CARA Science: Mid IR Site Characterizartion


CARA, with collaborators at Goddard Space Flight Center, installed and operated SPIRAC, a mid-Infrared Imaging Spectrometer and telescope mounted on top of the SPIREX telescope. The data are currently being reduced.


In the N band (8-13 microns) we expected to see gains of a factor of ~2.5 at the South Pole due to the lower temperature, and an additional factor of a few due to the lower water vapor. Reduction in aerosols and dust in the atmosphere could also help. An unknown quantity was the effect of wind-blown ice crystals, which could act as both a source of increased background and of decreased stability.

To test these conjectures, Craig Smith of the University College, UNSW, observed with his NIMPOL instrument at the South Pole during January 1996. NIMPOL (Smith, Aitken, & Moore 1994) is an imaging camera with a 128 x 128 Si:Ga array, a 1.6 degree beam on the sky, and N and Q (16-22 microns) broadband filters and 8-14 micron circular variable filter (CVF). The results (Smith and Harper 1997) exceeded all expectations: the 10.2-11.8 micron region was 20-40 times darker than a mid-latitude site. The improvement in the Q band was a factor of 4.5. Even better results could be expected during winter due to the reduced temperature and lower water vapor.

NIMPOL also showed that the mid-IR sky emission at the South Pole is so stable that it approaches photon shot noise. This eliminates the need to chop, thereby gaining an additional factor of 50-100% in observing efficiency. During a period of ice haze, NIMPOL recorded a 16% increase in N band emission, with some sky noise below 2 Hz.

Back to the main site testing page.