Calibration operations providing a lot of extremely interesting data
The first XMM-Newton Science Working Team meeting since the launch of the X-ray observatory last December has just been held at Vilspa on 5/6 April. It was an exceptionally well attended gathering. Convened by Project Scientist Fred Jansen, all those present had much to discuss, with new X-ray data and images to show.
Principal Investigators, mission scientists and their collaborators, the Instrument and Calibration Scientists, the staff from the Science Operations Centre at Vilspa and representatives from the Mission Operations Centre from Darmstadt all shared the same feeling of excitement now that XMM-Newton is approaching its operational phase.
Since the presentation of the first commissioning images on 9 February, the mission has moved into the calibration and verification phase. Giving an overview before the individual progress reports from the different instrument teams, Fred Jansen reflected the overall great satisfaction at the quality of the data that is now being obtained. He did not hide the many operational issues that are also being encountered.
Radiation but no damage
The possible impact of XMM-Newton’s radiation environment on the science instruments was one of the first issues to be examined. In addition to the expected effects of the Earth’s radiation belts and solar flares, XMM-Newton has been experiencing an occasionally higher than anticipated level of background radiation, most likely attributable to very low energy protons. These are being detected by the X-ray cameras but apparently not causing any damage to the detectors. Nevertheless mission controllers and science investigators remain vigilant; certain precautionary measures are being taken and the phenomenon is being investigated by space environment teams at ESTEC and also in the United States where teams on NASA’s Chandra X-ray observatory are faced with the same phenomenon.
Jean Clavel, the manager in charge of Science Operations at Vilspa, recapped on how the calibration phase operations were being conducted. Operations had suffered from a number of issues, which, at regular intervals, had made operations difficult. "Our job is actually very complex, human intensive and error prone" he emphasized. On the positive side, all this allows the Science Operations Centre to verify its procedures and ensure the safety of the science instruments in case of spacecraft/operations incidents. Staff in Vilspa and in Darmstadt are contributing a lot of overtime to ensure mission success, and Jean Clavel is confident that the efficiency of science operations will rapidly be seen to improve.
Investigators glad to be ‘playing with real data’
Martin Turner, Principal Investigator for EPIC had the honour of presenting some new images obtained by the X-ray observatory. Amongst them are some vivid X-ray colour pictures of the Crab nebula, seen by both the EPIC-PN and MOS cameras. All camera modes had been checked out and are working extremely well. The ongoing calibration showed that "the imaging and spectral resolution of the instruments is excellent".
The calibration of the Reflection Grating Spectrometers, for which Bert Brinkman is Principal Investigator, is progressing very fast. "The resolution of the RGS is excellent, as good as could be hoped for with a perfect alignment". He also displayed some new X-ray spectra, showing how much information can be learnt from one celestial source. The number of lines, betraying the presence of different elements in a source, "is really unbelievable and gives a flavour of the potential of our instrument".
Principal Investigator Keith Mason gave his status report for the Optical Monitor. The low level stray and scattered light issues have now been fully analysed, as have other minor telescope issues for which corrective solutions have been found. Calibration observations with the OM have been successful, although very limited because of some safety issues over the last two weeks. Focusing of the telescope has now been completed.
The Science Working Team meeting also heard several other presentations. Amongst them, Bernd Aschenbach, XMM-Newton Telescope Scientist,
stressed the perfection of the telescope optics being shown yet again by the calibration measurements. Philippe Gondoin recapped on the Mirror Calibration. Mission Operations staff from Darmstadt reminded everyone that despite the teething problems, flight operations were functioning very well.
Serendipitous science estimate
For the Science Survey Centre, Mike Watson presented the maturity of the different ‘pipelines’ that will be used to routinely process XMM-Newton data. He also gave a rough estimate of how well the mission will be performing for serendipitous science. Examination of the Hickson 16 group field (during a 50 kilosec exposure by the EPIC MOS-1 camera and presented at the ‘first images’ Vilspa press conference on 9 February) showed, as a 1st estimate, no less than 50 soft-band and 30 hard-band X-ray sources.
The calibration operations, viewing a fair number of X-ray sources for nearly a month, are also bringing in a considerable amount of science data which the scientists are eager to start publishing. This gave rise to a lively discussion on how and when the various teams could start releasing this "open data" and Project Scientist Fred Jansen reminded everyone of the ‘rules of the game’ that had been agreed.
Concluding the two day proceedings, Fred Jansen said that "we are having our expected fair share of problems. Such incidents are normal in the everyday life of a new mission. But for XMM-Newton there are no
show-stoppers". He confirmed the target of completing the calibration and performance verification phase by early summer. "A lot of hard work remains ahead for everyone, and especially for the Science Operations people here at Vilspa. But I expect that by mid-July, we will know when we will be able to start the routine observations phase."
* Smooth sailing through the first eclipse
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ESA Science News
13 Apr 2000
XMM-Newton’s built-in watchdog is put to the test
All satellites are designed to face up to unexpected circumstances. XMM-Newton, ESA’s new X-ray observatory launched last December, proved the reliability of its onboard systems when the spacecraft placed itself in standby mode on 2 April.
XMM-Newton entered what is formally called the "Emergency Sun Acquisition Mode" (ESAM) on Sunday 2 April when its onboard watchdog detected that something was not quite normal. As on two previous occasions, the satellite automatically responded by placing itself in a safe position with its solar panels pointing directly to the Sun to ensure continued power and its antennae towards Earth. It simultaneously alerted ground controllers.
This built-in watchdog functions by constantly checking all the spacecraft attitude parameters such as its orientation, any activity of the thrusters or reaction wheels used to maneuvre the spacecraft and by comparing them with pre-set tight limits. This routine, which is part of the satellite’s Attitude and Control System, is there to prevent for instance
the observatory’s sensitive X-ray detectors being blinded by pointing too close to the sun. It also allows the satellite to be able to survive several days without ground contact and recover without damage from what are called single-event upsets in its electronic systems that may be caused by deep space particle radiations.
Placing the satellite in this default standby mode, waiting patiently for instructions, ensures that ground controllers are able to fully appreciate the situation. There is, strictly speaking, no emergency and they can take their time to analyse the causes of the satellite’s unscheduled manoeuvre.
"XMM-Newton was being manoeuvred manually at the time" explains Robert LainÈ, XMM-Newton Project Manager. "A command to execute a new maneuver was sent too soon after the previous one. The satellite detected that this command conflicted with previous orders received. In such circumstances it immediately stops excuting the commanded maneuvre, closes the science instruments and places itself in safe mode within minutes".
"The Mission controllers in Darmstadt immediately understood what occurred. Once the satellite had reach its safe position, they went through their check list, verified that nothing was wrong on board then initiated the recovery procedure. This recovery involves checking and reloading one by one all the computers parameters needed for normal operations and that takes 10 hours. By now XMM-Newton has resumed normal science operations."
Such incidents always come as a surprise, says LainÈ, but must be expected. They occur several times during the in-orbit life of any satellite. There would be, on the contrary, cause for greater concern if the satellite failed to respond automatically to an anomalous command timing. And so far XMM-Newton is responding beautifully — even when things go slightly amiss!
* XMM-Newton operations in orbit
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