Photographing the Transit of Venus

1882 transit of Venus (left); 2003 transit of Mercury (right)
Left: Witnessed only five times since the invention of the telescope, Venus’s solar crossing is an eagerly anticipated event. This photograph of the planet’s last transit on December 6, 1882, was captured by a US Naval Observatory expedition. (The grid and spots are artifacts of the telescope system and emulsion.) Compare it with the composite view of the transit of Mercury (right) on the right obtained on May 7, 2003, by Dominique Dierick from Ghent, Belgium, with a 155-millimeter f/7 Astro-Physics refractor, a Nikon D100 digital SLR camera, and a Baader AstroSolar filter. With Venus nearly 2½ times larger than Mercury, and only half as far away form Earth, its silhouette will appear almost 5 times bigger this coming June 8th (58 arcseconds versus 12).
For the transit of Venus across the Sun on June 8th, amateur astronomers today have at their disposal a wide array of recording media, from modern high-speed ultrafine-grain film to high-resolution webcams and digital, CCD, and video cameras — high-tech, off-the-shelf equipment that 19th-century astrophotographers could only dream of. While we don’t expect any new scientific breakthroughs from this event, its appeal to amateur astronomers has not been diminished by this fact. Here are some tips and pointers on how to capture this rare and historic event for posterity.


Basic Requirements

Size of Sun and Venus on 35-mm film
Use this guide to see what telescope focal length (FL) you’ll need to photograph this year’s transit of Venus. On June 8th the Sun and Venus will measure 31.51-arcminutes and 0.97-arcminutes across, respectively. Here they're shown at their actual sizes relative to the 35-mm film format (24 by 36 mm). To get the size of the Sun in millimeters with your equipment, divide the effective focal length of your lens or scope by 109.1; for Venus, divide it by 3,544. For example, a refractor with a focal length of 4,000 mm will produce images of the Sun and Venus 36.7 and 1.1 mm across, respectively, at the film plane.
Sky & Telescope: Gregg Dinderman.
Photographing the transit is much like photographing sunspots. But first you have to decide what you want to record — the whole disk of the Sun or close-ups of Venus’s ingress (entrance) and egress (exit) along the solar limb to try and capture the so-called “black-drop” effect. In this curiuos phenomenon, as Venus’s silhouette makes contact with the limb, it seems to draw a thread of blackness that distorts the silhouette’s shape. It’s an elusive effect that was first reported by astronomers during the 1761 transit.

You’ll likewise need a proper, visually safe solar filter to cut down the Sun’s intense brightness and heat. Make sure your filter is securely mounted on the front of the telephoto lens or telescope objective. (Polarizing or photographic neutral-density filters are not safe for visual use.) For a list of filter manufacturers and dealers, see "Solar Filter Suppliers."

The focal length of the camera setup you should use depends mainly on what you want to capture. A standard 50-millimeter lens gives only a minuscule image (0.5 mm in diameter) of the Sun on film. To show the Sun’s disk (and that of Venus) reasonably large on film, you’ll need a telephoto lens or telescope with a focal length from 1,000 to 2,000 mm or more (see the diagram above), as well as a solid tripod or mounting.

For any telescope of a given focal length, the diameter of the Sun’s image is roughly equal to focal length (in mm) divided by 109. A standard 8-inch f/10 Scmidt-Cassegrain telescope with a 2,000-mm focal length yields a solar image 18 mm across, which nearly fills the frame of a standard 35-mm film. For close-up shots of the transit, you’ll want a telescope with about 4,000-mm effective focal length or more.

Be sure to test your equipment and practice your procedure well ahead of the actual event. You can begin by letting the camera’s light meter determine the exposure. Then try a variety of other exposures on either side of it (a technique known as “bracketing”). Keep careful notes and see which one comes out best. Such dry runs can also reveal potential problems with focus and vibration, as well as internal reflections and vignetting in your setup. Try to do your testing around the same time the transit will occur to determine the best exposure to use.

