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Holographic Filter Allows Astronomers to Read between the Lines
A prototype of a new holographic filter that should give astronomers a clearer window on the infrared universe was recently tested at the Palomar Observatory’s Hale Telescope.
For years astronomers observing in the near-infrared
have been frustrated by a naturally occurring atmospheric sky-glow. Bright
spectral OH lines dominate the near-infrared spectrum and make an astronomer’s
life difficult. The great number of OH lines has made producing a traditional
filter all but impossible.
At the
end of September, 2004 Caltech’s Sebastien Blais-Ouellette and Keith Matthews
used the 200-inch Hale Telescope to try out a new holographic filter with the
potential to remove these OH lines. The filter was fabricated by Ondax,
Inc. under a grant from the NSF (SBIR #0338906). The holographic filter uses a
property of light known as interference to reject specific wavelengths of
light. The new 1-inch filter was optimized to reject ten spectral bands of OH
sky glow (from 1.5 to 1.57 microns) while allowing the surrounding wavelengths
of light to come through.
Observations of distant quasars showed that prototype
filter is more than promising. The astronomers found that the prototype filter
almost doubled the efficiency of observing in this region of the spectrum giving
them the same signal to noise with a factor of 1.8 less observing time. This
has profound applications for use on large research telescopes: it allows
astronomers to observe much fainter objects than previously from ground based
telescopes. It costs tens of thousands of dollars a night to operate to operate
them and the use of a filter that can cut observing time nearly in half will
greatly improve their productivity.
The next generation of the filter could be available in a year or so. It should remove 30 to 50 OH lines across the whole 1.5 - 1.8 micron region that astronomers call the “H band” while reducing the observing time by a factor of two or three.
Keith Matthews is with the California Institute of Technology and Sebastien Blais-Ouellette is with Caltech and Photon etc., inc. (Canada).