The Images

There are no hard and fast rules regarding the telescope or CCD camera needed for asteroid work. To be effective, the system should record stars as faint as 18th magnitude with a single, 4-minute exposure. Almost any CCD camera on an 8-inch telescope can do this under a clear, dark sky.

While common sense tells us that a large field of view is helpful in any search program, a practical limit for asteroid work is set by image scale, which should be around 2 arcseconds per pixel. Having a smaller scale (covering more sky per pixel) will limit the positional accuracy of astrometric measurements. If the scale becomes too large, not only does the field of view become unnecessarily small, but the system's sensitivity drops, especially for moving objects that expose a given pixel for only a limited time. (A typical main-belt asteroid near opposition moves about 0.5 arcsecond per minute.)

Under average amateur observing conditions, pixel scales around 2 arcseconds give nearly optimum detectability of faint stars. Furthermore, at this scale frames taken about 10 to 15 minutes apart will show obvious asteroid motion when blinked.

To be useful for astrometry, the central time of an exposure should be known to an accuracy of one second (0.00001 day) or better if the object in question is moving rapidly. Some cameras automatically log the time of exposures directly from the host computer's clock, so the clock should be checked against radio or telephone time signals at the beginning of each night. Furthermore, users should note whether a camera logs the beginning or ending time of an exposure. The midtime is what you need to report, and for a single exposure it is simply half of the exposure duration added to the starting time. In the case of multiple exposures stacked to create a longer effective integration, the calculation is more complicated and good record keeping is necessary.

You also need to know the longitude, latitude, and altitude of your observing site to better than one arcminute. This information is readily obtained from topographical maps or a global-positioning-system (GPS) receiver.

Blinking Images

This is the key to finding objects, and there's a lot to be said for experience. A multitude of pitfalls await the unwary. Hot pixels, random cosmic-ray detections, and a host of other CCD artifacts can mimic moving asteroids and fool the novice. After blinking hundreds of images during the past two years I have a better appreciation for a story that circulates among comet hunters. The best of them seem to know instinctively when an object in the eyepiece is a comet and not some faint galaxy because "it looks like a comet!" Indeed, there's something about a real asteroid that just looks right when images are blinked.

A good way to rule out image artifacts is to have three or more images and blink them in various combinations. And for this it helps to have equal intervals between exposures. I was surprised how difficult it was to identify a faint object when blinking one set of images separated by 20 minutes and another pair separated by an hour. The eye-brain combination expects to see similar motion in both instances.

Even better than blinking pairs of images is to assemble sets of three or more into an animated movie loop. This way even the faintest objects appear to jump out of the screen. Axiom's MIRA software has a particularly effective routine for aligning images and creating animation loops.

Some computerized blinking routines "compress" several CCD pixels into a single screen pixel to get the image to fit a particular display window. Experience has shown that this compression tends to mask faint objects.