November 25, 2008
Cold, clear winter nights are ideal for astrophotography, but in my experience it is by far the most difficult photographic pursuit. Even something as simple as shooting a lunar eclipse can be trying. Given that our universe is constantly on the move, exposure times are always a consideration. Ideally one would want to use the shortest exposure time necessary to capture a sharp image. That said, using too high an ISO can also degrade image quality. The upside is that at least a lunar eclipse gives one time to experiment with various settings. Usually, long exposure noise reduction is not an issue either because most exposures will be under the applicable threshold.
When your subject is really on the move, the whole process gets quite a bit more dicey. The four most popular moving objects to image are meteor showers, the Space Shuttle, the International Space Station (ISS) and Iridium flares. I've tried all four with mixed success.
The best meteor showers to view are the Perseids in August and the Leonids in November. There are many others at various times throughout the year. The most valuable resource that I have found is the meteor shower page on Astronomy For The People. They have loads of data and lots of tips. Be prepared to make lots of long exposures and be frustrated. As moving objects, meteors are totally unpredictable except for a very general instruction to look in a certain direction. After three years of trying, I have yet to capture a meteor.
Taken individually, the Space Shuttle and the ISS are not very brilliant objects. When docked, however, they make a much brighter target. Fortunately, the position of either at any given time is predictable. You can find out where they are by consulting the Heavens Above website. Heavens Above is an invaluable compendium of data about the whereabouts of objects in space, both natural and man-made, including the Shuttle and the ISS.
Iridium flares are caused by the Sun glinting off the Main Mission Antenna of a relatively small communications satellite. These reflections can be up to 30 times brighter than Venus so they are quite visible. That said, the flares only last from 5 to 20 seconds before the satellite becomes invisible to the naked eye. Iridium flares are predictable and can be quite spectacular. Consult Heavens Above to find out when one will occur.
Getting a good image of any of these fast moving objects can be very difficult. Use a tripod and full manual mode. To give yourself the best edge, mount a lens of 50 mm or less. I like 24mm myself. Shoot in raw, use the highest ISO setting you feel comfortable with, and plan on making a manual exposure using a remote. Try to keep your exposure under 30 seconds for the Shuttle/ISS and Iridium flares to avoid "star trails," but don't worry if your shot looks over-exposed; exposing to the right means that you will have that much more data to work with. For meteor showers you may just have to live with motion distortion to get a good exposure as you will never know when one will show and you will have to use long exposures. Be aware that using long exposure noise reduction will double the time of your exposure.
Your sighting will happen very fast and you will have only one chance so you need to be absolutely sure where the object will be in the sky. I missed a rare double Iridium flare last winter because my camera was aimed too low on the horizon. Be sure you know exactly how many degrees above the horizon line you need to look and in what direction the object will be coming from. This is easier said than done, so do your research on Heavens Above or elsewhere on the Internet. Out in the field you may want to consider using some sort of aiming device like a Altazimuth or Equatorial telescope mount. Prices on these vary widely and the investment may not be worth it if you are a casual shooter.
A final caveat: If you will be disappointed that all your planning and hard work will only yield an image of a streak of light across the sky, try another pursuit. For many of us though, that streak of light represents something much larger and very satisfying.
Chris Lane is a lawyer and photographer who writes a photo-blog entitled Written By The Light.
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