Doing astrophotographyTripod-mounted photographs
. The equipment necessary for a tripod-mounted astrophotograph is minimal: A camera, lens, tripod, cable release (preferably with a lock), and a watch with a second hand (or a stopwatch). Focus the camera on infinity and lock open the shutter. Sometimes, astrophotographers include the horizon or an object in the foreground like a tree or mountain for a dramatic effect. It's really up to you. If star trails are your goal, try numerous exposures. And remember, long-exposure astrophotography requires a dark sky because light pollution fogs film.
Be sure to purchase a good tripod. This is an important photographic accessory. It's amazing how many people try to save money and end up with a shaky, inadequate tripod they eventually have to replace. A good tripod will work with any camera, so buy the best and strongest tripod you can afford.
Make sure the tripod you select has easy-to-operate controls that can be worked with gloves on in the winter. Ensure the tilt and pan movement locks are strong and will hold a front-heavy telephoto lens. A sturdy tripod also will allow you to mount two (or even more) cameras onto it at the same time using a bar or a plate. Finally, make sure the tripod you buy will point to the zenith.
The next necessity for tripod-mounted shots is a quality cable release. This item ensures that vibrations caused by your hand triggering the camera shutter don't ruin exposures. A cable release should be at least 18 inches in length to isolate the camera from the operator's motions. Several types of cable release locking mechanisms exist — pick one and use it.
Okay, you've mounted your camera to a tripod and pointed the lens at the sky. But you don't want to see "trailed" stars in your finished picture. Is there a way to figure out how long an exposure you can take without trailing? Yes, there is, and it can be determined with this formula x = 1000/fl*(cos d) where x
is the exposure time in seconds, fl
is the focal length in millimeters, and d
is the declination of object, in degrees. Declination (d
) is a factor because the closer you get to either Celestial Pole, the smaller the star trails are per unit time. Star trails are greatest when your camera is pointed at the equator.
Using this formula, the values in the table below were obtained.
|Maximum unguided exposure in seconds|
|declination||focal length of camera lens|
|(north or south)||28mm||35mm||50mm||135mm|
When setting the f-stop of your camera lens (how open the lens diaphragm is), remember these two words: wide open. Many astrophotographers stop down the lens (increase the f-stop) one or two clicks because the star images at the edges of the photograph are not pinpoints (due to lens irregularities). Rather than lose that light, which you will need if there are extended objects in the field (stars are pinpoint objects and are not affected by f-stop), shoot with your lens wide open and then either electronically crop the image or mask the slide with black photographic masking tape, which is available at photo-supply shops.