Chapter 1 | Chapter 2 | Chapter 3 | Chapter 4 | Chapter 5
Chapter 6 | Chapter 7 | Chapter 8 | Chapter 9
Appendix A | Appendix B


Chapter 1
Mission Overview

The Chandra X-Ray Observatory (CXO) combines an efficient high-resolution ( ≤ 1/2 arcsec) X-ray telescope with a suite of advanced imaging and spectroscopic instruments. The Observatory was successfully launched by the National Aeronautics and Space Administration's (NASA's) Space Shuttle Columbia on 1999-Jul-23, with Col. Eileen Collins commanding. Subsequently an Inertial Upper Stage and Chandra's Internal Propulsion System placed the Observatory in a high elliptical orbit . Chandra is the X-Ray component of NASA's four Great Observatories. The other components are the Hubble Space Telescope, the late Compton Gamma-Ray Observatory, and the decommissioned Spitzer Space Telescope.

1.1  Program Organization

The Chandra Project is managed by NASA's Marshall Space Flight Center (MSFC). The Project Scientist is Stephen L. O'Dell. Day-to-day responsibility for Chandra science operations lies with the Chandra X-ray Center (CXC) , Pat Slane, Director. The CXC is located at the Cambridge, Massachusetts facilities of the Smithsonian Astrophysical Observatory (SAO) and the Massachusetts Institute of Technology (MIT). The Chandra Operations Control Center (OCC) is located in Burlington, Massachusetts. The CXC uses the OCC to operate the Observatory for NASA.

1.2  Unique Capabilities

Chandra was designed to provide order-of-magnitude advances over previous X-ray astronomy missions with regards to spatial and spectral resolution. The High Resolution Mirror Assembly (HRMA) produces images with a half-power diameter (HPD) of the point spread function (PSF) of < 0.5 arcsec. Both grating systems-the Low Energy Transmission Grating (LETG) and the High Energy Transmission Grating (HETG)-offer resolving powers well in excess of 500 over much of their bandwidth which, together, cover the range from ≤  0.1 to 10 keV.

1.3  Observatory Overview

Figure 1.1: The Chandra Observatory with certain subsystems labeled.
An outline drawing of the Chandra X-ray Observatory is shown in Figure 1.1. Chandra consists of a spacecraft and a telescope/science-instrument payload. The spacecraft provides power, communications, command, data management, and pointing control and aspect determination. The principal elements of the observatory that will be discussed in this document are:
These and related elements of the Chandra Project are introduced briefly in the remainder of this chapter.

1.4  Pointing Control and Aspect Determination (PCAD)

The PCAD system controls the pointing and dithering of the observatory and provides the data from which both the relative and absolute aspect are determined. Dithering is imposed to spread the instantaneous image over many different pixels of the focal-plane detector to smooth out pixel-to-pixel variations. The dither pattern is a Lissajous figure (and can be seen quite clearly in the un-aspect corrected data from bright point sources). The amplitude, phase, and velocity depend on which instrument (ACIS or HRC) is in the focal plane.
Key elements of the PCAD system are the set of redundant gyroscopes, momentum wheels, and an aspect camera assembly (ACA) consisting of a four inch optical telescope with (redundant) CCD detector. The aspect camera simultaneously images a fiducial light pattern produced by light emitting diodes placed around the focal-plane instruments along with the flux from up to five bright stars that may be in the aspect camera's field-of-view. An interesting consequence is that the user may request that one of the targets of the aspect camera be at the location of the X-ray target. For bright optical counterparts, this option allows real-time optical monitoring, albeit at the price of a reduced-accuracy aspect solution-see Chapter 5 for further details. This option will be implemented only pursuant to a feasibility analysis during the planning and scheduling process.

1.5  HRMA

The HRMA consists of a nested set of four paraboloid-hyperboloid (Wolter-1) grazing-incidence X-ray mirror pairs, with the largest having a diameter of 1.2 m (twice that of the Einstein Observatory). The focal length is 10 m.
The mirror glass was obtained from Schott Glasswerke; grinding and polishing was performed at Hughes Danbury Optical Systems; coating at Optical Coating Laboratory; and the mirror alignment and mounting at Eastman-Kodak Co. The mirrors weigh about 1000 kg. Details of the HRMA and its performance are presented in Chapter 4.
The Chandra Telescope Scientist was the late Leon Van Speybroeck of the Smithsonian Astrophysical Observatory.

