Soon after the start of science operations of the Chandra X-ray Observatory, it became apparent that weakly penetrating
(0.1-0.5 MeV) protons in the Earth's radiation belt were causing an unexpectedly rapid increase in the charge-transfer
inefficiency of Chandra's front-illuminated CCDs. Fortunately, the Chandra team developed, implemented, and
maintains a radiation-protection program that successfully reduced the rate of degradation of the CCDs' performance to
acceptable levels. Since implementing this program, the average rate of increase of the charge-transfer inefficiency has
slowed to 3.2×10-6/y (2.3%/y) for the front-illuminated CCDs and 1.0×10-6/y (5.8%/y) for the back-illuminated CCDs.
This paper reviews the Chandra radiation-management program, reports the current status, and describes changes
planned or implemented since the previous paper on this topic.
The Chandra X-Ray Observatory was launched in July, 1999 and has yielded extraordinary scientific results. Behind the scenes, our Monitoring and Trends Analysis (MTA) approach has proven to be a valuable resource in providing telescope diagnostic information and analysis of scientific data to access Observatory performance. We have created and maintain real-time monitoring and long-term trending tools. This paper will update our 2002 SPIE paper on the design of the system and discuss lessons learned.
The CCDs on the Chandra X-ray Observatory are vulnerable to radiation damage from low-energy protons scattered off the telescope's mirrors onto the focal plane. Following unexpected damage incurred early in the mission, the Chandra team developed, implemented, and maintains a radiation-protection program. This program - involving scheduled radiation safing during radiation-belt passes, intervention based upon real-time space-weather conditions and radiation-environment modeling, and on-board radiation monitoring with autonomous radiation safing - has successfully managed the radiation damage to the CCDs. Since implementing the program, the charge-transfer inefficiency (CTI)
has increased at an average annual rate of only 3.2×10-6 (2.3%) for the front-illuminated CCDs and 1.0×10-6 (6.7%) for the back-illuminated CCDs. This paper describes the current status of the Chandra radiation-management program, emphasizing enhancements implemented since the original paper.
We have implemented a system to automatically analyze Chandra x-ray observations of point sources for use in monitoring telescope parameters such as point spread function, spectral resolution, and pointing accuracy, as well as for use in scientific studies. The Chandra archive currently contains at least 50 observations of star cluster-like objects, yielding 5,000+ sources of all spectral types well-suited for cataloging. The system incorporates off-the-shelf tools to perform the steps from source detection to temporal and spectral analyses. Our software contribution comes from wrapper scripts to autonomously run each step in turn, verify intermediate results, apply any logic required to set parameters, decide best-fit results, merge in data from other catalogs and to format convenient text and web-based output. We will outline this processing pipeline design and challenges, discuss the scientific applications, and focus on its role in monitoring on-orbit observatory performance.
The CCDs on the Chandra X-ray Observatory are sensitive to radiation damage, particularly from low-energy protons scattering off the telescope's mirrors onto the focal plane. In its highly elliptical orbit, Chandra passes through a spatially and temporally varying radiation environment, ranging from the radiation belts to the solar wind. Translating the Advanced CCD Imaging Spectrometer (ACIS) out of the focal position during radiation-belt passages has prevented loss of scientific utility. However, carefully managing the radiation damage during the remainder of the orbit, without unnecessarily sacrificing observing time, is essential to optimizing the scientific value of this exceptional observatory throughout its planned 10-year mission. In working toward this optimization, the Chandra team developed and applied a radiation-management strategy. This strategy includes autonomous instrument safing triggered by the on-board radiation monitor, as well as monitoring, alerts, and intervention based upon real-time space environment data from NOAA and NASA spacecraft. Furthermore, because Chandra often spends much of its orbit out of the solar wind (in the Earth's outer magnetosphere and magnetosheath), the team developed the Chandra Radiation Model to describe the complete low-energy-proton environment. Management of the radiation damage has thus far succeeded in limiting degradation of the charge-transfer inefficiency (CTI) to less than 3.5(10-6) and 1.3(10-6) per year for the front-illuminated and back-illuminated CCDs, respectively. This rate of degradation is acceptable for maintaining the scientific viability of all ACIS CCDs for more than ten years.
The Chandra X-ray Observatory was launched in July, 1999 and has yielded extraordinary scientific results. Behind the scenes, our Monitoring and Trends Analysis (MTA) system has proven to be a valuable resource. With three years worth of on-orbit data, we have available a vast array of both telescope diagnostic information and analysis of scientific data to access Observatory performance. As part of Chandra's Science Operations Team (SOT), the primary goal of MTA is to provide tools for effective decision making leading to the most efficient production of quality science output from the Observatory. We occupy a middle ground between flight operations, chiefly concerned with the health and safety of the spacecraft, and validation and verification, concerned with the scientific validity of the data taken and whether or not they fulfill the observer's requirements. In that role we provide and receive support from systems engineers, instrument experts, operations managers, and scientific users. MTA tools, products, and services include real-time monitoring and alert generation for the most mission critical components, long term trending of all spacecraft systems, detailed analysis of various subsystems for life expectancy or anomaly resolution, and creating and maintaining a large SQL database of relevant information. This is accomplished through the use of a wide variety of input data sources and flexible, accessible programming and analysis techniques. This paper will discuss the overall design of the system, its evolution and the resources available.