The strong emission from the airglow layer is predominantly O² (ATM) bands. The
O² (0,0) band at 7620Å is the strongest emission band. The O² in the upper atmosphere is
optically thick; therefore, below the bright O² layer seen in this image, the emission arises from
the O² "surface" down to an optical depth of about =1. From the vantage point of the orbiter,
the tangent height of the bright limb is at a range of about 2000 km. Above this layer the field
of view is into deep space while the emission below the bright layer originates from the
curved O² surface. The curvature is slight. The imager field of view below the bright layer has
been scaled in range in kilometers from the orbiter. The imager sees a surface about 700 km
long, from the limb tangent toward the orbiter. The look direction is toward the Shuttle wake.
The imager was not intended to give a high quality image; there is a fiber-optic interface
which shows as a weak interference pattern and degrades the image quality. However, the
dynamics which can be seen in the airglow layer are impressive.
This image shows one of the important features of the imager. There are five stars visible
in the field. These bright stars are easily identified. They provide an instantaneous calibration
of the scan platform azimuth and elevation position with respect to the orbiter, since the
instantaneous position and attitude of the orbiter is known. The scan platform movements are
commandable to an accuracy of 0.01°.
The image also provides an instantaneous tangent height independent of the scan
platform calibration. The calibration is validated by this type of imaging many times during the
flight. There are sufficient stars during any one-night transit to verify the calibration several
times, if it is necessary to remove the gyro drift of the orbiter ACS. We have prepared programs
to use with our standard UCAT data file for this purpose. One program (STARID) predicts the
right ascension and declination (RA & Dec) of a star appearing in the imager field of view.
Given the RA & Dec, a second program, "STARALT," calculates the AZ/EL position of the star
in an "ideal" GLO coordinate system. These programs can be supplied upon request.
Stars are also used to calibrate the spectrographs for sensitivity. As our tracking
capability improves we will use stars as occultation sources to probe the top side profiles of O²
and O³ during the night transits.