Mission Overview

This spacecraft was one of a series of automated astronomical observatories that was ground controllable in orientation and was placed in a low-earth orbit. This spacecraft carried two experiment packages, which were located centrally within the spacecraft, each viewing space from opposite ends. One experiment, the Wisconsin experiment package (WEP), consisted of four stellar photometers (1000 to 4250 A), two scanning spectrometers (1000 to 4000 A), and one nebular photometer (2000 to 3300 A). The other package, Celescope, consisted of four independent telescopic Schwarzchild cameras (1200 to 2900 A).

Built in an octahedron shape, 10 ft by 7 ft, the satellite weighed 4400 pounds. The design was dictated by the stringent requirement of the experiments for pointing accuracy, pointing stability, command capability, data handling, and thermal environment, and by the constraints of the orbit for ground-station contacts. Six two-gimbal star trackers were programmed by the onboard memory to acquire and track appropriate guide stars. Error signals were generated that drove the reaction wheels to obtain stellar stabilization. Coarse momentum wheels were used for slewing the spaecraft. Memories permitted storage of 200,000 bits of experimental data. All information was relayed to the central control station in Greenbelt, MD. For more details, see J. Sargent, IEEE Trans. Geosci. Elec., GE-8, p. 215, 1970.

Launch Information

Launch Date: 1968-12-07 at 08:38:00 UTC
Launch Vehicle: Atlas-Centaur
Launch Site: Cape Canaveral, United States

Trajectory Description
Type: Orbiter
Central Body: Earth
Epoch start: 1968-12-07 08:38:00 UTC

Orbital Parameters
Periapsis 759 km
Apoapsis 767 km
Period 100.1 minutes
Inclination 35 °
Eccentricity 0.00056
Mass: 2150.0 kg


High-Resolution Telescopes

This Smithsonian Astrophysical Observatory (SAO) experiment, referred to as the Celescope experiment, consisted of two major subassemblies: an optical package, containing four 12-in. Schwarzschild telescopes that used Uvicons to produce television pictures of star fields, and an electronics package. The purpose of Celescope was to measure the UV brightness of a large number of stars. The Uvicons were fitted with filters constructed by combining halves of filter blanks. Redundant data were obtained, thereby, over four different passbands. These passbands were from 1200 to 1500 A, 1375 to 1800 A, 1800 to 2800 A, and 2850 to 3250 A. When struck by UV radiation in the wavelength range to which it was sensitive, the photocathode of a Uvicon emitted electrons in proportion to the intensity of the radiation. The electrons were accelerated and electrostatically focused onto a target, forming a pattern that was half the size of the initial optical image at the focal plane. A time exposure could be made by allowing the electron "picture" to build up on the target. Operated by ground control, time exposures of up to 30-s duration could be made of a field 3 deg square. The optical parts of Celescope weighed 440 pounds, and its electronics 77. The only significant failure during the 16 months of operation was caused by overexposing one of the Uvicons to sunlight. The sensitivity of the other Uvicons decreased significantly but in a correctable manner, although this decrease was the primary reason that the operation of this experiment ceased in April 1970. Over 8500 2-deg by 2-deg star fields were observed, covering about 10% of the sky and yielding observations of some 5000 objects. For more details, see R. N. Watts, Jr., Sky & Tel., p. 280, May 1969; W. A. Deutschman, Pub. Astron. Soc. Pac., v. 84, p. 123, 1972.

The Wisconsin Equipment Pckage

This Wisconsin equipment package contained seven telescopes designed to make spectrophotometric measurements of selected celestial objects in the ultraviolet longward of 1050 A; a set of four stellar photoelectric photometers located behind 8-in. telescopes; a nebular photoelectric photometer located at the prime focus of a 16-in. telescope; and a set of two objective grating spectrometers. The stellar photometers were each located behind a filter wheel containing three filter passbands, a calibration source, and a dark slide. The filter passbands ranged from 1180 to 1370 A with effective wavelength 1330 A, and from 3810 to 4670 A with effective wavelength 4250 A. The filter passbands were arranged to provide redundant coverage so that the telescope responses could be cross-correlated. Two field stops were provided with angular diameters of 2 arc-min and 10 arc-min. Photons were detected by photomultipliers. These photomultipliers drove pulse counters and dc amplifiers, thus providing redundant output. Unfortunately, the analog channel of stellar photometer 4 failed shortly after launch and provided no useful data. The filters experienced some degradation in orbit. Corrections to be applied to the stellar photometer data are becoming available.

The nebular photometer was located behind a six-position filter wheel providing passbands from 1930 to 2230 A (effective wavelength 2130 A) to 3050 to 3570 A (effective wavelength 3330 A,) as well as a calibration slide and a dark slide. A pulse counter and dc amplifier similar to those used with the stellar photometers were used with this photometer meter. About 2-1/2 months after launch, a failure left the calibration source permanently in place, and no further data resulted from this detector. Spectrometer 1 covered the wavelength range from 1800 to 3800 A in 100 steps with resolutions of 20 or 200 A (switchable). The slit width of 20 A corresponded to 2 arc-min projected on the sky, and the slit height corresponded to 8 arc-min. Spectrometer 2 covered the wavelength range from 1050 to 2000 A in 100 steps with resolutions of 10 or 100 A. The slit width of 10 A corresponded to 2 arc-min projected onto the sky, and the slit height corresponded to 8 arc-min. Aside from the failures of the nebular photometer, the analog channel of stellar photometer 4, and the degradation of the filters, the instrument operated normally from launch to spacecraft shutoff in February 1973. For more details, see A. D. Code et al., Ap. J., v. 161, p. 377, 1970.