The Chandra Proposers' Observatory Guide

Prepared by:
Chandra X-ray Center
Chandra Project Science, MSFC
Chandra IPI Teams

Version 26.0
December 2023


1  Mission Overview
    1.1  Program Organization
    1.2  Unique Capabilities
    1.3  Observatory Overview
    1.4  Pointing Control and Aspect Determination (PCAD)
    1.5  HRMA
    1.6  Science Instrument Module (SIM)
        1.6.1  Aimpoints
    1.7  Ground System
    1.8  Orbit
    1.9  Particle Detector
    1.10  ACIS
    1.11  HRC
    1.12  HETG
    1.13  LETG
    1.14  Effective Area Comparisons
    1.15  Allocation of Observing Time
    1.16  How to Get Information and Help
2  Spacecraft, Telescope, Operations, & Mission Planning
    2.1  Introduction
    2.2  Spacecraft
    2.3  Telescope System
    2.4  Science Instrument Module (SIM)
        2.4.1  SIM Motions
    2.5  Electron Proton Helium Instrument (EPHIN)
    2.6  Operations
        2.6.1  Launch and On-orbit Verification
        2.6.2  The Ground System
        2.6.3  Commanding
        2.6.4  Telemetry
        2.6.5  SI Science Data
        2.6.6  Event Timing and the Spacecraft Clock
    2.7  Mission Planning
        2.7.1  The Long-Term Schedule
        2.7.2  Selecting Candidates for Short-Term Scheduling
        2.7.3  The Short-Term Scheduling Process
        2.7.4  Coordinated Observations
        2.7.5  The Chandra Cool Targets Program
3  Offset Pointing, Visibility, and other Constraints
    3.1  Introduction
    3.2  Offset Pointing
    3.3  Visibility
        3.3.1  Radiation Belt Passages
        3.3.2  Avoidances
        3.3.3  Thermal Constraints
    3.4  Other Instrument and Observation Considerations
        3.4.1  Instrument Considerations
        3.4.2  User-Imposed Constraints
4  High Resolution Mirror Assembly (HRMA)
    4.1  Introduction
        4.1.1  Description and Physical Configuration
        4.1.2  Sub-Assembly Calibration
        4.1.3  Operating Environment
        4.1.4  Heritage
    4.2  Calibration and Performance
        4.2.1  Calibration and Model
        4.2.2  HRMA Effective Area
        4.2.3  Point-Spread-Function and Encircled Energy Fraction
    4.3  Ghost Images
    4.4  Effects of Aspect and Instrument Uncertainties
    4.5  Optical Axis and Aimpoint
        4.5.1  Definitions
        4.5.2  Aimpoint on Chandra Detectors
        4.5.3  Optical Axis
        4.5.4  Permanent Default Aimpoints (PDA)
        4.5.5  Optical Axis and PDA positions
    4.6  References
5  Pointing Control and Aspect Determination System
    5.1  Introduction
    5.2  Physical Configuration
        5.2.1  ACA
        5.2.2  Fiducial Lights and Fiducial Transfer System
        5.2.3  IRU
        5.2.4  Momentum Control - RWA and MUPS
    5.3  Operating Principles
    5.4  Performance
        5.4.1  Celestial Location Accuracy
        5.4.2  Image Reconstruction
        5.4.3  Absolute Celestial Pointing and Aimpoint Stability
        5.4.4  PCAD 10-Second Pointing Stability
        5.4.5  Relative Astrometric Accuracy
        5.4.6  On-Board Acquisition and Tracking
    5.5  Heritage
    5.6  Calibration
        5.6.1  Pre-launch Calibration
        5.6.2  Orbital Activation and Checkout Calibration
        5.6.3  On-orbit Calibrations
    5.7  Operations
        5.7.1  PCAD Modes
        5.7.2  Operational Constraints
        5.7.3  Output Data
    5.8  Performing an Observation
        5.8.1  Star Acquisition
        5.8.2  Science Pointing Scenarios
        5.8.3  PCAD Capabilities (Advanced)
    5.9  Ground Processing
        5.9.1  Data Products
        5.9.2  Star Catalog
    5.10  References
6  ACIS: Advanced CCD Imaging Spectrometer
    6.1  Introduction & Layout
    6.2  Basic Principles
    6.3  Optical Blocking Filter & Optical Contamination
    6.4  Calibration
    6.5  Quantum Efficiency and Effective Area
        6.5.1  Molecular Contamination of the OBFs
    6.6  Spatial Resolution, PSF, & Encircled Energy
        6.6.1  PSF Anomaly
        6.6.2  ACIS PSF Broadening at Low Energies
    6.7  Energy Resolution
        6.7.1  Correcting the Energy Resolution of the CCDs
        6.7.2  Effect of the Focal Plane Temperature on CCD Energy Resolution
    6.8  Detector Gain
    6.9  Hot Pixels and Columns
    6.10  Cosmic Ray Afterglows
    6.11  Aimpoints
    6.12  Dither
        6.12.1  Gaps Between the CCDs, and Chip Tilts
    6.13  Operating Modes
        6.13.1  Timed Exposure (TE) Mode
