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The Astronomische Nederlandse Satelliet (ANS) was a collaborative effort between
the Netherlands and USA. The spacecraft was built by the Industrial Consortium Astronomical
Netheralands Satellite (Fokker-VFW and Philips) and it was lauched on an American Scout
rocket. ANS was launched from the Western Test Range, California, on August 30 1974 into a
high inclination (97.6°), sun-synchronous orbit. Due to a first-stage guidance failure
the desired 500 km circular orbit was not obtained. Instead the satellite was placed in a
elliptical orbit (initial perigee at 280 km and apogee at 1150 km period 98.2 minutes)
that caused initial problems with the background radiation and complicated the scheduling
of the observations.
The spacecraft was a three-axis stabilized with one axis always pointed
to the Sun (Z-axis). The experiments were mounted perpendicular to the Sun line (X-axis)
and the spacecraft could be pointed, with an accuracy of 1 arcminute, towards any position
in the sky perpendicular to the sun-axis. In 6 months, ANS made a complete circle in the sky.
The satellite contacted the ground station once every 12 hours. The observing time was
divided equally between the experiments on a one-orbit-per experiment basis. Typical data
acquisition on a known point source consisted of the detector pointing either at the source
continuously for ~1000 s, or pointing at the source for ~128 s, then pointing off for
background accumulation.
ANS was operational for one and a half year and the spacecraft re-entered the Earth’s
atmosphere June 14 1977.
The scientific payload consisted of an UV-spectrophotometer from the University of Groningen (1500–3300Å) and two X-ray experiments one from the University of Utrect (SXX; 0.16–7 keV) and the other from AS&E/MIT (HXX; 1.5–10 keV).
- The Soft X-Ray Experiment (SXX) consisted of 2 parts. The Utrecht soft X-ray detetcor,
sensitive from 0.2–0.28 keV, consisted of a grazing-incidence parabolic collector
(effective arae of 144 cm2) with a small-area 3.6 micro polypropylene proportional
counter in the focal plane. The sensitivity was such that 1 count/s (0.2–0.28 keV) was
about 0.53 photons/cm2/s/keV at 0.28 keV. The second part, known as the Utrecht
medium X-ray detector, was a proportional counter with a 1.7 μm titanium window. It was
sensitive in the 1–7 keV band, divided in 5 pulse-height channels, and an effective
area of 45 cm2.
- Hard X-Ray Experiment (HXX). The HXX also consisted of 2 major parts: the large area
detector (LAD) and the Bragg Crystal Assembly (BCA). The LAD was a pair of narrowly
collimated proportional counters sensitive to the energy range 1.5–30 keV, with a
total area of 60 cm2. Each counter in the pair was collimated by
10′ × 3°. There was a 4′ offset between the pair in the narrow
direction. The LAD energy data from 1-30 keV were distribute over 15 pulse height channels
and recorded with a resolution of either 4 and 64 seconds. A single channel collecting
data from 1&ndahsh7 keV could be read out every 1 or 4 or 16 sec. High time resolution
(uo to a 1 ms) was also available. One ANS HXX count in the 1.5–7 keV was equivalent
to 15 Uhuru counts. The Bragg Crystal Assembly is a pair of PET crystals and
proportional counters aligned to search for silicon emission lines near 2 keV.
ANS made three important discoveries: detection of the first coronal emission from Capella,
the detection of X-ray flares from the flares stars UV Ceti and YZ CMi and the discovery
(along with Vela 5A and 5B) of X-ray bursts. The first X-ray burst event was detected
from the globular cluster
NGC 6624.