1.6  Science Instrument Module (SIM)

The Science Instrument Module consists of the special hardware that provides mechanical and thermal interfaces to the focal-plane scientific instruments (SIs). The most critical functions from an observer's viewpoint are the capability to adjust the telescope focal length and the ability to move the instruments along an axis orthogonal to the optical axis.
The SIM houses the two focal instruments, the ACIS and the HRC. Each of these have two principal components: ACIS-I and -S and HRC-I and -S, respectively. The focal-plane instrument layout is shown in Figure 1.2. The SIM moves in both the X-axis (focus) and the Z-axis (instrument and aimpoint (1.6.1) selection). Note that the Y-Axis parallels the dispersion direction of the gratings.
Figure 1.2: Arrangement of the ACIS and the HRC in the focal plane. The view is along the axis of the telescope from the direction of the mirrors. For reference, the two back-illuminated ACIS-S chips are shaded. Numbers indicate positions of chips I0-I3 and S0-S5. SIM motion can be used to place the aimpoint at any point on the vertical dashed line denoting the spacecraft Z axis direction. Nominal detector aimpoints are indicated with small circles.

1.6.1  Aimpoints

Aimpoints are the nominal positions on the detector where the flux from a point source is placed. Note there is a slight (less than 20") distinction between the aimpoint and the on-axis position, which for most practical purposes can be ignored. The aimpoints are discussed in detail in the chapters about each instrument and in Chapters 4 and 5.

1.7  Ground System

The ground system consists of the CXC in Cambridge, MA, the OCC in Burlington, MA, the Engineering Support Center (ESC) at MSFC, and various NASA communications systems including the Deep Space Network operated for NASA by the Jet Propulsion Laboratory (JPL). See Section 2.6.2 for details.

1.8  Orbit

The Chandra orbit is highly elliptical and varies with time. In 2017-Dec the apogee height, after decreasing since mid-2012, reached a minimum of  ∼ 129,000 km; the perigee altitude, after rising since mid-2012, reached a maximum of  ∼ 19,000 km. The orbital eccentricity accordingly reached its minimum of  ∼ 0.68 in 2017-Dec. Since then, the eccentricity has increased, reaching its mission maximum of  ∼ 0.91 in 2023-July, at which time the apogee altitude reached its maximum of  ∼ 148,000 km and the perigee altitude its mission minimum of  ∼ 1060 km. Eccentricity has now begun to decrease again, a trend which will continue until  ∼ 2029. The orbit allows for high observing efficiency as the satellite spends most of the time well above the radiation belts ( ∼ 70%) and long observations (currently  ∼ 180 ksec) are made possible in principle by the orbital period of 63.5h (but see Section 3.3 for limitations due to spacecraft thermal considerations).

1.9  Particle Detector

There is a particle detector mounted near the telescope, called the Electron, Proton, Helium INstrument (EPHIN; see Section 2.5). This detector was used to monitor the local charged particle environment as part of the scheme to protect the focal-plane instruments from particle radiation damage; owing to performance degradation and erratic behavior in recent years, EPHIN is no longer used in this protective function as of 2013-Nov and was depowered in 2018-Sep.
The Co-Principal Investigators of the EPHIN instrument are Drs. Reinhold Muller-Mellin and Hoarst Kunow of the University of Kiel, Germany.

1.10  ACIS

ACIS is composed of two CCD arrays, a 4-chip array, ACIS-I; and a 6-chip array, ACIS-S. The CCDs are flat, but the chips in each array are positioned (tilted) to approximate the relevant focal surface: that of the HRMA for ACIS-I and that of the HETG Rowland circle for ACIS-S. ACIS-I was designed for CCD imaging and spectrometry; ACIS-S can be used both for CCD imaging spectrometry and also for high-resolution spectroscopy in conjunction with the HETG grating.
There are two types of CCD chips. ACIS-I is composed of front-illuminated (FI) CCDs. ACIS-S is composed of 4 FI and 2 back-illuminated (BI) CCDs. The BI S3 chip is at the best focus position and is normally used for ACIS-S imaging observations. ACIS-I is better when wider field (16 arcmin × 16 arcmin) and/or higher energy response is needed; ACIS-S imaging is better when low energy response is preferred and a smaller (8 arcmin × 8 arcmin) field of view is sufficient.
The efficiency of the ACIS instrument has been discovered to be slowly changing with time, most likely as a result of molecular contamination build-up on the optical blocking filter. The BI CCDs response extends to lower energies than the FI CCDs and the energy resolution is mostly independent of position. The low-energy response of the BI CCDs is partially compromised by the contaminant build-up. The FI CCD response is more efficient at higher energies but the energy resolution varies with position due to radiation damage caused by protons reflecting through the telescope during radiation-zone passages in the early part of the mission. Details on ACIS are given in Chapter 6.
The Principal Investigator is Prof. Gordon Garmire of the Huntingdon Institute for X-ray Astrophysics and Space Research.

