How-To: Astrophotography 101

So far we've only looked at what we can do with a fixed tripod, with our camera still while the sky rotates above us. We're limited in what we can shoot that way. Most of the really interesting things in the sky are quite dim, so we need much longer exposure times in order to see them in our images... but longer exposure times mean the stars leave trails in our images.

However, we can get longer exposure times without trails by mounting our camera to something that compensates for the Earth's rotation. If we could have some kind of rig that rotated around one axis (like the Earth does), align our rig's axis of rotation with the Earth's axis, and then rotate in the opposite direction the same speed as the earth rotates (about ½-degree per minute), then our camera would stay pointed at the same spot in the sky as the Earth rotated, and we could do longer exposures without having the stars trail.

Hmm, that sounds pretty complicated, though. How can we make such a rig? Actually it's very simple, and all you need are a couple of boards, a hinge, and a couple of bolts... it's called a "Barn Door" mount.

A Barn Door is simply two pieces of wood mounted around a simple hinge. At the end of the boards opposite the hinge, you put a screw-bolt through the bottom board in a threaded nut, so that it pushes the top board when you turn it. Using a simple formula, you can calculate how many turns per minute you need to give the screw to match the earth's speed of rotation. Line the hinge (the Barn Door's axis of rotation) up with the North Star (the Earth's axis of rotation), and you can very closely counteract the movement of the stars across the sky for fairly long lengths of time. With a Barn Door mount, you can use longer focal lengths (up to around 135mm) and exposure times of up to about 15 minutes and not see any star trailing -- all for about $10-$20 in materials.

Designs for Barn Doors vary from the ridiculously simple and inexpensive to deluxe versions with battery-operated drive motors (to save you from having to turn the screw manually). A simple single-arm version will track accurately for 10-15 minutes, while only-slightly-more-complex double-arm versions can track accurately for an hour or more! The Internet is rife with design possibilities; here are a few of the many that are worth looking at (I built the second one in the list below, and used it for the Milky Way photo above):

Anthony Galvan III's Simple Barn Door Mount

Dave Trott's Motorized Double-Arm Drive

The next step up in "tracking" mounts from a Barn Door is a German Equatorial Mount. A GEM (sometimes acronyms are very fitting) in its simplest form is a tripod head that has two axes that rotate 90-degrees from each other. When you align the main axis with the Earth's celestial pole, it can be driven to counteract the Earth's rotation. The other rotational axis lets you position the camera or telescope anywhere in the sky while still maintaining the first axis' alignment with the Earth. Commercial equatorial mounts vary greatly in size, quality, and price; a decent-quality mount with a drive motor (to keep you from having to turn it by hand) that will support a camera and short lens can be purchased for under $200, while a higher-quality mount that will support long lenses or telescopes (and often includes a computerized control that will automatically point it to specified celestial objects) can cost between $1,000 and $10,000.

Equatorial mounts can allow you to use longer focal lengths (using either camera lenses or telescopes) to "zoom in" on galaxies, star clusters, and nebulae. The image at left shows two telescopes mounted together on a mid-range (in both price and capacity) GEM, with the camera mounted on a 540mm FL telescope. Why two telescopes? Because once you start using focal lengths longer than 250mm or so, you must track the sky VERY accurately. Even a tracking error as little as a tenth of a degree can ruin an image, making stars look like little oblong streaks rather than nice, sharp points of light. Since even rather expensive GEM mounts are driven by mechanical gears, imperfect machining causes those gears to be slightly inaccurate, and produce tracking errors. To get a good image, these tracking errors have to be corrected. This is called "guiding."

Guiding an exposure involves looking through a separate telescope from the telescope or lens being used to take the picture. A telescope eyepiece with crosshairs embedded in it is used -- the photographer centers a star on the crosshairs, starts an exposure, and then carefully watches the star through the guidescope. When mechanical errors begin to move the star off the center of the crosshairs, the mount's drive controls can be used to put it back dead center. Keeping the guide star on the crosshairs during the exposure will ensure that tracking errors are minimized, and long exposures (up to several hours) can be made very accurately. Obviously, this can get quite tedious -- you must keep your eye glued to the guidescope for the entire exposure, and delaying too long in making corrections will ruin an image. Technology has offered a solution to manual guiding tedium -- a CCD guide camera. Commercial guide cameras are available, but even inexpensive digital Web cams can be easily modified to act as guide cameras. Attached to a computer, the camera takes an initial exposure, and notes the pixel position of a guide star (specified by the user). Then the shutter is opened on the imaging camera, and the guiding sequence started through the computer. It takes a new image every few seconds, and again notes the pixel position of the guide star. If it's moved from its original location, the computer sends signals to the equatorial mount to move it back to where it was, just as you would do manually. This "auto-guiding" can easily be more accurate than manual guiding, and doesn't require the photographer to sit at the guidescope for hours on end. The links section at the end of this article has resources for making your own auto-guider from an inexpensive Web cam.

With a good-quality GEM mount, even fairly short focal lengths can provide stunning "close-ups" of many celestial objects. The image at right is of the Andromeda Galaxy, taken with a 540mm telescope. While Andromeda (also known as M31) is one of the brightest galaxies we can see from earth, it's still very dim by photography standards. To make this image, the total exposure time was 12 hours!

Once you start using longer focal lengths, a whole universe of celestial objects become potential targets. But those longer focal lengths, while magnifying those smaller celestial objects, also magnify tracking errors -- making it more and more difficult to get good images of them. Taking "deep-sky" multiple-hour exposures of tiny, dim objects at long focal lengths will require investment in a high-quality GEM mount (which is expensive!), sophisticated tracking methods and equipment, and lots of practice. The details of this kind of work are beyond the scope of this beginning article. If you start with star trails or perhaps build a Barn Door mount, you can make wonderful night-sky images without investing a lot of money in equipment. But beware -- astrophotography can be addictive! Once you start getting beautiful night-sky pictures, it's quite likely you'll want to move up to higher levels, which means a significant investment of time and money. The results are very satisfying and beautiful -- but when your spouse starts asking why you need to spend $5,000 on some fancy tripod, don't say I didn't warn you!