Support Imaging, Pointing Prediction, Validation and Calibration

This image of the Earth airglow layer was taken by the GLO support imager. It was a bare CCD with pixels 0.022 mm x 0.022 mm at a focus of 75 mm. The pixel array used for this frame is 288 x 192 pixels. The purpose of the imager was to provide a pointing reference for the spectrographs, which are co-aligned with this center pixel of this imager. The field of view was 3.2° x 4.8°. The spectrograph slit position has been drawn on this image. It has an angular width of about 8°. In this image the scan platform oriented the slit at 19° to the tangent of the limb. The small angle increases the vertical resolution of the slit across the airglow layer. For normal observations, the angular width of the spectrograph field of view is divided into 24 contiguous \left(super\right) pixels \left(i.e. 8 1 pixels\right), giving an angular resolution along the slit of 0.35°. At the limb, 2000 km away, the linear spatial element is about 12 km. When the slit is 19° to the horizon the vertical resolution of the image super pixel is 4 km.

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.

Last Updated: 8 March, 1996