Design and model demonstration of a camera that removes the infra-red OH background from two dimensional images

Very deep images in the near infra-red 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 1-3 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 ∼40%. This would greatly enhance sensitivity to faint diffuse objects, such as distant red shifted galaxies. Our camera passes the two dimensional image through a large Littrow spectrograph. A long spectrum is formed, such that the full image width projects to a spectral resolution λ./δλ = 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 6×8 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 × 130 spatial resolution elements. A design optimized for use with a 2 m telescope has a grating size 50 cm × 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.