The present calibration of the Chandra Observatory is indebted to the work of many teams of scientists, engineers, and computer specialists within the Chandra project who have analyzed vast amounts of data over the past decade. Chandra calibration began many years before launch with the development, assembly, and testing of the individual scientific instruments. Commencing in December 1996, and continuing for 6 months, the assembled components of the Chandra Observatory (i.e., mirrors, gratings, and detectors) were extensively calibrated at the X-Ray Calibration Facility (XRCF) at MSFC, (see Newsletters #3, #4, #5, and #6 for discussions of prelaunch calibration). Ground testing is essential for absolute calibration, since only on the ground can we measure with certainty the flux and spectra of the incident radiation on the telescope.
There are, however, some characteristics of the observatory that can only be measured in-flight. For example, the X-ray source at XRCF was located 1700 feet from the mirrors. In-flight, the sources are essentially at infinity and the focal length is about 7 inches shorter. During the Orbital Activation and Calibration phase (OAC), which spanned the first three months after launch, the health of the instruments was checked along with measurements of many critical elements that could only be measured in-flight. Determining the focal point of the four onboard detectors was one of the first tasks completed after launch. The shorter focal length also necessitated in-flight measurements of the detector plate scales. In addition, the optical axis of the mirrors had to be located since the instruments were not assembled in the spacecraft in exactly the same manner as they were in the vacuum chamber at the XRCF.
During OAC we also established the framework for our monitoring program of the Chandra Observatory. We established a set of standard X-ray candles that we have periodically observed since launch. In addition to monitoring the Chandra instruments, these candles have helped cross calibration efforts with other X-ray observatories, in particular XMM-Newton. While there have been some deletions and additions to our yearly calibration plans, our set of standard candles has remained intact. The two most significant adjustments to our plan resulted from the radiation damage incurred by the front illuminated CCDs shortly after launch and the build-up of contamination on ACIS. In addition to observing cosmic sources, ACIS acquires data from its internal "55Fe" calibration source before and after every radiation belt passage. These data have been very helpful in monitoring the charge transfer inefficiency (CTI) of the CCDs, but the 55Fe source does not produce spectral lines below 1.5 keV. To monitor the ACIS gain at lower energies, we perform a raster scan of the oxygen rich supernova remnant E0102-72 every 6 months. Starting last spring, we added grating observations of PKS2155-304 at six month intervals to monitor the build-up of the contaminant on ACIS and determine its chemical composition.
The table below lists the targets we have repeatedly observed throughout the Chandra mission and the primary objectives of these observations. All calibration observations are immediately ingested into the public archive. These observations can be a valuable asset to observers in the analysis of their own Chandra observations.
Larry David, for the Chandra Calibration Team
| Calibration Targets | ||
|---|---|---|
| Target | Detector | Objective |
| Cas A | ACIS and HRC | Monitor the gain and spectral resolution of the CCDs. Monitor the QE and QE uniformity of the HRC. Cross-calibration between the focal plane detectors. |
| G21.5-09 | ACIS and HRC | Monitor the QE and QE uniformity of the detectors. Cross-calibration between the focal plane detectors and other X-ray telescopes. |
| E0102-72 | ACIS | Measure the effects of increasing CTI on gain and spectral resolution. Monitor the degradation in low energy QE. |
| HZ43 | LETG/HRC-I LETG/HRC-S | Monitor the HRC-I low energy QE and the throughput of LETG/HRC-S at low energies. |
| Coma | HRC-I | Monitor the QE, QE uniformity and degap map. |
| ArLac | HRC | Measure small scale gain variations across the detectors. |
| Betelgeuse Vega | ACIS, HRC | Monitor the optical/UV transmission of the detector filters |
| Capella | LETG/HRC-S HETG/ACIS-S | Monitor the gratings dispersion relation and LSF. |
| PKS2155-304 | LETG/ACIS-S HETG/ACIS-S LETG/HRC-S | Cross-calibration. Monitor the build-up of contaminant on ACIS and the HRC-S QE uniformity. |
| 3C273 | LETG/ACIS-S HETG/ACIS-S | Cross-calibration and monitoring the low energy absorption on ACIS. |