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).