A Tribute to ROSAT X-Ray Satellite – by Albert Lim (2000)

The name ROSAT came from the German word 'Rontgensatellit' in honour of the German physicist Wilhelm Conrad Rontgen who discovered X-rays in 1895. The ROSAT project was first proposed in 1975 by Max-Plank-Institute for Extraterrestrial Physics (MPE) and collaborated under joint agreements with 2 other international parties - the National Aeronautics and Space Administration (NASA) of USA and the British Science and Engineering Research Council (SERC) of UK.

Fig 01 - Artist  impression of  the ROSAT satellite on mission in space

Probably the most important and significant aspect of the German X-Ray observatory ROSAT is it's contribution towards the first truly successful all sky surveys in the soft X-ray band of 0.1 keV - 2keV (corresponding to wavelengths of 100 to 6 Angstroms) as well as in the adjacent extreme ultraviolet band of 0.04keV - 0.2 keV (corresponding to wavelength of 200 - 60 Angstroms). This survey was accomplished by ROSAT in scan mode from a 580 km Low Earth Orbit (LEO) utilising the X-Ray Telescope (XRT) and the Position Sensitive Proportional Counter (PSPS) on board ROSAT. These instruments were originally designed to be able to conduct high sensitivity survey of the sky and for detection of discrete sources in the 0.1 to 2.4 keV energy range. It soon became apparent to astronomers that the same instruments on board ROSAT would also be perfect for studying the diffuse X-ray background.
The extragalactic diffuse X-ray background was one of the first early discoveries in X-ray astronomy ; the existence of Galactic soft X-ray emission below 1 keV was first revealed in sounding rocket experiments even before the 70s. Astronomers have known since that the diffuse X-ray background must be made up of several components and these have considerable impact on our views of the intergalactic and interstellar space. The problem was that none of these components are well determined or even understood completely before ROSAT.

Fig 02 - ROSAT PSPC all sky X-Ray survey at 0.25, 0.75 and 1.5 keV

Fig 03 - ROSAT SXRB IJ band exposure all sky map-zero centered

Prior to ROSAT, astronomers already know the importance of such a survey. The complexity and apparent number of components in the diffuse X-ray background means that instruments of considerably improved angular and spectra resolution along with high statistical precision was required; these were simply not available until ROSAT came along. The relatively large effective area (2 degrees filed) and short focal length of the XRT in combination with the low background PSPC detector produced unprecedented signal-to-noise ratio for diffuse emission studies. At 0.25 keV for example, ROSAT image is 400% brighter and the background noise is 100 times lower than compared to the Einstein IPC (HEAO-2).  ROSAT's 2 degree field also assist in reducing survey time resulting in 100 times more detected photons than in any previous survey. As a comparison, see the diffuse source sensitivity comparison table below :

Table 1 : Diffuse Source Sensitivity Comparison

Abbreviations to above Table
ROSAT           -   The German 'Rontgensatellit'  X-ray satellite
Einstein             -   High Energy Astrophysical Observatory-2 (HEAO-2 - NASA)
ASCA              -  Advanced Satellite for Cosmology and Astrophysics(Japan & NASA)
Spectrum X      -  Spectrum X Gamma (USSR and later Russia)
AXFA              -  Advanced X-Ray Astrophysics Facility (NASA)
XXM               -  X Ray Multi-Mirror (ESA) 

In all, the importance of ROSAT and the sky surveys over it’s lifespan of nearly 9 years from mid 1990 till early 1999 leads to a quantum leap in X ray astronomy impressively documented by figures - more than 3,000 scientific contributions and articles, countless presentations based on ROSAT data and more than 1,000 research groups use and benefited from ROSAT observations. The ROSAT sky survey harvested over 60,000 X-ray sources - a phenomenal increase from 840 sources cataloged earlier by HEAO-1. ROSAT is destined to be remembered as the most successful and productive X-ray facility in the history of high energy astrophysics during the 20th century.
It must be realised however, that the phenomenal success and the importance of ROSAT and the extraordinary results that it accomplished were not at all accidental. It was based on 10 years of hard work and concerted efforts in design, planning, building and testing of the spacecraft and its instruments prior to launch. On 1^st June 1990, having overcome the precarious dangers of launch and once in orbit, ROSAT depended heavily on a highly technical team based in Germany, who were brilliantly able to save the satellite on numerous occasions near the brink of disaster. To have survived operations for nearly 9 years for an instrument originally designed for a 2 year lifespan speaks highly of ingenuity of the design and operations team. ROSAT is nothing short of a technological wonder even by today’s standard as are the team of experts who conceive and operated it.

Fig 04 - Left - The author at Carl Zeiss, Germany, on 21st August 1999 with personal friend Mr. Hasselmann (Director of Carl Zeiss Space Technologies Group). Top Right - Briefing session on the fabrication of the X-ray mirrors for the Wolter Type I telescope for ROSAT by Zeiss engineer. Below Right - A group of 16 from TASOS (The Astronomical Society of Singapore) attending a lecture on Zeiss Innovation in X-ray optics. We were then in Germany for the Total Solar Eclipse of 22nd Aug 1999.