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2.1     Overview

The Chandra X-ray Observatory (CXO) was launched on the Space Shuttle Columbia on July 23, 1999. The Chandra program is sponsored by NASA’s Science Mission Directorate (SMD) and managed by the NASA Marshall Space Flight Center (MSFC). The prime contractor responsible for developing the spacecraft and integrating the CXO was TRW. The science instruments were developed as follows:

• The Advanced CCD Imaging Spectrometer (ACIS), built by the Pennsylvania State University in collaboration with the Massachusetts Institute of Technology (MIT);


• The High Resolution Camera (HRC) built by the Smithsonian Astrophysical Observatory (SAO);


• The Low Energy Transmission Grating (LETG) built by the Scientific Research Organization of the Netherlands (SRON) in collaboration with the Max-Planck-Institüt für Extraterrestriche Physik (MPE); and


• The High Energy Transmission Grating (HETG) built by MIT.

Chandra has as its primary mission the study of the structure and emission properties of astrophysical sources of high-energy radiation. The scientific objectives of the Chandra Mission are to utilize the Observatory to:

• Determine the nature of celestial objects from normal stars to quasars;


• Understand the nature of physical processes that take place in and between astronomical objects; and


• Understand the history and evolution of the universe.

2.2     Science Payload

Chandra is comprised of the spacecraft, the X-ray telescope, and the Science Instrument Module (SIM). The spacecraft provides the power, attitude control, communications, etc. for the telescope and instruments. The X-ray telescope consists of an optical bench, the High Resolution Mirror Assembly (HRMA), an aspect camera system, and two objective transmission gratings: the High Energy Transmission Grating (HETG) and the Low Energy Transmission Grating (LETG). The HRMA is a Wolter Type I, 1.2-m diameter, 10-m focal length, iridium-coated X-ray telescope consisting of 4 nested pairs of cylindrical hyperboloid and paraboloid mirrors. At 1.5 keV, >85% of the on-axis, imaged and aspect-corrected X-rays are contained in a circle of diameter ~1.0 arc second.

Chandra carries two focal-plane scientific instruments mounted in the SIM: the ACIS, and the HRC. The SIM provides three functions: launch lock, translation (to interchange focal plane instruments), and focus. Only one of the two focal plane instruments can be placed at the telescope’s focus at any time; therefore, simultaneous observations with both focal-plane instruments cannot be accommodated.

The ACIS has two arrays of CCDs, one (ACIS-I) optimized for imaging wide fields (16x16 arc minutes) the other (ACIS-S) optimized as a readout for the HETG transmission grating. One chip of the ACIS-S (S3) can also be used for on-axis (8x8 arc minutes) imaging and offers the best energy resolution of the ACIS system.

The HRC is comprised of two micro-channel plate imaging detectors, and offers the highest spatial (<0.5 arc second) and temporal (16 msec) resolutions. The HRC-I is a single micro-channel plate and has a field-of-view of 31x31 arc minutes. The HRC-S consists of three contiguous segments, tilted slightly in order to conform to the Rowland circle of the LETG. The background rate is quite different in the two devices, being larger in the HRC-S.

The HETG is optimized for high-resolution spectroscopy over the energy band 0.4-10 keV. Two types of gratings are mounted in the HETG: medium-energy gratings (MEGs) covering the 0.4–5 keV band and high-energy gratings (HEGs) covering the 0.9–10 keV band. The MEGs are mounted behind the annular aperture of the outer two mirror pairs while the HEGs are mounted behind the apertures of the inner two mirror pairs. The two sets of gratings operate simultaneously so that the dispersed axes of the spectra cross at a shallow angle in the focal plane. The ACIS-S is the readout of choice for use with the HETG. The resolving power (E/DE) varies from ~800 at 1.5 keV to ~200 at 6 keV.

The LETG is optimized for high-resolution spectroscopy over the energy bandwidth ~0.09–4 keV. The LETG provides resolving power ~1000 at 0.1 keV and ~200 at 1.5 keV. The HRC-S is the only detector aboard the Observatory that can fully accommodate the LETG-dispersed spectrum.

Detailed descriptions of all of the instruments are contained in the Proposers’ Observatory Guide. Proposers should refer to that document for additional details before preparing a proposal.

