Guiding a Telescope for Imaging
We all do it -- ogle over the astrophotographs taken by the masters. The deep-sky images look so exquisite with their masses of detail and perfectly round, pinpoint stars. We know they didn't come easily.
Impressive celestial photography is a lot more feasible for amateurs now than it was a generation ago. But good results still require a willingness to learn the intricacies of cameras, focusing techniques, exposure times, films, and miscellaneous equipment. In particular, taking deep-sky pictures requires a skill that's involved in no other kind of photography: guiding on a star.
To get a sharp image from a long exposure, you must keep a guidestar centered on cross hairs for the entire time the shutter is open -- from a few minutes to an hour or more. While watching the star, you must often adjust the telescope's aim slightly to keep the star on the crosshairs and the image motionless on the film.
No telescope drive, no matter how well made, can eliminate the need for guiding. Any gear system contains some periodic error that shifts the telescope back and forth slightly in right ascension, typically with a period of 4, 8, or 15 minutes. If the telescope is not perfectly polar aligned it will probably drift slightly in declination too. The telescope tube may flex a little as it tracks the subject across the sky. Atmospheric refraction varies as an object's altitude changes, altering the object's apparent position enough to blur photographs after as little as a few minutes. The electricity running the drive motor may be slightly irregular. And the slowest component of atmospheric "seeing," or turbulence, can make a whole star field creep around slightly with a characteristic time of many seconds. You have to guide the telescope on a star to follow all of these image motions.
The aiming corrections required are so tiny -- just a few arcseconds -- that you cannot push the telescope by hand. Instead you need an electronic drive corrector that speeds or slows the telescope's drive motor. This allows fine guiding in right ascension (east-west), where the most frequent corrections are needed.
You also need a fine-motion control in declination (north-south). A turn-by-hand control will work only if the mounting is rigid enough so it doesn't wiggle at high power when you turn the knob. An electric declination motor usually gives much better results. Typically the buttons or joystick for controlling both right ascension and declination are mounted on a single hand paddle.
The first thing you need to do is polar align your mount. This means aligning the right-ascension axis parallel to the Earth's axis of rotation. Otherwise, even with perfect guiding, everything in the photograph will circle slightly around your guidestar, a problem known as field rotation.
How good a job of polar alignment do you need to do? That depends on the exposure time and image scale of the picture you are taking. Wide-field photography with a normal (50-millimeter) camera lens requires minimal alignment. Simply pointing the mount's polar axis at Polaris as best as you can judge is usually enough for normal-lens exposures of a few minutes.
When you start using longer lenses or a telescope, you need better alignment. Some mounts come with a polar alignment finder, a miniature telescope with an engraved reticle that you position on Polaris and surrounding stars. These devices work quickly and well. If you want the best possible alignment, however, especially for a permanently mounted telescope, use the declination drift method described in the article Accurate Polar Alignment; it takes a little time, but it's simple and needs no special equipment. Nothing else gets you closer to the celestial pole.