Mission Overview

The TD-1 satellite carried seven experiments devoted to astophysical studies. Its scientific mission was to make a systematic sky survey in the ultraviolet and high-energy regions of the spectrum. The experiments were divided into two main categories: five experiments--measuring ultraviolet, x and gamma rays, and heavy nuclei--scanned strips of the sky; the other two viewed along the sun-pointing x axis and measured solar x and gamma rays. The satellite was a triaxially stabilized platform with the x axis always pointed at the center of the sun with an accuracy of 1 arc min. The satellite rotated around this axis at a constant rate of 1 revolution per orbit during normal operations when sun sensors were used for stabilization but it was spun up during eclipse periods to maintain attitude. The sky-scanning instruments were able to scan a narrow band of the sky during each orbit and the whole celestial sphere in 6 months. Two and one-half complete scans of the celestial sphere were completed before the attitude control was lost in May 1974 following exhaustion of the on-board gas supply. Despite intermittent tape recorder failure, data coverage was achieved over 95 percent of the celestial sphere and many areas were observed during two or three separate scans. The spacecraft was a rectangular structure and comprised a bottom compartment containing the spacecraft subsystems and a top compartment containing the outward-viewing science instruments. It had a cross section of 1 by 0.9 m and was 2.2 m high; its mass was 473 kg including 120 kg of instruments.

Launch Information
Launch Date: 1972-03-12 at 01:55:00 UTC
Launch Vehicle: Delta
Launch Site: Vandenberg AFB, United States
Decay Date: 1980-01-09

Trajectory Details
Type: Orbiter
Central Body: Earth
Epoch start: 1972-03-12 00:00:00 UTC

Orbital Parameters
Periapsis 536.0 km
Apoapsis 557.0 km
Period 95.5999984741211 minutes
Inclination 97.5°
Eccentricity 0.0015150000108405948

Instrumentation

Stellar UV Radiation Experiment

This experiment consisted of a 1.4-m telescope with an attached spectrometer box. An off-axis paraboloid mirror (f/3.5, diam 275 mm) reflected starlight onto a system of two slits situated in the prime focal plane. One of the two slits fed the stellar light into a single photometric channel with a filter limiting the passband to 400 A centered at 2750 A. The other slit was much wider (11.9 x 17 arc-min), and led into the three-channel grating spectrometer. Once per orbit, the telescope, aligned along the Z-axis, scanned a great circle of the sky. Because of this motion across the sky, the primary image of a certain star entering the telescope's field of view moved across the photometer and spectrophotometer slots. While the star image traversed the wide spectrophotometer slot, its corresponding spectrum moved in the focal plane of the spectrograph across the three exit slits, behind which there were three pulse-counting photomultipliers. By employing the scanning motion of the satellite, a spectrum scanning action was achieved without the need for moving parts. The three exit slits of the spectrophotometer were fixed at the following wavelengths -- 1350 to 1760 A, 1760 to 2160 A, and 2150 to 2550 A. The wavelength region from 1350 to 2550 A was fully covered by the three channels in 3.3 s, yielding a total of about 60 data points. In each channel the spectrum was scanned at 19.4-A intervals, the effective passband during each integration interval having a full-width, half-maximum of 35 to 40 A. Just before the telescope was integrated into the satellite, the instrument was extensively calibrated in order to achieve an absolute photometric accuracy between 10 and 20 percent, a relative photometric accuracy within 10 percent and a wavelength calibration accurate to a few Angstroms. This experiment was designed to detect 20,000 stars, of which 6,000 should have given useful UV spectra. It was able to measure stars of magnitude 10.5. Two major objectives were the study of interstellar extinction and the preparation of a UV star catalog.

UV Stellar Spectrometer

This experiment consisted of a Cassegrain telescope (primary mirror 26 cm in diam) and a grating spectrometer that operated in three passbands (2260 to 2155 A, 2495 to 2590 A, and 2775 to 2865 A). When a star of sufficient brightness appeared in the telescope, the telescope locked onto it with a self-contained guidance system and then scanned three 100-A passbands in 0.5-A increments with an overall accuracy of 1 A and spectral resolution of 1.8 A.

Spectrometry of Celestial X-Rays 2-30 KeV (S77)

A 100-sq-cm proportional counter was used to measure the spectra of celestial X-ray sources in 10 channels between 2 and 30 keV. The proportional counter was located behind a crossed pair of slot collimators, which together yielded a 5- by 1-deg field of view. The proportional counter had a 0.5-mm beryllium window and a xenon filler gas. It was constructed in two parts, which were then anticoincidenced to remove the background due to cosmic-ray particles. Several months after launch the experiment was switched off when problems were encountered with the spacecraft's encoder. During the second scan period, on July 1, 1973, the experiment was successfully switched on. Calibration showed that the instrument had not suffered any degradation in sensitivity nor in energy resolution and the experiment was able to fulfill its scientific mission. During spin-up mode the experiment scanned the earth as well as the sky and was able to monitor X-ray radiation from the auroral zones.