2.3     Operation

After launch into low earth orbit by the shuttle Columbia, the initial Chandra operational orbit was achieved by use of Boeing’s Inertial Upper Stage and Chandra’s own propulsion system. There are sufficient expendables (control gas for momentum unloading) for 15-20 years of operation. The orbital period of about 63.5 hours allows for reasonably long, uninterrupted observations of up to ~180 ksec before the instruments have to be powered down as the satellite dips into the radiation belts. Approved longer observations are split into several orbit-sized observations on ingestion into the observation catalog. Information as to the begin and end times of Chandra orbits is available at http://cxc.harvard.edu/proposer/orbits.html.

The Observatory’s solar panels can rotate about an axis perpendicular to the optical axis so that at any time the Observatory can be pointed to any position in the sky except for avoidance regions around the Sun (46 degrees), Moon (6 degrees), and Earth (10 degrees). Both the Moon and Earth may be viewed if specially requested and as long as an accurate aspect solution is not required. In order to avoid over-heating the spacecraft components on the sunward side or excessive cooling of the propellant lines the maximum length of an exposure is dependent on the pitch angle at which the target is observed. Some pitch angles are excluded. Observations with exposure times longer than the maximum allowed at a given pitch angle will be segmented. Details of these restrictions are given in Chapter 3 of the Proposers’ Observatory Guide (http://cxc.harvard.edu/proposer/POG/index.html). However, pitch angle restrictions are evolving with time and proposers are urged to check the CXC website for current information.

The high elliptical orbit and the radiation belts that prevent the conduct of observations at low altitudes imply that most of observations are made nearer apogee, where the Earth, as seen from Chandra, appears to move only slowly through the sky. As a result, the Earth and its surrounding avoidance region constitute a portion of the sky that will be partially blocked from view, and long, continuous observations in this region (>30 ksec at the center of the region) will be difficult, although shorter observations are possible. The proposer is urged to read Chapter 3 of the Proposers’ Observatory Guide (POG) to become familiar with all Chandra observing constraints and to make use of the Observation Visualizer (ObsVis) and PRoVis  to see how these constraints might impact their observations.  For highly constrained observations, we recommend that the proposer contact the CXC Help Desk.

2.4     The Chandra X-ray Center (CXC)

The Chandra X-ray Center (CXC), funded by NASA via a contract to the Smithsonian Astrophysical Observatory (SAO) in Cambridge, MA, is responsible for planning and conducting all aspects of Chandra operations. The CXC’s main activities include:

• Proposal Solicitation and Review: Soliciting proposals for observing time and research funding, conducting peer reviews, and selecting proposals.


• Mission Planning: Based upon approved proposals, creating a timeline of science observations and detailed schedules of spacecraft activities.


• Instrument Calibration: By means of special observations and advanced data analysis, determining parameters and data products that characterize the science instruments.


• Mission Operations: Commanding the spacecraft, monitoring and assessing spacecraft and science instrument health and safety, and receiving science and engineering data from the spacecraft.


• Data Processing and Archiving: Processing spacecraft telemetry to produce science data products for users, and storing products in a permanent archive. Data in the archive are typically available to the public after the one-year proprietary period expires, while calibration data are available immediately.


• Supporting Data Analysis: Defining and producing software for use in analyzing Chandra data


• User Support: Assisting users to derive maximum benefit from the Chandra X-ray Observatory; maintaining and conducting the Chandra Users’ Committee; and producing documents and other materials on the use of the Chandra X-ray Observatory.


• Education and Public Outreach: Conducting a program of formal and informal education and public outreach using Chandra data and results.

SAO, through its management of the CXC, is responsible for scientific research of the highest technical merit utilizing the Chandra X-ray Observatory. In order to carry out this responsibility, NASA has directed SAO to engage the participation of the broader science community and has determined that this function will be accomplished by SAO allotting observing time and research funding to users in accordance with the following process conducted at appropriate intervals:

• Prepare and issue Calls for Proposals for observations with the CXO and for funding of activities including data analysis by General Observers; Archival and Theoretical Research; Postdoctoral Fellowships; Education and Public Outreach; and other research.


• Prepare and conduct independent peer evaluations of proposals, and select proposals for observation and funding as recommended by the peer review panels.


• Allocate funding to selected investigations as recommended by the peer review panels, determine the period of performance of each award, issue funding instruments on behalf of NASA in the form of grants, and administer the awards through closeout.

SAO is not responsible for transferring funds to NASA Centers and Other Federal Agencies whose proposals are selected for awards. NASA will be responsible for direct funding of research at NASA Centers and for executing appropriate inter-agency agreements with other federal agencies. However, the CXC provides the results of the CXO observations, as selected, to all investigators, including those at federal agencies.