Chandra Calibration Status Summary

as of June 2026

Detector/Grating	Calibration Product	Average Uncertainty	Comments	Additional Information
ACIS	Dector Gain	0.3% before 2022 0.5% after 2022	In general, the ACIS detector gain during a given observation depends on: 1) the date of the observation, 2) the focal plane temperature during the observation, and 3) the background rate. Early in the mission, the CCDs suffered radiation damage which produced an increase in the charge transfer inefficiency (CTI). This was addressed by developing a CTI correction algorithm for the ACIS CCDs. Continued radiation damage over the course of the mission has produced a slow increase in CTI and a subsequent decrease in gain. The CTI also increases with increasing focal plane temperature. In addition, background events act as sacrificial charge, filling in the radiation-induced traps, and reducing the CTI. The Chandra Calibration Team corrects for these effects by periodically releasing a time-dependent gain correction file, which accounts for the CTI and the ambient background rate during a given time period. Prior to 2022 a temperature-independent CTI correction was applied to all ACIS data. Due to the increasing focal plane temperature during observations, the Calibration Team released a set of temperature-dependent CTI corrections in 2022. The time-dependence of the ACIS gain has been monitored over the course of the mission with the ACIS external calibration source (ECS), which consists of three ⁵⁵Fe sources that produce L-shell emission lines of Fe (0.71 keV) and Mn (0.64 keV) and K-shell emission lines of Al (1.49 keV), Ti (4.5 keV), and Mn (5.9 keV). ACIS is exposed to the ECS whenever it is in the stowed position (i.e., when Chandra passes through the radiation belts). Due to the build-up of contamination on the ACIS filters, the L-shell lines of Fe and Mn are no longer useful for calibration. Between launch and 2016, updated ACIS gain tables were released quarterly by co-adding three months of ECS data. The half-life of ⁵⁵Fe is only 2.7 years, so it became necessary in 2016 to co-add six months of ECS data to achieve sufficient photon statistics to calibrate the ACIS gain. By 2022, it became necessary to supplement the ECS data with observations of astronomical targets. Since 2022, the ACIS calibration plan includes annual observations of the supernova remnant Cas A and the Perseus cluster of galaxies on the six imaging chips of ACIS (ACIS-I, S2 and S3). The remaining CCDs (ACIS-S0, S1, S4, and S5) are primarily used with the gratings when the photon energies are computed from the dispersion relation and not the detector gain. The 5.9 keV Mn emission line in the ECS and the 6.6 keV Fe line from the Perseus cluster of galaxies are now used to calibrate the high-energy gain. The Al and Ti lines in the ECS along with the 1.85 keV Si emission line in Cas A are now used to calibrate the mid-energy gain. Caveats: The gain on the BI chips has been more stable over the course of the mission compared to the FI chips due to the smaller relative increase in CTI. Throughout most of the Chandra mission (prior to 2022), the uncertainty in the ACIS detector gain remained less than 0.3%, even for warmer focal plane temperatures. For the FI chips, the uncertainty in the detector gain can approach 0.5% for observations taken after 2022, especially at large chipy and warm focal plane temperatures. Currently, high signal to noise observations are only carried out at focal plane temperatures less than -111C due to the greater uncertainty in the detector gain at warmer temperatures.	Web Pages Chandra Instruments & Calibration Page ACIS Chapter of the POG Papers/Memos Memo describing the time-dependent ACIS gain correction Mitigating Charge Transfer Inefficiency in the Chandra X-Ray Observatory Advanced CCD Imaging Spectrometer Temperature Dependence of CTI in Chandra X-Ray CCDs Presentations/Plots ACIS gain vs. time ACIS gain vs. FP temp
