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Hubble Space Telescope
COS Airglow Observations

To assist observers in evaluating the shape of geocoronal Lyman-alpha and O I line profiles observed with the G130M and G140L gratings, we have compiled at http://www.stsci.edu/hst/cos/calibration/airglow.html a list of airglow observations that are uncontaminated by the spectra of other astronomical sources. Such observations should allow users to model and subtract the line wings of the geocoronal lines from their spectra.

Observers should be aware the the continuing gain sag of the COS FUV detector, and the resulting pulse height dependent displacement of events along the direction perpendicular to the dispersion, can result in a significant distortion of the profile shape, (particularly for Lyman-alpha). However, this distortion should affect only the cross dispersion direction, and after summing along this direction the shape of the geocoronal profile as a function of wavelength should still be usable for fitting the observed profile in individual observations.

The COS/STIS team at STScI is also developing tools to make it easier to recalibrate COS time-tag data while excluding events taken during orbital day when the airglow lines are much brighter. These tools should be available within the next few months.