Spectrometry of Celestial X-Rays 2-30 KeV (S77)

A 100-sq-cm proportional counter was used to measure the spectra of celestial X-ray sources in 10 channels between 2 and 30 keV. The proportional counter was located behind a crossed pair of slot collimators, which together yielded a 5- by 1-deg field of view. The proportional counter had a 0.5-mm beryllium window and a xenon filler gas. It was constructed in two parts, which were then anticoincidenced to remove the background due to cosmic-ray particles. Several months after launch the experiment was switched off when problems were encountered with the spacecraft's encoder. During the second scan period, on July 1, 1973, the experiment was successfully switched on. Calibration showed that the instrument had not suffered any degradation in sensitivity nor in energy resolution and the experiment was able to fulfill its scientific mission. During spin-up mode the experiment scanned the earth as well as the sky and was able to monitor X-ray radiation from the auroral zones.

Solar Gamma-Rays in the 50- to 500-MeV Energy Range

A combination of scintillators and photomultipliers were used to detect solar gamma rays (photon energy ‹ 50 and › 500 MeV) while discriminating against charged particles. A directional accuracy of a few deg was achieved. The effective area of 100 sq cm allowed a background of 1.E-5 photons/sq cm-sec to be obtained while the dynamic range allowed fluxes up to 1.E-2 to be measured during solar flares.

Solar X-Ray Monitor

The solar hard X-ray, integrating, pulse-counting spectrometer measured 24 to 900 keV X rays in 12 approximately logarithmically-spaced energy channels. The instrument consisted essentially of a scintillation counter. A CSi (na) crystal of sensitive area of 5 sq cm and of thickness 15 mm was optically coupled to an EMR photomultiplier tube. Impulse height analysis was done by means of discriminators, because of low dead-time and redundancy advantages in comparison with a multi-channel analyzing system. The energy channel limits were 24, 34, 45, 63, 90, 125, 175, 270, 320, 420, 540, 670, and 900 keV, which were sufficient for the analysis of the flare-photon spectral distribution. The sampling rate was 1.2 s for the four low-energy channels and 4.8 s for the others. To absorb the background X rays from various sources a lead-tin-copper shield-collimator was placed around the counter. The full width at half maximum of the collimator was 26 deg. To minimize the effects of background particles, inside the collimator were two solid-state detectors mounted in front of and completely masking the scintillation crystal. All particles entering the scintillator within the field of view were detected by these two solid-state devices. Thus, pulses in the scintillator, which were coincident with particles detected in the solid-state units, could be rejected. In-flight calibration of the whole system utilized the detection of the decay products from the two radioactive sources SR(90) and AM(241).

Solar X-Ray Monitor

The solar hard X-ray, integrating, pulse-counting spectrometer measured 24 to 900 keV X rays in 12 approximately logarithmically-spaced energy channels. The instrument consisted essentially of a scintillation counter. A CSi (na) crystal of sensitive area of 5 sq cm and of thickness 15 mm was optically coupled to an EMR photomultiplier tube. Impulse height analysis was done by means of discriminators, because of low dead-time and redundancy advantages in comparison with a multi-channel analyzing system. The energy channel limits were 24, 34, 45, 63, 90, 125, 175, 270, 320, 420, 540, 670, and 900 keV, which were sufficient for the analysis of the flare-photon spectral distribution. The sampling rate was 1.2 s for the four low-energy channels and 4.8 s for the others. To absorb the background X rays from various sources a lead-tin-copper shield-collimator was placed around the counter. The full width at half maximum of the collimator was 26 deg. To minimize the effects of background particles, inside the collimator were two solid-state detectors mounted in front of and completely masking the scintillation crystal. All particles entering the scintillator within the field of view were detected by these two solid-state devices. Thus, pulses in the scintillator, which were coincident with particles detected in the solid-state units, could be rejected. In-flight calibration of the whole system utilized the detection of the decay products from the two radioactive sources SR(90) and AM(241).

Gamma-Ray Measurement

An optical spark chamber with counters and a vidicon system was used to measure gamma rays in the 70- to 300-MeV energy range. The sensitive area of the detector was 200 sq cm, and the efficiency for gamma rays was 16%. All of the sky was scanned in 6 months with a sensitivity capable of detecting a flux of › 1.E-6 photons/sq cm-s.

Science
Summary