        6.13.2  Alternating Exposures
        6.13.3  Continuous Clocking (CC) Mode
    6.14  Bias Maps
    6.15  Event Grades and Telemetry Formats
        6.15.1  Event Grades
        6.15.2  Telemetry Formats
    6.16  Pile-Up
        6.16.1  Other Consequences of Pile-Up
        6.16.2  Pile-Up Estimation
        6.16.3  Reducing Pile-Up
    6.17  On-Orbit Background
        6.17.1  The Non-Celestial X-ray Particle Background
        6.17.2  The Total Background
        6.17.3  Background Variability
        6.17.4  Background in Continuous Clocking Mode
    6.18  Sensitivity
    6.19  Bright Source X-ray Photon Dose Limitations
    6.20  Limitations on the Number of Required and Optional CCDs
    6.21  Observing Planetary and Solar System Objects with ACIS
        6.21.1  The Sun, Earth, and the Moon
        6.21.2  Observations with ACIS-I
        6.21.3  Observations with ACIS-S
    6.22  Observing with ACIS-the Input Parameters
        6.22.1  Required Parameters
        6.22.2  Optional Parameters
        6.22.3  Non-ACIS Parameters Relevant to an Observation with ACIS
        6.22.4  Choosing CC Mode for Bright Source Observation
        6.22.5  Warm ACIS Observations
7  HRC: High Resolution Camera
    7.1  Status of the HRC Detectors
    7.2  Introduction and Instrument Layout
    7.3  Basic Principles
        7.3.1  Aimpoints
        7.3.2  Drift Correction
    7.4  Shutters
    7.5  Dither
    7.6  Spatial Resolution & Encircled Energy
    7.7  Energy Resolution
        7.7.1  Non-Dispersive Energy Resolution
        7.7.2  Dispersive Energy Resolution
    7.8  Gain Variations
    7.9  UV/Ion Shields
    7.10  Quantum Efficiency and Effective Area
    7.11  On-Orbit Background
        7.11.1  HRC-I
        7.11.2  HRC-S
        7.11.3  Temporally Variable Background
        7.11.4  Limiting Sensitivity
    7.12  Instrument Anomalies
    7.13  Calibration
    7.14  Operational Considerations and Constraints
        7.14.1  Total Count Limits
        7.14.2  Count-Rate Limits
    7.15  Observing with HRC - Operating Modes
        7.15.1  Timing Mode
        7.15.2  Edge and Center Blanking
    7.16  References
8  HETG: Chandra High Energy Transmission Grating
    8.1  Instrument Overview
        8.1.1  Examples of Observations with the HETGS
        8.1.2  Scientific Objectives and Grating Heritage
        8.1.3  HETGS Operating Principles
        8.1.4  HETG Physical Configuration
    8.2  Instrument Characteristics
        8.2.1  HETGS Effective Area
        8.2.2  HETGS Line Response Function
        8.2.3  Background
        8.2.4  Absolute Wavelength
        8.2.5  Comparing 0th and 1st Orders
    8.3  Calibration Status
    8.4  HETG Operations
        8.4.1  Flight Events and Anomalies
        8.4.2  Operational Constraints
        8.4.3  Output Data
        8.4.4  Performance Monitoring, Health and Safety
        8.4.5  Thermal Response Time
        8.4.6  Observation Frequency/Duty Cycle
        8.4.7  Radiation Considerations
        8.4.8  Operating with a Warm ACIS-S Array
    8.5  Observation Planning
        8.5.1  General Considerations
        8.5.2  Choice of Focal-Plane Detector
        8.5.3  Complications from Multiple Sources
        8.5.4  Extended Sources and Spatial-Spectral Effects
        8.5.5  Optimizing Detection of Isolated Emission Lines: Choice of Spectrometer
    8.6  Simulations with MARX
    8.7  References
9  LETG: Low Energy Transmission Grating
    9.1  Instrument Description
        9.1.1  Scientific Objectives
        9.1.2  Heritage
        9.1.3  Operating Principles
        9.1.4  Physical Configuration
    9.2  Calibration
        9.2.1  Pre-Launch Calibration
        9.2.2  In Flight Calibration
    9.3  LETGS Performance
        9.3.1  Usage
        9.3.2  Wavelength Coverage and Dispersion Relation
        9.3.3  Resolving Power
        9.3.4  Grating Efficiency
        9.3.5  Effective Area
        9.3.6  Background
        9.3.7  Example Data and Support Structure Diffraction
    9.4  Observation Planning
        9.4.1  Detector Choices
        9.4.2  Other Focal-Plane Detector Considerations
        9.4.3  General Considerations
    9.5  Technical Feasibility
        9.5.1  Simple Calculation of Exposure Times and Signal-to-Noise Ratio for Line and Continuum Sources
    9.6  References
I   Appendices
A  Contact Information
    A.1  Contact Information
    A.2  CDO Staff
B  Acronym List

List of Figures

