The Drift of Saturn's North Polar Spot Observed by the Hubble Space Telescope.

Polar projections of 50 images of Saturn at 889 nanometers and 25 images at 718 nanometers taken by the Hubble Space Telescope in November 1990, as well as 3 images at each wavelength taken in June 1991, have been examined. Among them, 31 show the north polar spot, which is associated with Saturn's polar hexagon, in locations suitable for measurement. In each image, planetocentric coordinates of the polar spot were determined, and the movement of the spot with respect to Saturn's system III rotation rate was studied. During the period of observation, the polar spot had first a short-term westward movement and then a long-term eastward drift. The rate of the long-term drift was -0.060 +/- 0.008 degrees per day with respect to system III, approximately 50 percent greater than previously determined from Voyager. The original 1980 and 1981 Voyager data were combined with the new Hubble images to form an 11-year base line. The eastward drift over the longer period was -0.0569 degrees per day. The long-term drift could be due to uncertainty in the standard value of the internal rotation period, which is 810.7939 +/- 0.148 degrees per 24-hour day. The short-term movement in November 1990 has a rate that is greater in magnitude but opposite in sign and probably represents a real, transient motion of the spot relative to the internal rotation system.

Eruption prediction aided by electronic tiltmeter data at mount st. Helens.

Telemetry from electronic tiltmeters in the crater at Mount St. Helens contributed to accurate predictions of all six effusive eruptions from June 1981 to August 1982. Tilting of the crater floor began several weeks before each eruption, accelerated sharply for several days, and then abruptly changed direction a few minutes to days before extrusion began. Each episode of uplift was caused by the intrusion of magma into the lava dome from a shallow source, causing the dome to inflate and eventually rupture. Release of magma pressure and increased surface loading by magma added to the dome combined to cause subsidence just prior to extrusion.

Ultraviolet downconverting phosphor for use with silicon CCD imagers.

The properties and application of a UV downconverting phosphor (coronene) to silicon charge coupled devices are discussed. Measurements of the absorption spectrum have been extended to below 1000 A, and preliminary results indicate the existence of useful response to at least 584 A. The average conversion efficiency of coronene was measured to be ~20% at 2537 A. Imagery at 3650 A using a backside illuminated 800 x 800 CCD coated with coronene is presented.

Infrared images of jupiter at 5-micrometer wavelength during the voyager 1 encounter.

A coordinated program to observe Jupiter at high spatial resolution in the 5-micrometer wavelength region was undertaken to support Voyager 1 imaging and infrared radiation experiment targeting. Jupiter was observed over a 5-month period from Palomar and Mauna Kea observatories. The frequency of observations allowed the selection of interesting areas for closer Voyager examination and also provided good short-term monitoring of variations in cloud morphology. Significant global changes in the 5-micrometer distribution are seen over this time period.

Two-dimensional silicon vidicon astronomical photometer.

We have developed and successfully used an integrating two-dimensional silicon diode array vidicon photometer which is exceptionally well suited for use with telescopes. The video signal is read out from the 1-cm silicon target of the vidicon through a current mode preamplifier and then converted to digital form and stored on magnetic tape. The 256 x 256 element frames are recorded at 20,000 eight-bit words/sec. The vidicon tube has a published quantum efficiency ranging from 85% at 0.5 micro to 6% at 1.1 micro and must be cooled to about -65 degrees C to eliminate thermal dark current. The minimum detectable signal in the present system is about 1000 carriers per resolution element, limited by preamplifier and other system noise. The system is used as a single-frame camera. The large dynamic range (>10(3)), linear response, high quantum efficiency over a large spectral region, and low cost of the system make it well suited for digital direct image and spectroscopy as well as for a laboratory digitizer of two-dimensional m terial.

A method for obtaining differential 8-13 micro spectra of the moon and other extended objects.

A method is described for measuring spectral emissivity differences in the 8-13 micro region between two points on the lunar surface which may have different temperatures. Spectral emissivity differences of 0.5% on the moon can be detected in this wavelength region, excluding the strong 9.2-10 micro atmospheric ozone band where the accuracy is reduced to 2%. The application of this method to the bright planets is discussed.