Film Considerations

In general, color-negative emulsions (those used for prints) offer greater exposure latitude than transparency (slide) films do; that is, they record features over a wider range of brightness with a single exposure. When choosing the film’s speed (ISO rating), bear in mind that the faster the film, the shorter the exposure. Short exposures tend to minimize blurring due to vibrations and tracking errors. But fast films are relatively grainier than slow emulsions.

For those traveling overseas to see the transit, bear in mind that newer, more powerful security X-ray machines are now in use at many airports. These units can damage film, especially high-speed emulsions. If possible, ask that your films be hand-inspected; never pack your film in your checked luggage.

Focusing Issues

Mercury near the Sun's limb
This image showing Mercury about to exit the Sun’s face on May 7, 2003, was made by Enrico Perissinotto of Talmassons, Italy, using a 130-mm Astro-Physics refractor working at f/12, a Baader filter, a Herschel wedge prism, and a Philips ToUcam Pro webcam. This image scale is ideal for capturing the so-called black-drop effect.
Focusing is especially critical when you use telescopes and superlong telephoto lenses that don’t have a fixed infinity setting. In some 35-mm cameras you can replace the viewfinder with a magnifier to aid in focusing. Once you achieve optimum focus, place a piece of adhesive tape on your lens’s focus ring or your telescope’s focus knob to prevent it from accidentally being moved during the transit. The same technique also applies when you set zoom lenses, which can slip without warning, especially when aimed high in the sky.

If the Sun’s image fills the frame, focus to make the solar limb look sharp where it will actually fall on the film (near the edge of the frame); don’t move the limb to the field’s center to focus on. Schmidt-Cassegrain telescopes in particular focus a little differently at the center of the field than they do at the edge. Be sure to recheck your focus as the transit progresses since changing temperature can cause the focus to shift slightly.

Mountings

Whether you’re traveling by land, sea, or air to your observing site, try to keep your mount as portable, light, and easy to assemble and operate as possible. Portability is especially essential if you need to relocate in a hurry to escape clouds.

In central Europe and the Middle East the Sun’s altitude will be up to 76° at the end of the transit — make sure your camera tripod can be aimed this high. A geared head allows for slow-motion control and is ideal for manually tracking the Sun, which moves across the sky by its own diameter every two minutes or so.

To improve a tripod’s stability, hang some weights under its center post. You can also set the tripod legs on rubberized footpads to dampen vibrations. The mirror slap in single-lens reflex (SLR) cameras can blur images, especially at slow shutter speeds. To reduce camera shake, operate the shutter button with a long cable release or use the camera’s delay timer. Lock the viewfinder mirror up beforehand if possible. Last, choose your site so it’s shielded from direct breeze; erect a windbreak, if needed.

The Digital Alternative

Nikon Coolpix with adapters
For digital cameras with threaded lens barrels, such as the Nikon Coolpix 990 shown here, you can purchase various lens adapters to attach the camera to the telescope. Since you get instant results, digital cameras, like video camcorders with their autofocus and autoexposure features, can take the guesswork out of imaging the transit — what you see is what you get.
Sky & Telescope:Craig Michael Utter.
Digital cameras operate basically the same as a conventional camera except that they use a CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor) chip instead of film for capturing images. A digital camera’s resolution is measured by the number of pixels in its image (expressed in millions of pixels, or megapixels). The more pixels an image has, the better quality the image will be, so get the model with the most megapixels that you can afford.

Digital cameras range from the consumer-level point-and-shoot cameras costing between $200 and $1,000 to "prosumer" digital SLR cameras (about $1,000 and up), which look and feel like 35-mm SLRs and feature fully manual controls and detachable lenses. They use removable memory cards to store images. High-resolution pictures require more memory, so try to buy cards with the largest capacity in order to avoid running out of memory at a critical time. Use your camera’s built-in preview screen so you can save memory by deleting the bad images.

