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Ground based photometry in the 20-micron spectral region has historically been limited to observations made with the Q filter, bandpass 17 to 25 microns. The highly variable nature of the atmospheric extinction in this part of the infrared spectrum and the extremely wide bandpass of the Q filter limit the photometric accuracy. Generally, errors in the 10 to 20 percent range are quoted. In this paper, we present photometric observations made with a high-resolution (R=200) grating spectrometer designed to operate from 17 to 28 microns. We present a technique which allows an estimate of the sky's current extinction by analysis of the sky's background emission. We estimate that this technique affords photometric accuracy in the 2 to 5 percent range and is extremely useful in improving photometric accuracy in less than ideal conditions.
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