ACIS Calibration

The Chandra project concluded its major pre-launch calibration effort with the delivery this Fall of the Science Instrument Team's calibration reports. The ACIS report, entitled ``Science Instrument (SI) Calibration Report for the AXAF CCD Imaging Spectrometer (ACIS),'' edited by team leaders Mark Bautz and John Nousek, describes in detail the performance of the camera and the team's calibration and data analysis strategies. The ACIS pre-flight calibration tests were carried out in two principal phases. During the first phase, the camera's components (CCDs, filters, electronics, spares etc.) were calibrated in the MIT and Penn State University laboratories. Additional tests on selected CCDs and the optical blocking filters were conducted at calibrated synchrotron light facilities. The camera components were illuminated by X-rays with a flat illumination pattern, which provided for high signal-to-noise, high throughput X-ray measurements across the entire CCD and filter assemblies. The quantum efficiencies, energy resolution functions, electronic gain tables, charge transfer inefficiencies, filter transmissions and so forth for each CCD and filter were derived from these measurements. In addition, the report examines the effects of event charge spreading among pixels and its consequences for event detection, charge collection efficiency, and so forth.

The report describes an X-ray camera of the highest quality. The energy resolution of the FI (front illuminated) devices ranges between $\simeq 50-170$ eV for energies between 0.5-8 keV respectively (for single pixel events). The FI devices have negligible charge transfer inefficiency. Quantum efficiency maps of the FI devices on a $32\times 32$ pixel grid are remarkably uniform. Peak-to-peak QE variations of less than a few percent across each device are seen, and they are virtually free of cosmetic defects. The popular BI (back illuminated) devices sacrifice the high energy resolution, high degree of spatial QE uniformity, and high charge transfer efficiency for a several-fold increase in quantum efficiency in the important soft X-ray band below 1 keV. Observers who plan to use the BI devices should take great care with their data analysis to reap the maximum benefit out of the extra photons below 1 keV.

End-to-end tests of the telescope were conducted at the X-ray calibration facility (XRCF) in Huntsville, AL., during Winter 1996 and Spring 1997. During this second calibration phase, the assembled camera was illuminated by X-rays focused by the High Resolution Mirror Assembly (HRMA) onto the camera's contoured focal surface. In order to approximate a X-ray point sources at infinity, several X-ray sources were placed roughly 500 meters from the HRMA entrance aperture at the end of a horizontal, evacuated tube. Gravity distortions were removed from the horizontally-mounted HRMA using a system of weights and pulleys. The telescope was illuminated with monochromatic X-rays at about a dozen energies ranging from 0.2-8 keV with a variety of fluxes and focal plane locations. The HRMA and camera were tipped and tilted using independent mountings to approximate the telescope's response to off-axis point sources. A series of tests was designed by the Instrument Team and CXC scientists to measure the point response function (PRF), the telescope's effective area, and the effects of event pileup in the camera. For those who have become accustomed to fuzzy X-ray imagery, the results of the XRCF tests will seem remarkable. The on-axis FWHM of the ACIS + HRMA PRF is $\mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$ }}}\hbox{$<$ }}}1$ arcsec at 1 keV, including residual gravity and finite source distance blurring, and is nearly constant with energy. The $70\%$ encircled energy radius is $\leq 1.2$ arcsec at all energies. We are certainly pleased with the telescope's performance at the XRCF.

In addition to the calibration report, the ACIS Instrument Team developed a computer model of ACIS's CCDs. This model is being used by CXC scientists to generate response matrices for the CCDs, and the model will be available to the community within MARX. The ACIS calibration report crowns the delivery of a superb CCD camera to the community. I encourage you to browse through the report which can be obtained through our website.

One of our jobs here at the CXC is to distil and synthesize the information contained in the calibration document and in technical memoranda prepared by CXC scientists into the data that you will use to calibrate your Chandra science data. You can locate the most up-to-date calibration data via the World Wide Web. The data are presented under a matrix of hyperlinks with descriptive titles such as ``ACIS Response Matrices,'' ``Energy Resolution,'' ``Pileup,'' etc., and a brief description of the information tied to the hyperlink. The webpage will appear on your computer monitor as in Figure 2. For example, when you activate the ``ACIS QE'' hyperlink using your mouse, you will be presented with a page showing a series plots containing the quantum efficiencies of the ACIS CCD detectors as a function of energy. Hardcopy is available by clicking ``postscript'' with your mouse, as are machine-readable copies of the data. Additional information can be found in reports describing the data analysis. This website is intended to serve as a focal point for the latest Chandra calibration information, and it will be updated frequently.

  

Figure 2: ACIS Calibration Webpage.

LINK TO POSTSCRIPT FILE for Figure 2

In addition to serving as the community interface for Chandra-related calibration information, CXC scientists actively participated in the planning and execution of the calibration measurements in the MIT laboratories and at the XRCF. Since that time, we have been analyzing the calibration data in cooperation with the ACIS team. For example, CXC scientists have performed detailed examinations of the ACIS effective area, the PRF, event pileup, electronic gain and charge transfer inefficiency of the CCDs. Reports relating to some of these analyses can be accessed through our website. Finally, the calibration group has prepared a detailed plan and timeline for the orbital calibration which will occur during the first few months of operation. During this phase, several critical calibration observations will be obtained. These include focusing the camera and measuring the PRF and the effects of event pileup. In addition, the relative effective areas of the CCDs will be measured and monitored using observations of clusters of galaxies, and the electronic gain, energy resolution, and charge transfer efficiency will be measured and monitored using radioactive calibration targets. Details of the orbital calibration plan can be accessed through our website. I encourage you to browse our website at:

http://asc.harvard.edu/cal/Acis/WWWacis_cal.html

- B.R. McNamara