ACIS	Spectral Resolution	35 eV @ 1.5 keV 80 eV @ 6.0 keV	The spectral resolution of the ACIS CCDs was initially calibrated with ground-based data. The early radiation damage, and subsequent increase in CTI, produced a broadening of the ECS lines, with the FWHM of the emission lines increasing with chipy (i.e., distance from the read-out). This was partially accounted for by the CTI correction algorithm developed early in the mission. The spectral resolution of the CCDs is also sensitive to the focal plane temperature, with the FI CCDs being more sensitive to temperature changes than the BI CCDs. As the average focal plane temperature continued to increase during the mission, it became necessary to develop a set of temperature-dependent rmfs. In 2024, a set of temperature-dependent rmfs was released for the primary FI CCDs used for imaging (ACIS-I and S2). The remaining CCDs are primarily used with the gratings when the photon energies are computed from the dispersion relation and not the detector gain. Work continues on developing a set of temperature-dependent rmfs for the less temperature sensitive S3 CCD. The quoted uncertainties given below for the spectral resolution of ACIS were determined by fitting a large sample of ECS data over a broad range of focal plane temperatures. Each of the three primary ECS lines (Al-Ka, Ti-Ka, and Mn-Ka) was fit in many regions on a CCD with a Gaussian profile and the appropriate CALDB rmf for the given location and focal plane temperature. The rms scatter in the best fit FWHM of the gaussian model across a CCD is then taken to be the uncertainty in the spectral resolution of the CCD at a given focal plane temperature. Caveats: In general, the uncertainties in the ACIS spectral resolution have all of the same dependencies as the detector gain listed above. Based on the latest set of temperature-dependent rmfs in the CALDB, the uncertainty in the FWHM is about 35 eV at 1.5 keV for the FI chips, and is fairly independent of focal plane temperature. Based on the latest set of temperature-dependent rmfs, the uncertainty in the FWHM at 1.5 keV for the FI chips varies from about 70 eV at a focal plane temperature of -119 C, to about 90 eV at a focal plane temperature of -111 C.	Web Pages Chandra Instruments & Calibration Page ACIS Chapter of the POG Presentations/Plots ACIS FWHM gain vs. FP Temp
ACIS	Effective Area	5%	The ACIS effective area is a convolution of the HRMA effective area, the transmission of the optical blocking filters, and the ACIS QE. Over the course of the mission, out-gassed molecular material has continued to accumulate on the cold ACIS filters. This is the primary mechanism that has lead to a changing ACIS effective area over time. The optical depth of the contaminant on the ACIS filters has been monitored with periodic imaging observations of the rich cluster of galaxies Abell 1795 and the supernova remnant E0102-72, and grating observations of the blazar Mkn 421. To account for the build-up of contamination on the ACIS filters, the Chandra Calibration Team has released updates to the ACIS contamination model on approximately a yearly timescale. The contaminant primarily absorbs soft X-Ray photons, i.e., below ~2 keV. Above ~2 keV, the contaminate is mostly transparent and high energy effective area of ACIS has remained very stable. The uncertainty in the ACIS relative effective area is based on the rms scatter in broad band fluxes for a sample of steady calibration sources (e.g., supernova remnants and clusters of galaxies) that have been periodically observed with ACIS over the course of the mission. The uncertainty in the absolute ACIS effective area is a more difficult problem and can best be accessed through cross-calibration studies with other X-ray missions, such as those conducted by the International Astronomical Consortium for High Energy Astrophysics (IACHEC). Caveats: The attached plots show the scatter in flux measurements within several energy bands for the Abell 1795 observations. These observations were taken with both ACIS-I and ACIS-S, at many different locations on the detectors, and at many times over the course of the mission. The observed scatter in these plots is the best estimate of the systematic uncertainty in the relative ACIS effective area. The quoted uncertainty of 5% is a conservative estimate over 0.5-7.0 keV bandpass. The attached plots show that there is some variation with energy band. Both FI and BI chips suffer from a steady loss in QE at all energies due to continued radiation damage and the subsequent increase in CTI. The average QE loss has been about 0.2% per year for the BI chips and about 0.1% per year for the BI chips. This reduction in detector QE is accounted for in default CIAO processing and leads to a negligible increase in the uncertainty in the ACIS effective area. All Chandra observations are consistent with no degradation in the effective area of the High Resolution Mirror Assembly (HRMA) over the course of the mission.	