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Software Makes Light Work of Optical Design

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Ruth A. Mendonsa

It's not surprising that astronomers in California and Hawaii are watching the skies. They are searching for planets -- not the ones most of us are familiar with, but as-yet-undiscovered planets around nearby stars. The scientists are using the high-resolution echelle spectrometer designed and constructed by Steve Vogt, an astronomer at the Lick Observatory at the University of California at Santa Cruz. Vogt credits Focus Software Inc.'s ZEMAX optical design software as an easy-to-use tool that helped him with the instrument's design, construction and final alignment.
Vogt first used the software to help construct the $4 million spectrometer for the Keck Observatory's 10-m telescope in Hawaii. He had previously built a similar spectrometer, the Hamilton spectrograph, at the Lick Observatory's 3-m telescope, and he has since used ZEMAX to upgrade the performance of some of its optics. The astronomers are using both instruments for their exploration beyond our solar system.
The high-resolution spectrometer operates in the 0.3- to 1.1-µm range, feeding light via a complex system of gratings and lenses to a 2048 3 2048-pixel charge-coupled device detector. It can examine objects 10 to 100 times as faint as can be measured by any similar instrument.

Vogt needed a user-friendly optical-design and -analysis package to build these instruments. He found ZEMAX to be powerful and easy to learn. He said, "Its tolerancing features were particularly useful for final alignment tasks. I don't like to invest lots of time using products with overly lengthy manuals to read, or overly steep learning curves to climb. I got up to speed and became productive quickly with ZEMAX."
The spectrometers' use for planet search is described in a March 1998 article in Sky and Telescope, written by R. Paul Butler and Geoffrey Marcy (the latter was one of Vogt's first graduate students). The authors said that the detection of planets outside our solar system has been all but impossible because they are lost in the glare that surrounds a star's image.
To remedy this, the astronomers are using an indirect method of detection: They are employing the Doppler effect to detect tiny "stellar wobbles" that result from a gravitationally bound planet swinging its star around in a mirror image of its own orbit. This allows them to determine the existence of an unseen orbiting body. The size of the wobble indicates the planet's mass. The planet's orbital period is determined by the amount of time it takes for the star to complete one wobble.
The authors say that since 1995 eight extrasolar planets of sunlike stars have been positively detected.

Photonics Spectra
Apr 1998
Accent on ApplicationsApplicationsenergySensors & Detectorsspectroscopy

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