Liquid metal mirror for optical measurements of orbital debris

A telescope employing a liquid metal (mercury) mirror that is three meters in diameter was developed for the purpose of measuring the population of small orbital debris. The telescope is installed at a 9,000 ft site in the mountains of New Mexico at a latitude of 32.98 degrees, and has completed final optical performance tests. A Ford 2048 × 2048 CCD detector is used at the image plane. With this detector, the limiting stellar magnitude of the telescope was measured to be 23 at a S/N of about 3, with a nominal full width, half maximum for stellar images of 1.3 arcsec. Safety problems with toxic mercury vapor were minimal at operating temperatures below 60 F, greatly simplifying operations with the mirror. Performance of the telescope for debris at geosynchronous orbit (GEO) altitudes is estimated to be in the range 5 to 10 cm using the existing CCD detector. However, at its present location in New Mexico, the telescope cannot be used for detection of debris in the GEO region, since it cannot see debris with inclinations below 32 degrees. Performance of the telescope for debris in low earth orbit depends on the kind of detector that is used. Efforts were made to use the CCD detector for fast-moving low earth orbit debris by reading out the detector at the same rate and in the same direction as the debris object crosses the field of view of the telescope. In this mode, the telescope was capable of detecting debris as small as 3 centimeters at 900 km altitude. However, this mode is very inefficient, since debris moving in other directions is not detected. Observations have also been done using an intensified CCD video camera as the detector. The limiting detectable size for this detector was about 4 centimeters at 500 km, and 7 centimeters at 1000 km. A design was developed for the optimum detector for fast-moving debris in low earth orbit based on currently available technology. This is a 1024×1024 element CCD detector operating at 200 frames per second, with pixels binned to 16×16. This detector should be capable of detecting 1 cm debris at 500 km and 1.5 cm debris at 1000 km. New data processing algorithms will be required to process the data stream from this detector.