The Chandra Proposers' Observatory Guide

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

Version 25.0
December 2022


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
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, the 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  Image created with ObsVis showing ACIS field-of-view overlaid on an optical image.
    3.2  Example of offset pointing with HRC-I
    3.3  Example of offset pointing with HRC-S
    3.4  Chandra visibility showing contours of fractional visibility averaged over the 12-month interval of Cycle 25
    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, calculated for an on-axis point-source
    4.7  The radii of encircled energy fraction as functions of X-ray energy
    4.8  Simulated HRMA/HRC-I images of on-axis mono-energetic point sources with aspect blurring
    4.9  Normalized radial profiles (units arcsec−2) of the Her X-1 scattering wings.
    4.10  Spectral hardening of the diffuse mirror scattering halo
    4.11  HRMA Focal Surface
    4.12  HRMA/HRC-I encircled energy average radii for circles enclosing 50% and 90% of the power at 1.49 and 6.40 keV as a function of off-axis angle.
    4.13  Azimuthal dependence of the HRMA/ACIS-I encircled energy
    4.14  Simulated HRMA images
    4.15  Simulated HRMA 6.4 keV images
    4.16  A simulated 1.49 keV point source at an off-axis
    4.17  The PSF anomaly
    4.18  Ratio of counts collected from different segments around the PSF
    4.19  As Figure 4.18, for on-axis HRC-I observations of AR Lac.
    4.20  Simulated images of off-axis sources.
    4.21  The HRMA/HRC-I on-axis fractional encircled energy
    4.22  The HRMA/HRC-S on-axis fractional encircled energy
    4.23  Fractional encircled energy as a function of angular radius.
    4.24  SIM
    4.25  Optical Axis
    4.26  Optical Axis
    4.27  ACIS-I aimpoint
    4.28  ACIS-S aimpoint
    4.29  ACIS-I dither box on ≤128 pixel subarray
    4.30  ACIS-S3 dither box on ≤128 pixel subarray
    4.31  HRC-I aimpoint
    4.32  HRC-S aimpoint
    5.1  Aspect camera assembly
    5.2  Spectral response of the ACA CCD
    5.3  Fiducial Transfer System
    5.4  Cumulative histogram of celestial accuracy for Chandra X-ray source locations 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  Differential histogram of dark current distribution for the ACA CCD in 1999-Aug and 2022-Oct
    6.1  A schematic of the ACIS focal plane
    6.2  A scematic of the ACIS focal plane: ACIS-I and ACIS-S arrays.
    6.3  The quantum efficiency, convolved with the transmission of the appropriate optical blocking filter
    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 measured at 0.66 keV from observations of the galaxy cluster Abell 1795. Red squares denote observations with the target on ACIS-S and blue disks denote observations with the target on ACIS-I. The solid curve shows the optical depth of the contaminant as modeled in the most recent contamination model.
    6.8  ACIS-S spectrum of the galaxy cluster Abell 1795 at six different epochs ranging from 2000 to 2021. The decreasing count rates in the spectrum of a stable calibration source demonstrate the loss of effective area due to the build-up of the contaminant on the ACIS OBF.
    6.9  The difference between the optical depth at 0.66 keV measured near the center of the ACIS-S chip S3 and that measured near the readout at the edge of the same chip is shown against time. Black circles are measurements made using the ECS data and red squares are made from observations of the galaxy cluster Abell 1795. The solid curve shows the optical depth of the contaminant as modeled in the most recent contamination model.
    6.10  The optical depths of C, O, and F were measured from LETG/ACIS-S blazar spectra within 120-240 rows of the top or bottom of ACIS-S. The oxygen content has varied significantly over the mission and was consistent with zero from 2003 to 2009, while the fluorine fraction has been relatively stable.
    6.11  The ratios of the deposition rate in oxygen atoms and fluorine atoms per year to that for carbon over the Chandra mission as measured by LETG/ACIS-S observations of blazars.
    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  An example of the application of the CXC CTI-corrector in two energy bands.
    6.16  Energy resolution (FWHM in eV) versus row number (CHIPY) for Al-Kα and Mn-Kβ lines for both FI and BI CCDs for several different focal plane temperatures. The error bars are large at the warmest temperatures because of the relative lack of calibration data at these temperatures compared to lower temperatures.
    6.17  Contours of constant 50% encircled energy at 1.49 keV around the ACIS-I aimpoint
    6.18  Contours of constant 50% encircled energy at 1.49 keV around the ACIS-S aimpoint
    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  The Radial Distribution of the Core of the PSF for Different Incident Fluxes
    6.24  Pile-Up Fraction versus Rate
    6.25  MARX simulations of the effects of pile-up on spectral shape
    6.26  Enlarged view of an area of the FI chip I3 hit by a cosmic ray event
    6.27  Energy spectra of the charged particle ACIS background with ACIS in the stowed position
    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  Total background rates under certain conditions as a function of time for a FI and a BI chip
    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  ACIS-I imaging, nominal aimpoint (4 Required CCDs, 1 Optional CCD)
    6.39  ACIS-I imaging, nominal aimpoint (4 Required CCDs, 1 Optional CCD)
    6.40  ACIS-I imaging, nominal aimpoint (4 Required CCDs, 2 Optional CCDs)
    6.41  ACIS-S imaging, nominal aimpoint (4 Required CCDs, 1 Optional CCD)
    6.42  ACIS-S imaging, nominal aimpoint (4 Required CCDs, 1 Optional CCD)
    6.43  ACIS-S imaging, nominal aimpoint (4 Required CCDs, 2 Optional CCDs)
    6.44  ACIS-S spectroscopy, nominal aimpoint (4 Required CCDs, 2 Optional CCDs)
    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  Fractional encircled energy as a function of radius for an on-axis point source observed with the HRMA/HRC-I
    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  The HRC-I and the center section of the HRC-S UV/Ion shield effective area as a function of wavelength
    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  The average residuals to fits to the HETGS data for BL Lac objects using logarithmic parabola continuum models. For most of the HETGS range, the systematic deviations are not significant or are less than 3%. See Marshall (2012) for details.
    8.13  HEG (upper panel) and MEG (lower panel)
    8.14  HEG (upper panel) and MEG (lower panel) "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  A `collision' between two sources
    8.29  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  The 1st order spectrum for an 80 ksec observation of the AGN NGC5548.
    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:12/14/22
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