, trial and error is needed to find the correct exposure. For example, a good starting point for Jupiter and Saturn is ¼- to ½-second exposure with typical Schmidt-Cassegrain telescopes. Keep exposures short to minimize blurring due to atmospheric turbulence.

The number of exposures you can take is limited by the camera's memory capacity. Image files are stored either on a memory chip in the camera itself or on a removable card. When the card is full, the images are offloaded to a computer with either an optional external card reader or a high-speed USB cable. The images are then archived on the computer's hard disk, a floppy or Zip disk, or a CD-ROM.

Use your camera's highest image-quality setting when saving the image. This means using the camera's full resolution and applying the least image compression. Most cameras save images using JPEG compression to reduce file size and store more images within the available memory. The more you compress an image, however, the more it shows artifacts such as degraded color or noticeable "graininess." Use the lowest compression possible or save the image with an uncompressed format. Don't be surprised if you can store more astronomical than "daytime" images in the same amount of memory. The dark background of a typical astronomical scene compresses far more than conventional images. You can also experiment with the black-and-white mode if your camera offers this feature. Because of the way color CCDs work, images shot in black and white can appear sharper than color images.

So, what is the best advice for using a digital camera for astro-photography? It is simply, "Just try it."
The August 2001 issue of Sky & Telescope magzine (page 128) describes in detail many of the tricks and techniques involved in taking astronomical images in this manner. But when digitally imaging a lunar eclipse, the key is to shoot, shoot, and shoot some more. A digital camera gives instant results, and since a lunar eclipse is a leisurely affair you can keep on trying until you acquire a good image.

References
Fred Espenak

Alan M. MacRobert and Paul Deans: Sky & Telescope Magazine Website