Overview of Chandra Calibration

Larry David

The CXC Calibration Team is responsible for monitoring the performance of the High Resolution Mirror Assembly (HRMA), the Advanced CCD Imaging Spectrometer (ACIS), the High Resolution Camera (HRC), and the Low and High Energy Transmission Gratings (LETG and HETG, respectively). This has been accomplished through an extensive calibration program that includes periodic observations of several standard X-ray candles (e.g., HZ43, SNR G21.5-0.9, E0102-72.3, and Abell 1795) as well as collecting data from the ACIS External Calibration Source (ECS).

HRMA

At launch, all calibration products were derived from ground-based measurements. The mirrors and gratings have required the fewest refinements to their calibration, and these corrections are applied to all data taken over the course of the mission. Only the detectors have time-dependent calibration products. The primary adjustment to the HRMA effective area occurred in 2009, when it was determined that the surfaces of the mirrors were covered with a thin layer of molecular contaminant. Periodic on-axis observations of AR Lac with the HRC-I have shown that the imaging properties of the HRMA have remained stable.

Transmission Gratings

The primary targets used to calibrate and monitor the performance of the gratings are HZ43 (a white dwarf), Mkn 421 (a blazar), and Capella (a chromospherically-active stellar binary). Since Mkn 421 is a variable source, interleaved observations with different detector/grating combinations have been carried out at periodic intervals to cross-calibrate between the gratings. No in-flight data have shown the need to adjust the 1st order transmission efficiency of the LETG. The most recent adjustments to the 1st order transmission efficiencies of the HETG were completed in 2009 to maintain consistency with changes to the HRMA effective area.

Since the LETG orders cannot be separated with the HRC-S, the higher order transmission efficiencies of the LETG must be calibrated using ACIS-S/LETG data. A sample of LETG/ACIS-S observations of bright continuum sources was used to refine the transmission efficiencies of the LETG higher orders in 2011. A calibration project is currently underway to refine the higher order transmission efficiencies of the HETG. Periodic observations of Capella have shown that the dispersion relation and Line Spread Function (LSF) of the LETG and HETG have remained stable.

ACIS

The response of the ACIS detectors has been affected by radiation damage, the build-up of contamination on the optical blocking filters, and an increase in focal plane temperature. Immediately after launch, ACIS suffered radiation damage by soft protons during passages through the Earth’s radiation belts. Since October 1999, ACIS has been stowed during belt passages to prevent further damage. While ACIS is in a stowed position, it is illuminated by the ECS; data from the ECS have been heavily used to monitor the ACIS detectors. However, the ECS is a 55Fe source with a half-life of 2.7 yr, and it is of limited use at the present time.

To correct for the effects of radiation damage, a CTI correction algorithm was developed and released to the public early in the mission. Recently, an updated temperature-dependent CTI correction file was released to the public in CALDB 4.11.1, improving the CTI correction up to a focal plane temperature of −105 °C. The recent improvements in the ACIS CTI correction allow for higher S/N observations at warmer focal plane temperatures.

ACIS observations of astronomical sources and the ECS have shown that molecular contamination has been building up on the optical blocking filters since launch. The uncontaminated ACIS QE is still based on ground-based flat field measurements. All time-dependent effective area corrections are made relative to the ground calibration. Early in the mission, the depth of the contaminant was monitored using line ratios in the ECS data, but the low energy lines eventually became too heavily absorbed to serve as a useful diagnostic tool. Since that time, periodic observations of the rich cluster Abell 1795 and “Big Dither” LETG/ACIS-S observations of Mkn 421 have been used to monitor the build-up of contamination. An ACIS contamination model was developed early in the mission, but changes in the composition and build-up rate of the contaminant have necessitated updates to the contamination model on a roughly yearly time scale. However, both the composition and build-up rate of the contaminant have remained fairly stable over the past five years.

The ACIS gain has shown a slow and steady decline over the course of the mission, with some dependence on the solar cycle. All time-dependent gain corrections are made relative to the ACIS gain measured during the first three months following the instrument cooling to −120 °C (January–March 2000). For most of the mission, time-dependent changes to the ACIS gain were calibrated with the ECS. As the ECS continued to fade, it became necessary to transfer the gain calibration to the supernova remnant Cas A and the Perseus cluster of galaxies.

HRC

The gain and QE of the HRC detectors have been monitored with periodic observations of the white dwarf HZ43 (a soft X-ray source) and the supernova remnant G21.5-09 (a harder X-ray source). These observations have shown a steady decline in the gain and QE of the HRC detectors over time. To remedy this situation, the high voltage (HV) of the HRC-S was increased in 2012 and the HV of both the HRC-S and HRC-I was increased in 2021. The 2021 HV increase restored the HRC detectors to their operating state of approximately three years prior. Another HV increase for both HRC detectors was recently performed in summer of 2024. Updated QE files have been released on an annual cadence to account for changes in the QE and HV, and new HRC QE tables will be released shortly, following the recent HV increase.