Astrophotography
EQUIPMENT Photographing the night sky is interesting for anyone with even a slight interest in astronomy. You don’t even need much equipment to take picture of star trails, comets and moonscapes. The main requirement for the camera is the ability to take time exposures and accept a cable release or remote control cable. Cameras that can take time exposures usually have a “B” shutter speed that stays open for as long as the release is pressed. A locking cable release lets you lock the shutter open for long exposures. A sturdy tripod is a must, because even slight movement of the camera during the exposure will reduce sharpness. Light from stars, comets and other objects in the night sky is very dim. Setting a digital camera to a moderately high ISO setting (or ISO 400 or faster film if using a 35mm camera) will help keep exposure times from being unduly long.
A midrange zoom lens will work fine for recording star trails, meteor showers and comets. Because celestial objects appear so small and far away, telephoto lenses won’t dramatically increase their size and visual impact in photos. However, a telephoto lens (preferably 300mm or longer) will be helpful when photographing sunsets or the moon. In fact, the sun or moon will be very small in pictures taken with a midrange lens. Wide-angle lenses can be used in astrophotography to cover a greater section of sky or to include more land objects, such as buildings or trees. Cameras can be mounted on some telescopes for high-magnification photos. The telescope usually mounts on an SLR camera in place of the lens. A camera attachment supplied by the telescope manufacturer is used, plus a T-mount adapter to fit the specific camera make/model onto the camera attachment.
TIPS ON TECHNIQUE The ideal condition for taking pictures of the night sky is crisp, clear weather, and a moonless sky away from the bright light of cities. “Light pollution” from cities and the moon creates a grayish sky that gets even lighter during long exposures. Aerial haze reduces the number of celestial objects that are visible and also produces a grayish sky.
EXPOSURE TABLE SUBJECT
EXPOSURE TIME*
Comets Star Trails Meteors Auroras
10 sec. to 15 min. 5 to 30 minutes 10 to 30 minutes 1 sec. to 2 min. (depending on brightness)
Full moon Half moon
1/500th f16 1/250th f11
If shooting film, start a new roll with a daylight or flash picture of a person or some other common subject. This gives the film processing lab a reference point for cutting the film. Tell the lab to print all negatives or mount all slides. Otherwise, the lab may think the pictures are “mistakes” and not print all negatives or mount all slides. Since depth of field is not a factor in astrophotography, the lens should be set at or near its widest aperture. Exposure is controlled by the length of time the shutter is open. Doubling the exposure time or reducing it by half results in a one f-stop change. Multiplying it by four or reducing it to one-fourth gives a two f-stop change. Several pictures at various exposures (called bracketing) should be taken to make sure that you get some good results. Stars, comets and the moon move in the sky because of earth’s rotation. During time exposures, this movement is recorded on film as streaks. The longer the exposure, the longer the star trails will be. If you are really interested in astronomy, you may want to consider the purchase of a telescope with a motorized drive and mount system. This device compensates for earth’s rotation, so stars and planets can be photographed as stationary, rather than moving objects. Pictures of the night sky are often more interesting if something on land is in the foreground. A hill, building, farm or a few trees will add more depth and impact. Avoid having streetlights and other bright ambient lights in the pictures. Because of the long exposure time of the celestial objects, lights on the ground will severely overexpose the landscape.
COMETS Some comets appear only once while others (such as Halley’s) appear on a regular cycle. Dim comets will be difficult to record. To photograph bright comets, use fast film or a high ISO setting, a wide open lens aperture and an exposure time between 1 and 15 minutes if the sky is really dark. If it’s twilight, reduce the exposure to 10-60 seconds. For optimum viewing, go to a rural area away from city lights on a night with no aerial haze or bright moonlight.
STARS TRAILS Bright stars are easy to photograph, making them good subjects for your first effects at astrophotography. Because the earth rotates, star pictures show the stars not as points of light but as trails. You can make interesting circular star trails by photographing the sky around the North Star using an exposure time of at least five minutes. Silhouettes of trees, buildings or hills along the horizon line will add an interesting base to your photos.
METEORS Meteors are bits of matter that burn as they rapidly enter the earth’s atmosphere. They are a challenge to photograph as each meteor is visible for only a few seconds and may appear anywhere in the sky. Meteor showers appear during certain times of the year. Astronomy guides list the dates for various meteor showers and where to look for them.
AURORAS Also called the Northern Lights, auroral displays vary greatly in brightness, color and intensity, so bracketing the exposures is important. It’s a good idea to use a high ISO setting or ISO 800-1000 film. Light patterns in auroral displays move, so shorter exposure times resulting from the high speed film yield sharper images.
MOON Our closest neighbor, the moon is the brightest object in the night sky. To the human eye, the moon is very big and prominent. However, a picture of the moon taken with a 50mm lens will be disappointing. The size of the moon on film will only be about the size of the period at the end of this sentence. A 300mm or longer telephoto lens will produce pictures showing surface detail. Compared to distant planets and stars, the moon is very close and bright so the exposure is far shorter. In fact, the exposure for a full moon is the same as for a sunlit subject here on earth. (f16 at the shutter speed closest to the ISO speed. For example, f16 1/125 at ISO 100.) Increase the exposure two f-stops for a half moon and 3-1/2 f-stops for a crescent moon. You’ll get a totally black sky when photographing the moon in total darkness. By taking moon pictures at twilight while there’s still some light in the sky, you can keep detail in both the moon and any landscape that is part of the foreground.
Courtesy of— Porter’s Camera Store
ph. 1-800-553-2001
www.porters.com