Spacewatch Observing


This is the "control room" where the observers sit in front of several computers and take data.
Click image to enlarge!


     Observers at Spacewatch currently use the 0.9-meter Steward Observatory telescope atop Kitt Peak to survey the sky for asteroids. The dry climate and prevailing winds ensure clear skies most of the time, and when it does cloud up it rarely stays cloudy for more than a couple of days. The limiting magnitude during a good night is 21.5.

     There are two major observing routines: surveying and astrometry (measurement of the positions and motion) of known objects. While surveying, we try to cover as much sky as possible with our usual limiting magnitude in order to find new asteroids and comets. Astrometry of known objects takes two forms: recovery of objects that haven't been observed for a while, and followup of important objects that have been recently discovered. We put high priority on objects fainter than V=20.5 magnitude because they are less likely to be observed by other stations. Potentially Hazardous Asteroids (PHAs), Virtual Impactors (VIs) as defined by the JPL NEO Office (http://neo.jpl.nasa.gov/risk), faint objects on the Minor Planet Center's Confirmation Page (http://cfa-www.harvard.edu/cfa/ps/NEO/ToConfirm.html), fast-moving objects (FMOs), and objects originally discovered by Spacewatch receive priority for recovery and followup by Spacewatch.

     Usually at the 1.8-meter Spacewatch Telescope, drift scanning is used, whereby the telescope is left pointing motionless for the duration of the exposure. The effective exposure time is 137 seconds when pointed at the celestial equator, and increases with the inverse of the cosine of the survey region's declination. The instantaneous field of view on the CCD is 34x34 arcminutes, but the result of a survey scan is a long, narrow strip of sky 34 arcmin wide. Survey scans are usually 1/2 hour long and recovery and followup scans can be shorter if a large area doesn't have to be searched for the object. This process is done three times on the same region of sky to allow the software to detect movements between the passes. Scans composed of three passes are thus completed in about 1.5 hours.

     At the 0.9-meter Spacewatch telescope, drift scans were also used from 1984 April 22 through 2002 April 22. In 2002 October we began making "staring" observations with a new mosaic of four CCDs on the 0.9-m telescope that cover 2.9 square degrees in each exposure. That will allow us to cover sky seven times faster than with the old single CCD detector on that telescope. The large light gathering power of the 1.8-meter telescope and the large area coverage of the 0.9-m telescope complement each other, providing Spacewatch with comprehensive capabilities for surveying and followup of important solar system objects.

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     Observers sit in the warmth of the control room as the data is sent down to the computers for the observers to analyze it. Using software developed by Spacewatch, the observer is able to automatically detect and verify asteroid discoveries in near real-time. Shown above is a typical computer display the observers interact with. There are also separate computers that control telescope movement, shutter operation, and communicate with the outside world.



The animation to the left shows the motion of asteroids among the background stars. Here, four ordinary main belt asteroids move with respect to the background stars over the course of an hour. Usually, however, asteroids are not this close together on the sky. Finding asteroids and comets is merely a matter of finding objects that move rapidly among the background stars. (North is to the right, and east is at bottom.)








Sometimes an asteroid moves so quickly that it leaves a streak across the image during the exposure, as is shown at right. When this happens, the observer must be ready to follow the object immediately and re-observe it. Fortunately, the software provides this functionality, making it easy to follow these fast-moving objects (FMOs). In this image, the observer was following up on the discovery of asteroid 1994 GK (the longer trail near top right) when he was surprised to find another fast moving asteroid just 3.8 arcminutes away, later designated 1994 GL. Although close together on the sky, the two objects were moving in opposite directions with 1994 GK being 4.2 million kilometers from Earth (about 10 times the distance to the moon) and moving about 3 times as fast across the sky as 1994 GL which was 5.8 million kilometers away. These two asteroids were both between about 50 to 100 meters in diameter.


Last Update: November 07, 2002

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