Transit Videography

Nothing offers instant gratification in the field better than a video. All of today’s camcorders use highly sensitive CCD or CMOS detectors. The Mini-DV or Digital 8 format offers the highest resolution compared to 8-mm, VHS, VHS-C, Hi-8, or S-VHS formats. The compact size and light weight of 8-mm camcorders make them ideal for travel. There are dozens of models and prices from which to choose, with features such as flip-out color LCD viewfinders and image-stabilized optics.

Some camcorders now have zoom lenses with up to 32x optical and 64x (or more) digital magnification. Optical zoom is more important since it increases image scale on the detector. (Digital zoom simply enlarges the pixels.) The easiest way to determine the actual size of the Sun in your camcorder is to shoot brief footage of it, zooming in to the highest power. If your camcorder doesn’t have enough magnification, consider adding a high-quality teleconverter (3x or more) to the front of the lens or shooting through a telescope with a wide-field, long-eye-relief eyepiece.

As with still cameras, you need a proper solar filter over your camcorder or scope when recording the transit. Keeping the Sun centered in the field of view at high magnifications will be a lot easier if the telescope is on a motor-driven, polar-aligned equatorial mount.

Take 2- to 3-second clips every two to five minutes to produce a time-lapse sequence that compresses the 6.2-hour-long transit into just minutes. High-end camcorders have manual controls for adjusting the gain, f/stop, and shutter speed so you don’t overexpose the Sun’s disk and cause blooming (streaking) of the image. Again, it's best to test your setup well in advance. On transit day, be sure to use a freshly charged battery. Keep a spare one as backup.

Mounting the camcorder directly to the telescope requires that you use a beefy mount and rebalance the scope; holding it with your hands can create shaky footage. A better alternative would be to attach a small black-and-white video camera, such as Orion's Electronic Imaging Eyepiece or Meade's Electronic Eyepiece to the telescope’s 1¼-inch eyepiece holder. These cameras are compact, lightweight, and inexpensive (around $70), and run on a single 9-volt battery. You record the video output separately with either your camcorder or a portable VCR.

After the transit, be sure to remove the tape from the camcorder for safekeeping; don’t forget to label the videocassette and “lock” it or break its tab so you won’t accidentally erase your recording.

Webcams and CCDs

Webcam attached to a telescope
To use a webcam for solar imaging, unscrew its supplied lens and attach an adapter that mates the camera to a properly filtered telescope; use a Barlow lens to increase the magnification. You can either machine your own adapter or buy one commercially.
Sky & Telescope: Craig Michael Utter.
Webcams and astronomical CCD cameras are also ideal for capturing transit images, though they do require a computer to operate and download images. This entails an additional power requirement, which can be an issue during the hours-long transit.

As with digital cameras, the output of these devices is already in digital format, so anyone in the field who has Internet access can e-mail images anywhere in the world or post them on a Web site. In addition to still images, webcams can send live streaming (continuous) video of the transit in real time over the Internet or another network. For more information about webcam astro imaging, see "Shooting the Planets with Webcams" in Sky & Telescope: June 2003, page 117.

Owing to their sensitivity, filtered solar images from CCD and video cameras can still be greatly overexposed. When this happens, you have to add a neutral-density or variable polarizing filter in front of the camera to further cut down the brightness.

Image Processing

With the advent of powerful desktop computers and commercial image-processing software, astrophotographers can now take a range of exposures, select the best ones, and use popular graphic-arts programs such as Adobe Photoshop or Paint Shop Pro to “stack” (combine) them into a single image or stitch them together to create a movie.

With just a few mouse clicks you can easily retouch any film defects (dust specks, scratches, or fingerprints) and adjust the image contrast, brightness, and hue. About a dozen or more images can stacked to improve image quality and produce a smooth composite, which can be printed on a high-quality photo printer or sent to a photo lab to produce prints or slides. The potential for imaging is only limited by your imagination.

If you miss this transit, don’t despair. Remember that transits of Venus come in pairs, so you don’t have to wait more than a century for the next one — you’ll get another chance on June 5–6, 2012.