Papers/Memos The evolution of the ACIS contamination layer on the Chandra X-ray Observatory Concordance: In-Fight Calibration of X-Ray Telescopes without Absolute References XMM-Newton and Chandra cross-calibration using HIFLUGCS galaxy clusters Cross Spectral Calibration of Suzaku, XMM-Newton, and Chandra with PKS 2155-304 Cross-calibration of the X-ray instruments onboard the Chandra, INTEGRAL, RXTE, Suzaku, Swift, and XMM-Newton using G21.5-0.9 SNR 1E 0102.2-7219 as an X-ray Calibration Standard Presentations/Plots Abell 1795 fluxes
HRC	Effective Area	3%	The HRC effective area is a convolution of the HRMA effective area, the transmission of the UV Ion shield, and the HRC. As mentioned above the HRMA effective area has been stable over the course of the mission. Periodic observations of Vega have shown that the transmission of the UV Ion shield has also been very stable. The QE of both HRC detectors has been monitored throughout the mission with periodic gratings and imaging observations of steady sources (primarily the white dwarf HZ43 for low energy calibration and the supernova remnant G21.5-09 for high energy calibration). The QE of both HRC detectors has changed over the course of the mission due to a steady decline in the gain of both HRC detectors. As the gain decreases, more events fall below the Low Level Discriminator (LLD) and these events are not telemetered to the ground. This QE loss due to the decreasing detector has a wavelength dependence, with the longer wavelength photons affected the most. The HRC-S QE has been slowly declining since launch, while the HRC-I QE was stable until about 2015 and then began to decline. Due to the decreasing detector gain and subsequent lower count rates, the High Voltage of the detectors has been raised several times to increase the observed count rates of the steady sources used for HRC QE calibration. For the HRC-S, the High Voltage was increased in 2012, 2021, and 2024. For the HRC-I, the high voltage was increased in 2021 and 2024. The latest increase in High Voltage restored the HRC detectors to their sensitivities approximately two years prior. Due to the steady decline in QE of the HRC detectors, QE tables for both detectors have been released to the public via the CALDB on roughly an annual timescale. In addition, new QE tables for the HRC detectors are released anytime the High Voltage is raised. The rms scatter in the computed fluxes of HRC-I and HRC-S observations of HZ43 and G21.5-09 taken over the course of the mission is approximately 3%. Caveats: The HRC-S QE loss due to the decreasing detector has a wavelength dependence, with the longer wavelength photons affected more significantly.	Web Pages Chandra Instruments & Calibration Page HRC Chapter of the POG Papers/Memos An Update to the HRC-I MCP QE Model In-flight Performance and Calibration of the Chandra High Resolution Camera Adjustments to the HRC-S High Voltage QE loss attributable to gain drop on HRC Presentations/Plots HRC observations of HZ43 HRC observations of G21.5-09
LETG/HRC-S	Absolute Wavelength Scale	0.013 Å	The LETG/HRC-S dispersion relation depends on how well the HRC-S follows the Roland Torus, the gaps between the three HRC-S plates, and the HRC-S de-gap map. The LETG/HRC-S dispersion relation has been very stable, but has been fine tuned with updates to the gaps between the three HRC-S plates, and updates to the HRC-S de-gap map in 2012 and 2018. The coronal line source Capella has been observed with the LETG/HRC-S at periodic intervals over the course of the mission to monitor the dispersion relation. The uncertainty in the absolute wavelength of dispersed LETG/HRC-S spectra is computed from the rms scatter between the lab and observed wavelengths for emission lines detected in LETG/HRC-S observations of Capella. The quoted uncertainty is derived from emission lines detected over the full wavelength range of LETG/HRC-S spectra. Caveats: There are some trends in the accuracy of the absolute wavelength calibration with distance along the dispersion axis. The rms scatter between lab wavelengths and derived wavelengths is slightly less on the central HRC-S plate (λ < 60 Å) compared to the outer HRC-S plates (see attached figure).	Web Pages Chandra Instruments & Calibration Page LETG Chapter of the POG Papers/Memos The Dispersion Relation of the LETG+HRC-S HRC-S/LETG: Degap and Wavelength Corrections Degap Update for HRC-S2 Characterizing Non-linearities in the Chandra LETG+HRC-S Dispersion Relation Presentations/Plots LETG Absolute Photon Energies