    1.1  The Chandra Observatory
    1.2  Arrangement of the ACIS and the HRC detectors in the focal plane
    1.3  Point source on-axis effective areas for HRMA/ACIS and HRMA/HRC
    1.4  Effective areas of the grating spectrometers
    2.1  A schematic of the Science Instrument Module
    3.1  Effects of SIM motions and pointing offsets
    3.2  Example of offset pointing with HRC-I
    3.3  Example of offset pointing with HRC-S
    3.4  Cycle 26 Chandra visibility contours
    3.5  Solar pitch sensitivity of spacecraft components
    4.1  The 4 nested HRMA mirror pairs and associated structures
    4.2  Residuals near the Ir M edges
    4.3  The HRMA effective area as measured during the ground calibration
    4.4  The HRMA effective area versus X-ray energy
    4.5  The HRMA effective area versus off-axis angle
    4.6  The fractional encircled energy as a function of angular radius
    4.7  The radii of encircled energy fraction as functions of X-ray energy
    4.8  Simulated HRMA/HRC-I images of on-axis point sources
    4.9  Normalized radial profiles of the Her X-1 scattering wings
    4.10  Spectral hardening of the diffuse mirror scattering halo
    4.11  HRMA Focal Surface
    4.12  Encircled energy radii versus off-axis angle
    4.13  Azimuthal dependence of the HRMA/ACIS-I encircled energy
    4.14  Simulated HRMA 1.49 keV images
    4.15  Simulated HRMA 6.4 keV images
    4.16  A simulated 1.49 keV point source at an off-axis
    4.17  Evolution of the HRMA 90% ECF
    4.18  The PSF anomaly
    4.19  Ratio of counts collected from different segments around the PSF
    4.20  As Figure 4.19, for on-axis HRC-I observations of AR Lac
    4.21  Simulated images of off-axis sources
    4.22  The HRMA/HRC-I on-axis fractional encircled energy
    4.23  The HRMA/HRC-S on-axis fractional encircled energy
    4.24  Fractional encircled energy as a function of angular radius
    4.25  Layout of Chandra detectors
    4.26  Optical axis positions since launch
    4.27  Optical axis positions since launch with error ellipses
    4.28  ACIS-I3 Full-Frame PDA & Dither Box
    4.29  ACIS-S3 Full-Frame PDA & Dither Box
    4.30  ACIS-I dither box on ≤ 128 pixel subarray
    4.31  ACIS-S3 dither box on ≤ 128 pixel subarray
    4.32  HRC-I aimpoint
    4.33  HRC-S aimpoint
    5.1  Aspect camera assembly
    5.2  Spectral response of the ACA CCD
    5.3  Schematic of the Fiducial Transfer System
    5.4  Cumulative histograms of source location accuracy for each SI.
    5.5  Difference between observed and planned aimpoint
    5.6  Intra-observation aimpoint drift
    5.7  Chandra relative astrometric accuracy
    5.8  ACA CCD dark current distribution
    6.1  A schematic of the ACIS focal plane
    6.2  A schematic of the ACIS focal plane: ACIS-I and ACIS-S arrays
    6.3  Quantum efficiency of the ACIS CCDs
    6.4  HRMA/ACIS effective area versus energy (log scale)
    6.5  HRMA/ACIS effective area versus energy (linear scale)
    6.6  Vignetting as a function of energy and off-axis angle
    6.7  Optical depth of the contaminant at 0.66 keV
    6.8  Evolution of ACIS-S spectrum of Abell 1795 from 2000 to 2023
    6.9  Optical depth difference between S3 chip center and edge
    6.10  ACIS filter accumulation rate for C, O, and F
    6.11  Ratios of contaminant O and F deposition rates to C
    6.12  Measured ACIS on-axis encircled energy versus radius
    6.13  The PSF anomaly illustrated with an ACIS-S observation of NGC 6397
    6.14  The ACIS pre-launch energy resolution versus energy
    6.15  Effect of the CXC CTI-corrector on energy resolution
