Part of the pointing problems were, strangely, because the telescope thought it was in the wrong place. The telescope has to know where it is on the earth: at the North Pole, Polaris is straight overhead, but at Apache Point, Polaris is 57.2 degrees off north, and the telescope didn't seem to know that. The problem was that the telescope had been installed with the slightest tilt, about one millimeter off perpendicular. That one millimeter on earth, translated to the sky, meant that when the telescope thought it was looking straight up at Polaris, it was actually looking 1 arc minute off to the side. One minute of arc translated to the 8,000-mile-diameter earth, Jim figured, meant the telescope thought it was living about a mile away from Apache Point. Jim thought the sensible solution was to just lie to the telescope, to tell it that it was a mile west, out in the rift valley. The UW engineers thought this was a jaw-droppingly brilliant solution.


Pointing was one of only two things the telescope had to do; the other thing was tracking, and it couldn't track either. The advantage of drift scanning is that the telescope sits still and the earth turns it across the sky with a steadiness and smoothness no telescope controls could hope for. The disadvantage is that drift scanning works effortlessly only when the telescope aims at the celestial equator, the great circle drawn on the sky directly over the earth's equator, where the stars move in long arcs that look like straight lines and slip evenly across the CCDs. But aim the telescope up off the celestial equator, and the stars move in increasingly tight arcs until, near Polaris, they are moving in small circles. So to get the star moving evenly across the CCD, the telescope needs to track, to move automatically to counter the earth's motion.

A year after first light, in May 1999, the observers were still reporting they couldn't get the telescope to track. It would refuse to move at all, it would move at inconstant speeds, it would oscillate violently, it would start to move and then slip its leash and run away to the zenith. One night that May, Dan Long—one of the few observers who was not a PhD astronomer, just experienced and natively smart—was out on the cliff with the telescope and later wrote up a report: "I heard the sound the alt lvdt makes when the windscreen changes position with respect to the altitude. I looked up with a flashlight to see oscillation in altitude. By the time I got to a stop button, the oscillations were so violent that the turnbuckle rods in the windscreen were slapping the sides of the windscreen making a loud clang. At least that is what I hope the noise was and not the telescope hitting the windscreen."

As a result of the telescope's bad behavior, most of the data during that first year came from drift scans at the celestial equator —nothing was wrong with that except the data were all coming from a strip near the horizon, and the Sloan was supposed to be surveying the whole sky. Many of these problems were in the software, much of which had been handed off from the UW engineers to Fermilab, whose software writers, said Jim Gunn, "had never seen a telescope." So Princeton took over the software: Robert Lupton, whose PhD thesis had been the code that analyzed the data from PFUEI and Four-Shooter, went to Apache Point for three months and rewrote the code until it worked.


So far, the problems had been more or less as expected. But on October 19, 1999, an Apache Point engineer named Jon Brinkmann was showing another engineer, John Briggs, where on the telescope the spectrographs were mounted. Briggs looked up through the open mounting hole and said, "Jon, is that a crack?" Brinkmann and Briggs moved their heads back and forth, to see if it was just a trick of lighting; it wasn't. Brinkmann called French. "French," he said, "do you know there's a crack in the secondary?"

"Is that really a crack," French said, "or is there something on the mirror?" So they crawled up inside the Tinkertoys to the mirror, and sure enough, it was a crack. French walked over to the operations building to get Bruce Gillespie and brought him back. "See anything weird about that mirror?" Briggs said to Gillespie. "Look at the center."

"Shit," Gillespie said. "Oh shit."

1 Comments

1. 1. WilliamStoertz 08:04 AM 8/21/10

   Classical! A very good story to encourage young budding scientists to keep at it, learning to overcome all these various problems, from living situation and technical glitches to adjustments needed in the fundamental theories themselves. Good testimony!

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