LETG/HRC-S	Effective Area	10%	The LETG/HRC-S effective area is a convolution of the HRMA effective area, the transmission efficiency of the LETG, the transmission of the UV ION shield of the HRC-S, and the QE of the HRC-S. Only the HRC-S QE has varied with time (see the HRC section above). All other components have been very stable over the course of the Chandra mission. While the transmission efficiency of the LETG has been stable, the transmission efficiency of the gratings was updated to improve internal cross-calibration in 2004 and 2011. To maintain a stable LETG/HRC-S effective area calibration, the CXC calibration team carries out yearly LETG/HRC-S observations of the white dwarf HZ 43, a stable soft X-ray point source. Since all hard X-ray point sources are variable, a set of interleaved observations were completed each year that cycle through all the grating/detector combinations while Chandra observes the same target over most of the mission. In most cases, the target was the blazar Mkn 421. These interleaved observations were discontinued recently due to the imposed limitations with HRC-S observations. The interleaved observations were used to cross-calibrate between the different grating/detector combinations. Over a broad wavelength range, the uncertainty in the relative effective areas of all grating/detector combinations is approximately 10%.	Web Pages Chandra Instruments & Calibration Page LETG Chapter of the POG Papers/Memos Cross Spectral Calibration of Suzaku, XMM-Newton, and Chandra with PKS 2155-304/a> Presentations/Plots XMM-Newton/Chandra Blazar Flux Comparison 2015 CUC Calibration Report Interleaved Gratings Observations
HETG/ACIS-S	Absolute Wavelength Scale	0.011 Å MEG 0.006 Å HEG	The HETG/ACIS-S dispersion relations (both the HEG and MEG) have been very stable over the course of the Chandra mission. As with the LETG, the HETG dispersion relations have been monitor with periodic observations of the emission-line source Capella. The uncertainty in the absolute wavelength of dispersed HETG/ACIS-S spectra is computed from the rms scatter between the lab and observed wavelengths for emission lines detected in HETG/ACIS-S observations of Capella.	Web Pages Chandra Instruments & Calibration Page HETG Chapter of the POG Papers/Memos In-Flight Calibration of the Chandra High Energy Transmission Grating Spectrometer Presentations/Plots Measuring the Accuracy of Chandra/HETGS Wavelength Scale with Capella Data
HETG/ACIS-S	Effective Area	10%	The HETG/ACIS-S effective area is a convolution of the HRMA effective area, the transmission efficiency of the HETG (both the MEG and HEG), the transmission of the optical blocking filter on ACIS-S and the QE of ACIS-S. The transmission efficiencies of the HEG and MEG have been very stable, but adjustments to the HEG and MEG transmission efficiencies were released in 2004, 2005, and 2011 to improve cross-calibration between the different detector/gratings combinations. Changes to the transmission of the ACIS optical blocking filters due to the build-up of molecular contamination have been accounted for through the approximately yearly updates to the ACIS contamination model. All grating/detector effective areas, including the HETG/ACIS-S, were monitored with a set of yearly interleaved observations of the blazar Mkn 421. These observations are discussed in the LETG section above. Over a broad wavelength range, the uncertainty in the relative effective areas of all grating/detector combinations is less than 10%.	Web Pages Chandra Instruments & Calibration Page HETG Chapter of the POG Papers/Memos Updating the Chandra HETGS Efficiencies using In-Orbit Observations Cross Spectral Calibration of Suzaku, XMM-Newton, and Chandra with PKS 2155-304/a> Improving the Relative Accuracy of the HETGS Eﬀective Area Calibration Properties of the CHANDRA HETG 0th Order in TE-Mode Presentations/Plots XMM-Newton/Chandra Blazar Flux Comparison 2015 CUC Calibration Report Interleaved Gratings Observations