    6.16  Energy resolution variation with temperature and position
    6.17  ACIS-I aimpoint 50% encircled energy contours
    6.18  ACIS-S aimpoint 50% encircled energy contours
    6.19  Examples of Subarrays
    6.20  An Example of a Trailed Image
    6.21  Schematic ACIS Grade Calculator
    6.22  Pile-Up Effects at a Single Energy
    6.23  Effect of pile-up on the radial distribution of the PSF
    6.24  Pile-Up Fraction versus Rate
    6.25  MARX simulations of the effects of pile-up on spectral shape
    6.26  FI and BI chips after a hit by a cosmic ray event
    6.27  Energy spectra of the charged particle ACIS background
    6.28  Fraction of ACIS background events as a function of grade
    6.29  ACIS-S3 spectrum of the non-X-ray background
    6.30  Conditional total background rates with time for ACIS chips
    6.31  ACIS background counting rate variability
    6.32  Cumulative probability of background variability
    6.33  Spectra of background flares
    6.34  Pitch sensitivity of spacecraft components
    6.35  PSMC temperature as a function of spacecraft pitch angle
    6.36  DPA temperature as a function of pitch angle
    6.37  DEA temperature versus pitch angle
    6.38  First ACIS-I Imaging recommended chip set
    6.39  Second ACIS-I Imaging recommended chip set
    6.40  Third ACIS-I Imaging recommended chip set
    6.41  First ACIS-S Imaging recommended chip set
    6.42  Second ACIS-S Imaging recommended chip set
    6.43  Third ACIS-S Imaging recommended chip set
    6.44  Fourth ACIS-S Imaging recommended chip set
    6.45  Fifth ACIS-S Imaging recommended chip set
    6.46  Sixth ACIS-S Imaging recommended chip set
    6.47  ACIS-S spectroscopy recommended chip set
    7.1  A schematic of the HRC focal-plane geometry
    7.2  A schematic cross-section of the HRC-S MCP array
    7.3  A schematic of the HRC Microchannel-Plate detector
    7.4  Schematic representation of the HRC position determination
    7.5  HRMA/HRC-I fractional encircled energy versus radius
    7.6  PSF anomaly illustrated with HRC-I observations of Capella
    7.7  HRMA/HRC-I Encircled energy as a function of source off-axis angle
    7.8  Monitoring the gain and gain correction across the HRC-I detector
    7.9  Monitoring the gain and gain correction across the HRC-S detector
    7.10  HRC-I and HRC-S UV/Ion shield effective areas
    7.11  The predicted HRC-I and HRC-S effective area
    7.12  HRC-I background variability
    7.13  HRC-I background intensity variability
    7.14  HRC-I background spectrum variability
    7.15  PI filtering to reduce HRC-I background
    7.16  As Figure 7.15, for sources with thermal spectra
    7.17  As Figure 7.15, for sources with blackbody spectra
    8.1  HETGS observation of Capella, ObsID 1318.
    8.2  HETGS Capella spectrum, MEG m=−1
    8.3  Schematic layout of the HETGS
    8.4  The Rowland geometry
    8.5  The HETG support structure (HESS)
    8.6  Cross-sections of the MEG and HEG membranes
    8.7  The HETGS HEG effective area
    8.8  The HETGS HEG effective area: linear scale
    8.9  The HETGS MEG effective area
    8.10  The HETGS MEG effective area: linear scale
    8.11  HRMA-HETG-ACIS-S combination first-order effective area
    8.12  Average residuals of fits to HETGS observations of blazars
    8.13  HEG and MEG efficiencies as a function of energy
    8.14  HEG and MEG "Banana Plots"
    8.15  HETGS pile-up and higher-order events
    8.16  HEG Line Response Functions
    8.17  MEG Line Response Functions
    8.18  HETGS zero order and Frame transfer Streak (Trailed Image)
    8.19  HEG and MEG resolving power