1.11  HRC

The HRC is composed of two microchannel plate (MCP) imaging detectors: the HRC-I designed for wide-field imaging; and HRC-S designed to serve as a read-out for the LETG. The HRC-I is placed at right angles to the optical axis, tangent to the focal surface. The HRC-S is made of three flat elements, the outer two of which are tilted to approximate the LETG Rowland circle. The HRC detectors have the highest spatial resolution on Chandra, matching the HRMA point spread function most closely. Under certain circumstances, the HRC-S detector also offers the fastest time resolution (16 μs). Details concerning the HRC are in Chapter 7.
The current Instrument Principal Investigator is Dr. Ralph Kraft of the Smithsonian Astrophysical Observatory, who was appointed to this position following the untimely passing in 2015-Aug of the original HRC Principal Investigator, Dr. Stephen Murray of SAO.

1.12  HETG

The HETG , when operated with the ACIS-S, forms the High-Energy Transmission Grating Spectrometer (HETGS) for high resolution spectroscopy . The HETGS achieves resolving power (E/∆E) up to 1000 in the band between 0.4 keV and 10.0 keV. The HETG is composed of two grating assemblies-the High Energy Grating (HEG) and the Medium Energy Grating (MEG)-on a single structure that can, by command, be placed in the optical path just behind the HRMA. The HEG intercepts X-rays from only the two inner mirror shells and the MEG intercepts X-rays from only the two outer mirror shells. The HEG and MEG dispersion directions are offset by 10 deg so the two patterns can be easily distinguished. Details are presented in Chapter 8.
The Instrument Principal Investigator for the HETG is Prof. Claude Canizares of the MIT Kavli Institute for Astrophysics and Space Research.

1.13  LETG

The LETG when operated with the HRC-S, forms the Low Energy Transmission Grating Spectrometer (LETGS). The LETGS provides the highest spectral resolution on Chandra at low (0.08-0.2 keV) energies. The LETG is comprised of a single grating assembly that, on command, can be placed in the optical path behind the HRMA. The LETG grating facets intercept and disperse the flux from all of the HRMA mirror shells. Details are given in Chapter 9.
The LETG was developed at the Laboratory for Space Research in Utrecht, the Netherlands, in collaboration with the Max-Planck-Institut für Extraterrestrische Physik in Garching, Germany. The original Instrument Principal Investigator was Dr. Albert Brinkman (retired) and the current Principal Investigator is Dr. Jelle Kaastra of the Laboratory for Space Research.

1.14  Effective Area Comparisons

The effective areas of the imaging instruments are shown in Figure 1.3. The ACIS curves allow for the expected degradation of the ACIS efficiency caused by molecular contamination predicted for the middle of Cycle 26. A comparison of the effective areas of the grating spectrometers are shown in Figure 1.4. Note that the data from the HEG and MEG are obtained simultaneously. The comparisons shown here are based on the most recent calibration at the time of issuance of this document and are subject to revision. The proposer is urged to read the detailed material in the appropriate chapters and examine the CXC web site (see Section 1.16) for updates.
Figure 1.3: Comparison of the on-axis effective areas for observing a point source (integrated over the PSF) of the HRMA /HRC-I, the HRMA/ACIS(FI), and the HRMA/ACIS(BI) combinations. The ACIS curves show the predicted values for the middle of Cycle 26.
Figure 1.4: Comparison of the total first-order (positive and negative orders combined) effective areas of the LETG and HETG (HEG and MEG are shown separately and summed) spectrometers. HEG and MEG spectra are obtained simultaneously and can sometimes be usefully combined. For a given energy in the range of overlap the resolving power of the HEG is approximately twice that of the MEG, which in turn is approximately twice that of the LETG. The LETG extends to much lower energies than reached by the HETG+ACIS-S combination, especially when used with the HRC-S detector. For full details on spectrometer performance and observation planning see Chapters 8 (HETG) and 9 (LETG).

1.15  Allocation of Observing Time

Observing time is awarded through the NASA proposal and peer review process. The prospective user must submit a proposal in which the observation is described and justified in terms of the expected results. The proposer must also show that the observation is well suited to Chandra and that it is technically feasible. Refer to the Call for Proposals (CfP, for more information.

1.16  How to Get Information and Help

The CXC web page ( provides access to documents, proposal preparation tools, and proposal submission software.