Plots and Figures

ACIS Detector Gain vs. Time

ACIS-I3 ACIS-S3

Caption: Ratio of the computed photon energies for the ECS Mn-Ka emission line relative to the lab energy over the course of the Chandra mission for each of the four nodes on the I3 and S3 CCDs. The data are color codded by the chipy position of the events (i.e., from the read-out to the top of the CCD). A greater scatter is evident in the most recent data. The dotted lines show an uncertainty of ± 0.5%.

ACIS Gain vs. Focal Plane Temperature

ACIS-I3 @ Al-Ka ACIS-I3 @ Mn-Ka

ACIS-S3 @ Al-Ka ACIS-S3 @ Mn-Ka

Caption: Ratio of the computed photon energies for the ECS Al-Ka and Mn-Ka emission lines relative to the lab energies vs. chipx along the top 1/4 of the I3 and S3 CCDs. The curves are color coded by the focal plane temperature. The horizontal lines show discrepancies of ± 0.5%.

ACIS FWHM vs. Focal Plane Temperature

FP Temp = -119:117

ACIS-I3 @ Al-Ka ACIS-I3 @ Ti-Ka ACIS-I3 @ Mn-Ka

Caption: The uncertainty in the FWHM for the three ECS emission lines (Al-Ka, Ti-Ka, Mn-Ka) across the I3 CCD for focal plane temperatures between -119 and -117 C. The color scale is the uncertainty in the FWHM divided by the CALDB predicted FWHM.

Uncertainties in the I3 FWHM

Caption: Uncertainty in the FWHM on I3 for focal plane temperatures between -119 and -117 C.

FP Temp = -117:115

ACIS-I3 @ Al-Ka ACIS-I3 @ Ti-Ka ACIS-I3 @ Mn-Ka

Caption: The uncertainty in the FWHM for the three ECS emission lines (Al-Ka, Ti-Ka, Mn-Ka) across the I3 CCD for focal plane temperatures between -117 and -115 C. The color scale is the uncertainty in the FWHM divided by the CALDB predicted FWHM.

Uncertainties in the I3 FWHM

Caption: Uncertainty in the FWHM on I3 for focal plane temperatures between -117 and -115 C.

FP Temp = -115:113

ACIS-I3 @ Al-Ka ACIS-I3 @ Ti-Ka ACIS-I3 @ Mn-Ka

Caption: The uncertainty in the FWHM for the three ECS emission lines (Al-Ka, Ti-Ka, Mn-Ka) across the I3 CCD for focal plane temperatures between -115 and -113 C. The color scale is the uncertainty in the FWHM divided by the CALDB predicted FWHM.

Uncertainties in the I3 FWHM

Caption: Uncertainty in the FWHM on I3 for focal plane temperatures between -115 and -113 C.

FP Temp = -113:111

ACIS-I3 @ Al-Ka ACIS-I3 @ Ti-Ka ACIS-I3 @ Mn-Ka

Caption: The uncertainty in the FWHM for the three ECS emission lines (Al-Ka, Ti-Ka, Mn-Ka) across the I3 CCD for focal plane temperatures between -113 and -111 C. The color scale is the uncertainty in the FWHM divided by the CALDB predicted FWHM.

Uncertainties in the I3 FWHM

Caption: Uncertainty in the FWHM on I3 for focal plane temperatures between -113 and -111 C.

Abell 1795 Fluxes

0.5 - 1.0 keV 1.0 - 2.0 keV

2.0 - 4.0 keV 4.0 - 6.0 keV

Caption: Abell 1795 fluxes in four different energy bands for observations taken at different locations on I3 and S3 over the course of the mission. The on- and off-axis observations are color coded. The rms scatter in the fluxes is also shown in the Figures.

HRC Observations of HZ43

HZ 43 Count Rates HZ43 Fluxed

caption Left: Observed HRC count rates in LETG/HRC-S and LETG/HRC-I observations of HZ43. Shown in the figure are the 0th order and ± first order count rates for HRC-S observations and the 0th order count rate for HRC-I observations. Right: Fluxed data for the observations shown in the left panel of the figure using the current CALDB

HRC Fluxed Data for Observations of G21.5-09

HRC Fluxed Data for Observations of G21.5-09

Caption: Fluxed data for HRC-I observations of G21.5-09 in several regions of the supernova remnant using the current CALDB.

LETG Absolute Energies

Caption: Difference between observed and lab energies for the emission-lines observed in dispersed LETG/HRC-S spectra of Capella.

Interleaved Gratings Observations

Caption: Fluxed light curve for Mkn 421 during a 90 ksec observation. The observation is bookended by HETG/ACIS-S observations. Between these observations, the LETG is inserted and the focal plane detector is cycled between the HRC-S and ACIS-S. This plot shows that even though the flux of Mkn 421 varies throughout the observation, the flux smoothly between the various detector/grating combinations with the current CALDB.