    8.20  MEG Cross dispersion profiles
    8.21  HEG Cross dispersion profiles
    8.22  HETGS Enclosed power in rectangular apertures
    8.23  HETGS spectral resolution: extended sources
    8.24  HETG grating spectral resolution: off-axis
    8.25  HETGS background count spectra
    8.26  Expected HETGS spectral resolution with HRC-I
    8.27  Observed HETGS spectral resolution with HRC-I
    8.28  Idealized sketch of a `collision' between two sources
    8.29  Simulated spectral contamination caused by a second source
    8.30  HETGS spatial-spectral effect example
    9.1  The LETG Grating Element Support Structure.
    9.2  A detail of the LETG Grating Element Support Structure.
    9.3  Two grating modules in the LETG GESS.
    9.4  A schematic picture of the LETG facet structure.
    9.5  The HRC-S array elements and the Rowland circle.
    9.6  LETGS effective areas and the choice of detector and Y-offset
    9.7  LETG spectral resolving power.
    9.8  Observed LETG zeroth-order LRF
    9.9  LETGS Line Response Function
    9.10  LETGS zeroth order profile goodness of fit versus β
    9.11  LETG spectral resolving power for extended sources.
    9.12  LETG spectral resolving power for off-axis sources.
    9.13  LETG grating efficiency.
    9.14  LETG+HRC-S Cross-dispersion and Extraction window
    9.15  LETG+ACIS-S Spectral Extraction Efficiency
    9.16  LETGS zeroth-order effective area
    9.17  LETGS 1st-order effective area.
    9.18  LETG/HRC-S effective area for higher orders.
    9.19  LETG/HRC-S/LESF effective area for higher orders.
    9.20  LETG/ACIS-S effective area for higher orders.
    9.21  Solar cycle and HRC-S background
    9.22  LETG+HRC-S background
    9.23  HRC-S detector image of LETGS observation of Capella
    9.24  Detail of LETG/HRC-S Capella image
    9.25  HRC-S/LETG image of Capella positive order dispersion
    9.26  Extracted Capella spectrum
    9.27  Sirius AB, zeroth order image
    9.28  ISM Transmittance in LETGS bandpass
    9.29  Observed and Simulated 1st order spectrum of an AGN
    9.30  MARX simulation of spectra showing the effect of source extent

List of Tables

    2.1  Spacecraft Parameters
    4.1  Chandra HRMA Characteristics
    4.2  HRMA Encircled Energy Performance
    4.3  Optical Axis & PDA Locations
    4.4  Dimensions of Pointing Error & Total Dither Boxes
    5.1  Aspect System Requirements and Performance
    5.2  Star Acquisition and Tracking Success
    5.3  PCAD modes
    5.4  Default dither parameters
    5.5  Aspect pipeline data products
    6.1  ACIS CCD Information
    6.2  Table of ACIS Characteristics
    6.3  Nominal Optical Blocking Filter Composition and Thicknesses
    6.4  Aimpoint Spectral Resolution: 2012-Apr to 2012-Jun
    6.5  CCD Frame Time (sec) for Standard Subarrays
    6.6  ACIS flight grades and ASCA Grades
    6.7  Telemetry Saturation Limits
    6.8  ASCA-Grade Distributions at 1.5keV for Different incident fluxes
    6.9  Approximate on-orbit background counting rates
    6.10  Total quiescent background rates
    7.1  HRC Parameters
    7.2  HRC-I sensitivity
    7.3  HRC-S sensitivity
    7.4  Current and past HRC-I calibration targets
    7.5  Current and past HRC-S calibration targets
    8.1  HETG(S) Parameters
    8.2  Table of HETGS Gap Locations
    8.3  Comparison of HETGS 0th and 1st Orders
    9.1  LETGS Parameters
    9.2  Routine LETGS Calibration Monitoring Observations
    9.3  LETG Position-Dependent Spectral Coverage
    9.4  Instrumental Absorption Edges

Last modified:12/13/23
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