1 June 1994 Design and model demonstration of a camera that removes the infrared OH background from two-dimensional images
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Abstract
Very deep images in the near IR could be recorded from ground based telescopes, if the strong night sky emission lines of OH were blocked and the continuum transmitted. For example, in the H band, of width 330 nm, 87 line groups contribute nearly all the emission. If 1 to 3 nm bandwidth sections of spectrum were removed at each group, the total background--lines plus continuum--would be reduced by 95 to 99.8%, depending on the continuum strength, while the light from the object under observation would be reduced by only approximately 40%. This would greatly enhance sensitivity to faint diffuse objects, such as distant redshifted galaxies. Our camera passes the 2D dimensional image through a large Littrow spectrograph. A long spectrum is formed, such that the full image width projects to a spectral resolution (lambda) /(Delta) (lambda) equals 1500. After reflection at a mirrored `staircase' along the focused spectrum, the light retraces its path through the spectrograph to form a high resolution white image at the original entrance. By placing a comb-like mask along the dispersed spectrum, the OH lines are removed from the rebuilt image, while an optical arrangement involving a polarized beam-splitter efficiently separates the original and rebuilt images. A small scale demonstration in the laboratory with a 10 cm grating proves the method. The source was a resolution chart 6 x 8 mm at f/15. Working in the optical with 10% bandwidth, the spectral resolution was up to 50,000 for individual image resolution elements, 370 for the full rebuilt image. A CCD image of the result image shows 100 x 130 spatial resolution elements. A design optimized for use with a 2 m telescope has a grating size 50 cm x 66 cm and 20% bandwidth. It gives a nominal field of 3 by 5 arcmin and is limited by seeing rather than aberrations. The 2.5 m paraboloidal reflector needed for such an instrument could be built inexpensively as a spinning liquid mirror.
© (1994) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Robert Content, Robert Content, James Roger P. Angel, James Roger P. Angel, } "Design and model demonstration of a camera that removes the infrared OH background from two-dimensional images", Proc. SPIE 2198, Instrumentation in Astronomy VIII, (1 June 1994); doi: 10.1117/12.176730; https://doi.org/10.1117/12.176730
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