The Guiding Demon

Hell Really Exists

Hell Really Exists

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This used to be the number one killer of good astrophotos. Back in the bad ol' pre-CCD days it was really tiresome to keep a dim star exactly centered in the field of a crosshair eyepiece. You could sit out there in the cold for hours, doing one-hour exposures, pushing hand controller buttons whenever the guide star drifted, unable to get up and stretch your legs for fear of missing one of your drive's periodic error excursions. Today, most imagers use autoguide, either with built-in guide chips or separate guide cameras and guide scopes. Once an autoguidingsystem is working, it is just like heaven. Start the exposure, get up, wander around the field or backyard, go inside for a snack, maybe even take a short nap.

The problem for most new imagers, however, is getting the system going. Most autoguide programs have quite a few variables to set, things like "aggressiveness" and "track time." Read the guide program/camera's manual carefully and prepare to do a lot of experimenting. If you're a lazybones, however, you may want to try a freeware guiding program called PHD Guiding. "PHD" in this case does not mean "doctor of philosophy," but, instead, "push here dummy." PHD works with most CCD/guide cameras, and in most cases no fiddling with settings is required. Find a guide star, focus the guide scope, and push the "go" button.

What do astro-imagers use as guide cameras? With the right software, almost any electronic camera, even a video camera, can be pressed into service to watch a guide star and report its movements to the guide program. Imagers get good results with webcams and cheap "obsolete" CCD cameras. There's no need to run out and buy a camera specifically designed for this purpose (though several are available). The best choice for a guide camera is one with enough sensitivity to show plenty of guide stars in almost any field. A webcam probably won't be sensitive enough to do that, and neither will the Meade LPI. A modified webcam can. One of the best and most popular guide camera choices lately? The Meade DSI. The original is inexpensive, especially used, and is sensitive enough to offer plenty of guide star choices with 1- to 2-second exposures.

The guide camera must be connected to the mount so guiding commands can be sent to it to keep the star precisely centered. There are two ways of making this connection. If, like SBIG cameras, the guide camera has an "ST4" compatible output and the mount has an ST4 ("autoguide") input, the scope is interfaced directly to the camera. Shoestring Astronomy can provide an ST4 adapter for the computer to enable cameras without autoguide outputs to be used. If the mount doesn't have an autoguide port, guiding must be done via the computer's RS-232 output and the mount's serial input (usually on the hand controller). Which is better, ST4 or RS-232 guiding? Some folks swear "ST4" is better, since the mount is being guided by simple "switch closures" that don't involve software commands. But generally speaking, both methods work equally well.

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