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| © Paul LeFevre |
| Quarter Moon shot with a Canon 20D, ISO 400, at 1600mm focal length. Click photo to launch a gallery of images. |
There's one night-sky target we haven't covered yet. I saved it until last because it's one of the easiest to take pictures of... our Moon. While the moon appears to move across the sky at nearly the same rate as the stars, it's very bright -- about the same brightness as a daylight scene on Earth (since the light we see from the Moon is just reflected sunlight, that makes sense!). Since it's so bright, it doesn't require the long exposure times that are needed for stars, galaxies, etc. But the Moon can be a great target to photograph.
The issue with taking pictures of the Moon is really focal length of the lens you're going to use. On 35mm film or full-frame digital cameras, it takes a focal length of about 2000mm to fill the frame with the Moon. APS-C DSLRs need about 1200mm to do the same. While you can get satisfactory Moon pictures with shorter focal lengths, the closer you can come to those frame-filling ideals, the better.
Proper exposure values for the Moon are easy to figure out -- just use the "sunny 16" rule. At f/16, use a shutter speed that is the reciprocal of your film or sensor's ISO value; at ISO 200, you'd use 1/200th sec. at f/16. Adjust the shutter speed up or down according to how many f-stops above or below f/16 your lens is operating at. Use a sturdy tripod, use your camera's mirror lock-up function (if it has one), and use a remote or cable release. Focus at infinity, and shoot away. It's best NOT to use your camera's auto-exposure modes; most camera exposure meters see all that black surrounding the Moon, and add exposure time to compensate, which over-exposes the Moon itself.
If your goal is to shoot frame-filling photos of the Moon showing lots of detail, the best time to shoot isn't when it's full: at full Moon, the Sun is lighting the Moon face-on. Just as with your on-camera flash, straight-on lighting is flat, boring, and doesn't provide any shadow details. Try taking Moon pictures at first or third quarter (when the Moon appears half-lit, as above). At these times, the Sun provides side-lighting, showing relief on lunar surfaces and providing interesting shadows. Try not to shoot Moon pictures when it's low on the horizon. At those positions you're shooting through the thickest part of Earth's atmosphere, and the turbulent, moving atmosphere makes it difficult to see fine details on the lunar surface. Wait until the Moon is high in the sky, where the atmosphere is thinnest, even if that means getting up at 2AM to shoot!
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| © Paul LeFevre |
| Moonrise over Tokyo, with Mount Fuji at left. Shot with a Canon 5D DSLR at ISO 50, 0.6 second at f/22 with a 135mm lens. Click photo for larger image. |
Another way to shoot pictures of the Moon is to include it in landscape shots for dramatic impact. You can use much shorter focal lengths than for the frame-filling detail shots, but stay on the longer side (100mm or above) or the Moon will look very small in the image. The same "sunny 16" exposure guide applies, with a caveat: when the Moon is near the horizon, the thick layer of atmosphere you're looking through to see it will dim its light, sometimes dramatically. When it's near the horizon, start with about two stops more exposure than when it's high in the sky, and bracket your exposures if you can. For the moonrise image above I used about three stops more than the "sunny 16" guideline. A full Moon will rise in the east about the same time the Sun is setting in the west; a crescent Moon will rise just before sunrise in the east or set just after sunset in the west. Balancing the near-daylight exposure for the Moon with twilight can be tricky, making sunrise and sunset the best times to shoot. Just be careful not to over-expose the Moon or you'll wind up with a white blob that shows no detail.
I hope this How-To has piqued your interest in the possibilities of astrophotography. Don't put that camera away when it gets dark, get outside and take pictures! A whole universe of wonderful images awaits you.
Resources and Links
To do astrophotography well, you need to get to know the night sky -- when and where the Sun and Moon will rise and set, which constellations are in the sky at different times of the year, and which celestial objects are visible. Here are some internet resources to help get you up to speed:
NASA/JPL's Ephemeris
An ephemeris gives the positions of the Sun, Moon, and planets. The NASA/JPL site lets you enter your position on earth to get specific information for your location, at any time in the present, future, or past.
Jim Kaler's Constellation Maps
Provides maps of the constellations and their brightest stars, with different maps for the different seasons of the year.
Sky and Telescope's Interactive Star Charts
An on-line star chart customizable for your specific location and time.
The sky is full of interesting objects to photograph besides stars -- galaxies, nebulae, clusters, and more. French astronomer Charles Messier catalogued 110 of the brightest of these objects in the late 1700's, and his Messier List is a good starting point to find targets of interest. The Andromeda Galaxy above, for example, is also known as Messier 31 (M31). You can get a list of all 110 objects, along with information about where to find them in the sky, at: SEDS Messier Catalog.
If you start going for long exposures at long focal lengths on an equatorial mount, you're going to need to guide your shots -- as mentioned in the text, a simple Web cam can be easily modified to be used as an "autoguider." Ash's Astro Pages has several articles and tips for using Web cams for guiding and astro-imaging.
If you're using a DSLR to do your astrophotography, Michael Covington offers a comprehensive Web page with lots of tips to help you get started: Michael Covington's DSLR Astrophotography.
Paul LeFevre is a photographer, writer, and astronomer living near San Diego, California. He can be reached at plefevre@hughes.net.
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