Mid Infrared Observing Conditions at the South Pole

Measurements of the mid infrared sky brightness at the South Pole throughout the winter of show that the sky background is extremely low and stable For of the time the ux in the to m window is below Janskys per square arcsecond Typical background levels in this window during clear conditions are of the order of Janskys per square arcsecond This is almost an order of magnitude better than any other site on earth The lower limit to the sky background across most of the N window appears to be set by the aerosol content of the sky rather than by residual water vapor These data were acquired remotely using an automated instrument housed in the AASTO Automated Astrophysical Site Testing Observatory Subject headings site testing atmospheric e ects Introduction Astronomical observations in the mid infrared from the ground are limited by an atmospheric thermal background ux that is orders of magnitude greater than that of the objects studied The di culties faced are most eloquently described by Low Rieke Observing at m has been likened to observing visually through a telescope lined with luminescent panels and surrounded by ickering lights as though the telescope dome were on re So dominant is the local thermal background that any improvement in site quality will translate directly into improved sensitivity for astronomical observations The most important parameters a ecting the quality of a site are atmospheric temperature stability and water vapor content The antarctic plateau promises signi cant advantages for astronomy in the thermal infrared that is for wavelengths greater than about m The high altitude low water vapor content cold atmosphere and the stable air mass can potentially provide conditions that are superior to any other ground based site In the K dark window m the low temperature provides a dramatic decrease in the background radiation Near infrared wavelengths are on the Wien side of the Planck function at atmospheric temperatures with the result that radiation is extremely sensitive to changes in temperature Measurements made at the South Pole in the K dark window show reductions in background ux relative to that at temperate sites by factors of between and Ashley et al Nguyen et al Phillips et al In the L window m the background ux is typically times lower than that at temperate sites Ashley et al Phillips et al A preliminary exploration of the bene ts this confers to wide eld thermal infrared imaging is currently being conducted with the SPIREX Abu experiment a cm telescope equipped with a pixel InSb array camera Fowler et al Terrestrial thermal emission peaks in the mid infrared and so the emission from the sky is not as sensitive to changes in temperature At the South Pole calculations show that the lower temperatures should be responsible for a reduction of only a factor of about two However larger reductions in the mid infrared could result from the decrease in the water vapor content of the atmosphere producing lower emissivity Summer observations of the thermal emission of the atmosphere over the South Pole in the mid infrared have already been reported by Smith Harper and show a greater than expected decrease in sky background In this paper we report measurements of the mid infrared sky brightness at the South Pole taken throughout the winter of The results con rm that the typical sky background is exceptionally low suggesting that the bene ts already seen in the near infrared will extend right across the infrared spectrum Method Instrument Results presented here were recorded with the Mid Infrared Sky Monitor MISM A detailed description of this instrument appears in Storey et al only a brief overview of its operation will be given here The MISM detects the di erence in the ux received between two beams each degrees wide and degrees apart in the sky A re ective chopper chops between these beams at a frequency of kHz After passing through a lter wheel the beam is incident on a HgCdTe detector maintained at K by a Stirling cycle microcooler The elevation of the two beams can be rotated in a plane from one horizon to the other passing through zenith For calibration a black body is placed in the path of one of the beams The temperature of this black body is around Celsius ie approximately degrees above ambient and is accurately recorded with a platinum resistance thermometer The MISM lter wheel contains the following lters a standard astronomical N lter to m a circular variable lter CVF covering to m at a resolving power of a CVF covering to m at a narrow band lter covering to m allowing sensitive measurements to be made in this region of particularly low sky brightness reported by Smith Harper To facilitate a broad campaign of site testing across a wide range of wavelengths and at a variety of high plateau antarctic sites an Automated Astrophysical Site Testing Observatory AASTO has been developed Storey Ashley Burton Storey The AASTO is a self heated self powered autonomous laboratory that hosts a suite of instruments each designed to provide data on a particular characteristic of the atmosphere of relevance to astronomy The MISM was installed in the AASTO at the South Pole in January Di culties with the thermal generator and power supply of the AASTO meant that no results were obtained with the instrument in the winter of Over the antarctic summer of the AASTO was brought back into operation allowing the MISM to be operated remotely over the antarctic winter of via telnet sessions from Sydney Commands for the rotator lter wheel integration times and power and ampli er settings are included in software macros Data are recorded in two modes skydips at xed lter positions and spectra across the CVFs These are described below Skydips are recorded by moving the rotator so that the instrument scans from one horizon to the other passing through zenith They are calibrated using the readings obtained when one of the beams passes across the calibration black body Each reading is integrated over seconds The angle between readings is either or degrees ie or times smaller than the resolution of the beam Skydips are taken with a xed position on the lter wheel either through a broad band lter or at a position on the CVF corresponding to the wavelength of a major emission feature of a speci c atmospheric constituent namely carbon dioxide ozone or water Spectra are recorded by holding the rotator at a xed zenith angle and scanning through the full range of the CVF Readings are integrated over or seconds with the longer integration needed for the to m CVF because of lower signal levels Step sizes on the lter of m and m are used for the to and the to m CVFs respectively The absolute sky ux is derived from the di erential readings by combining the results from appropriate rotator positions see x Knowing the ux from the calibration black body and assuming a ratio between the ux at the zenith and that at degrees the zenith ux can be calculated Table shows a typical operation sequence for the MISM Theory Thermal emission from the atmosphere in the mid infrared can be modelled by assuming that the emission comes from a single at emissive layer at a uniform temperature Under Table Typical operating sequence for the MISM SKYDIPS Wavelength microns Comments to N window to N dark window Carbon dioxide emission Ozone emission Water emission SPECTRA Rotator Position Zenith angle of Beam Zenith angle of Beam zenith black body calibrator Zenith angles are written as positive on one side of the zenith and negative on the other this assumption the variation in down welling radiation with zenith angle is determined solely by the optical path length at the angle viewed which is proportional to the secant of the zenith angle By converting the optical depth into emissivity the following relation between ux F and angle can be found