Mission Overview

The Astronomical Netherlands Satellite (ANS) was an earth-orbiting, sun-synchronous satellite, designed as an astronomical observatory. The spacecraft was attitude-controlled by magnetic coils interacting with the earth's magnetic field, by reaction wheels, and by a so-called “yo-yo” (a device for initially despinning the spacecraft by ejecting two masses that carry away most of the angular momentum). Attitude sensing was carried out by solar sensors (coarse, intermediate, and fine), horizon sensors, a star sensor, and a magnetometer. Two guide stars near the object being observed served as the final pointing references. Experiments on board observed celestial objects in UV and X-ray wavelengths. During its observing lifetime of 20 months (September 1974 to June 1976), ANS measured the positions, spectra, and time variations of galactic and extragalactic X-ray sources in the energy range 2 to 15 keV, and obtained over 18,000 observations of about 400 objects in the UV range 1500 to 3300 A.

Launch Information
Launch Date: 1974-08-30 at 14:07:39 UTC
Launch Vehicle: Scout
Launch Site: Vandenberg AFB, United States
Decay Date: 1977-06-14

Trajectory Details
Type: Orbiter
Central Body: Earth
Epoch start: 1974-08-30 14:07:00 UTC

Orbital Parameters
Periapsis 266.0 km
Apoapsis 1176.0 km
Period 99.19999694824219 minutes
Inclination 98.0°
Eccentricity 0.06404799968004227

Instrumentation

UV Telescope

The ultraviolet five-band photometer of ANS consisted of a Cassegrain telescope followed by a grating spectrometer of the Wadsworth type. The telescope primary mirror had a diameter of 22.5 cm and a light-collecting surface of 266 sq cm. A rectangular slit in the focal plane of the telescope provided a field of view of 2.5 sq arc-min to the photometer. The curved refraction grating of the spectrometer had five fixed slits in its focal surface. The position and width of these slits determined the central wavelength and wavelength range of the passbands. The five passbands, which did not overlap, had central wavelengths from 1545 A to 3294 A. The transmitted light reached photomultipliers--one behind each slit--that were used in a pulse-counting mode. The instrument was sensitive to stars as faint as apparent visual magnitude +11. The primary aim of the experiment was to measure the absolute intensities of a large number of objects in the five passbands and, thus, to make possible a better classification of hot stars.

Soft X-Ray Experiment (SXX)

The instrumentation consisted of a mylar-window proportional counter (44- to 55-A passband), located at the focus of a grazing incidence ring paraboloid telescope, and a titanium-window proportional counter (passbands of 27- to 35-A, 4- to 12-A, and 2- to 4-A) located behind a honeycomb collimator. The sensors, which observed X rays from cosmic sources, required an instrument-pointing accurary of 0.1 deg. Part of the experiment became inoperable on June 21, 1975, when the mylar-window on the 44-55 A proportional counter apparently ruptured.

Hard X-Ray Experiment (HXX)

This experiment was designed to detect cosmic X-ray emissions in the energy range from 1 to 30 keV. The principal scientific objectives of the experiment were (1) to gather spectral data with an energy resolution of 20%, (2) to detect silicon emission lines at about 1.8 and 2.0 keV at an energy resolution of 0.15%, (3) to study periodic and random intensity variations of sources over a time range of 4 ms to several minutes, (4) to obtain data on X-ray light curves, and (5) to define positions of sources with a precision approaching 1 arc-min.

The experimental package contained three major components: (1) a collimator assembly, (2) a large area detector (LAD) unit for measuring 1- to 30-keV X-rays, and (3) a Bragg-crystal spectrometer tuned for detection of the silicon lines. The LAD and Bragg spectrometer detectors were very sensitive, being able to detect 3E-3 photons/(sq cm-s). X-ray incident on the front face of the package passed through the collimator assembly onto either the LAD or a series of four Bragg crystals that were oriented at about 45 deg with respect to the incident beam. The collimation in front of the LAD was a combination of a fine collimation (10 arc-min FWHM) and coarse collimation (3 deg FWHM). The two counters composing the LAD were pointed in directions that differ by about 4 arc-min. This allowed for the possibility of guiding on strong X-ray sources in the event guide stars were not available. Also, it made for a uniform response of the combined output of the detectors within this 4 arc-min range.

Each detector consisted of an argon-filled proportional counter with a 9.4-mg/sq cm beryllium window. The effective collection area of each counter was about 40 sq cm, after correction for the collimator transmission, and each had a detection efficiency in excess of 10% from 1.8 to 20 keV. The output from the LAD counters was processed by a 15-channel logarithmic pulse-height analyzer, all channels of which were recorded in memory either every 4 s or 64 s. Higher time resolutions of 1 to 4 ms were possible through the use of a scheme that recorded the time of arrival of the first six events occurring each second in the LAD. In addition, a single channel analyzer was used to record the integrated counts in the 1.3- to 7-keV range in 1-, 4-, or 16-s intervals.

Only the coarse collimator fed X-rays onto the four Bragg pentaerythritol (PET) crystals. The diffracted X-rays were then detected by two argon-filled proportional counters with 4.7-mg/sq cm beryllium windows. When projection effects and peak reflectivity of the crystal were accounted for, the effective detection area of each counter was 6 sq cm within the 2-eV resolution of the crystal. The output from a Bragg detector was filtered by an eight-channel logarithmic pulse-height analyzer operating in the energy interval from 1 to 4.2 keV. For both the LAD and Bragg detectors, effective non-X-ray event rejection was accomplished by pulse-shape discrimination of the proportional counter signals.

Science

The data produced a spectrum of the diffuse background over the energy range 14-200 keV.

Summary