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From supermarket to dome control.

From supermarket to dome control

Practically every supermarket product carries a striped patch that can be read by a laser detector at the checkout counter. This code allows a store's computer to identify the product. A modified form of this technology is now showing up in astronomical observatories--to keep a dome's slit properly lined up with the observatory's telescope.

At many observatories, astronomers no longer endure chilly nights standing at a telescope's eyepiece. Instead, they work in comfortable control rooms that may be miles away. But when images of stars suddenly begin to dim, it's often hard to tell what's causing the problem. Clouds may be passing overhead, ice may have formed on the detector or the observatory's rotating dome may have shifted enough to block the telescope's view.

The dome problem was particularly severe at the Lick Observatory's heavily used 1-meter telescope on Mt. Hamilton in California. Originally built a century ago to house a 12-inch refracting telescope, its wooden, copper-sheathed dome, starting in the late 1970s, had to cover a much larger reflecting telescope. The slit opening was widened but it was still barely large enough for the new telescope. "It was an extremely tight fit," says Robert Kibrick, Lick's senior programmer. "The positioning of the dome in front of that telescope was more critical than on any other telescope we've had."

Moreover, the dome isn't perfectly spherical; when it rains, swelling wood further distorts the dome's shape. And when the dome rotates, it sticks and slips. "Its speed can be incredibly irregular", says Kibrick. All this meant that conventional methods to track the dome's position weren't accurate enough.

Rubber rollers set against the dome's rim, for example, could easily slip, sending incorrect information to an encoder that translates roller movements into dome positions.

The answer that Kibrick and Calvin R. Delaney came up with was to glue the equivalent of a "universal product code" along the dome's circumference. They used two tracks. One carries an evenly spaced pattern of vertical black and silver bars. Two sensors determine the dome's speed and direction of travel. The second track is tagged at regular intervals so that the dome's actual position can be determined. Each coded "label," read by a third sensor, defines one of 18 positions.

The track patterns were generated by computer and then printed on sheets of transparent plastic. It took two people only 45 minutes to paste the tracks along the dome's 72-foot circumference. The result, says Kibrick, is an inexpensive, highly reliable means for establishing dome position. A similar encoder is to be installed at Lick's 3-meter telescope.

Kibrick reported his work last month at an International Society for Optical Engineering meeting in Tucson, Ariz.
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Title Annotation:bar code technology used to line up observatory dome's slit
Publication:Science News
Date:Apr 5, 1986
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