The ACIS team collected nearly 40 twenty-four hour days of calibration data at the X-ray Calibration Facility (XRCF) at the Marshall Space Flight Center. We present an overview of the types of measurements made, and sample results of the calibration, with an emphasis on joint flight ACIS/HRMA measurements. We apply the results of these measurements to considerations in selecting ACIS flight observing considerations.
A preliminary version of the ACIS team summary report on the ACIS XRCF calibration results is available via ftp from:
ftp.astro.psu.edu in the file /pub/nousek/cal_report.v220.ps.gz
The ACIS team collected calibration data utilizing the HRMA telescope during three data collection intervals. Following the XRCF nomenclature these occured during Phases F, G and H. During Phases F and G the HRMA focussed X-rays were collected on two flight-like CCDs (one front side illuminated (FI), and one back side illuminated (BI)) in a camera utilizing electronics very similar to the MIT CSR lab cameras. This camera, named the ACIS-2C, served as a proxy to enable data collection while the ACIS flight instrument finished the final stages of certification and thermal vacuum testing.
The ACIS-2C camera, using chips of identical design and fabricated in the same lots at Lincoln Lab as the flight ACIS chips, has differing processing electronics and does not hold the chips in the same orientation as the flight unit. Thus these ACIS-2C data are useful for understanding the chip level interaction between the mirror effects and the CCDs, but do not fully calibrate the effect of the flight electronic processing and flight array orientation effects.
Measurements were collected using the flight ACIS instrument and the HRMA at XRCF for eight days during Phase H (see Table 1 for the times of ACIS data collection). These data are the most directly relevant data as they use the flight mirrors and detectors, but must still be adjusted for the effects of finite conjugate distance, gravitational loading on the mirrors and a slightly differing CCD temperature operating point than planned for flight (-115 C vs. -120 C).
Following the departure of the HRMA mirrors to meet the schedule of assembly into the spacecraft, the XRCF was used to collect data with ACIS without any focussing optics. These data, collected during Phase I, fit naturally into the sub-assembly lab calibration data, and are discussed there.
Table 1: XRCF Data Collection Phases Involving ACIS
Times are given in local (Central) time. Note these refer to
the start and end of ACIS data collection intervals, not necessarily
to the start and end of the XRCF Phase.
Point Response Function (PRF) tests measure the core (PI) and wings (PW) of the PRF on-axis and at several off-axis positions, at the point of ideal focus determined by the Shutter focus/Plate focus measurements (X=0). Two types of data are collected: the core PRF is done in single photon mode in the core of the PRF; the wings PRF is done with higher fluxes, which produces photon mode in the wings but integration mode data in the core. Note that the shutter focus test data at X=0 are identical to the PRF core test for Al, so we do not repeat that test. Measurements are made at medium (Al K), high (Cu K), and low (O K) energy. We changed from the rehearsal energy of Fe to Cu because of the higher response above 6 keV of the HRMA versus the TMA. The Al K inner core test is deleted because the plate focus at X=0 provides the same data. C K is included for an ultra-low energy point for the BI chips. Tests were also made to estimate the PRF using the ACIS and ACIS-2C continuous readout mode.
Sub-pixel position measurements were also performed as part of the PRF test suite. We moved the PRF by sub-pixel amounts to sample the digitization error resulting from pixel size undersampling of the PRF and to test split X-ray event reconstruction models of sub-pixel interaction location. For FI chips, tests were performed at high (Cu K) and low (O K) energy to test for differing split event fractions. For BI chips, Cu K was also used as the high-energy test, but C K was chosen for the low-energy test for differing split event fractions.
See the poster by Townsley et al. for a discussion of the PRF results.
Effective Area (EA) tests measure the total effective collecting area over the PRF at many energies in photon counting mode. We performed these tests at many energies due to the need for detailed sampling of energy space. The tests were done with the system substantially defocused (by roughly 40 mm), to minimize photon loss due to pileup. Measurements were made on-axis and at several off-axis positions.
In analysis of these data it is crucial to include the effects of data selection criteria such as event grade, incident flux rate and readout mode. See the poster by Chartas et al. for a discussion of the Effective Area results.
Count-rate Linearity (Pileup) tests measure the effect of increasing the level of photons per pixel on the PRF, the EA, source centroiding, and photon detection. We used two methods to vary photon density: longer readout times for constant beam flux, and brighter beam flux at constant readout times. The data are being compared to see if the pileup is different between these two methods. For FI chips, this test was repeated at medium energy (Al K) for minimum event splitting and at high energy (Cu K) for maximum event splitting. For BI chips, it was repeated at medium energy (Al K) for minimum event splitting and high energy (Cu K) for comparison with FI; we added low energy (O K) and ultra low energy (C K) for maximum BI event splitting. It was important to repeat these tests separately for each chip because the results depend intimately on the event splitting characteristics of the chip.
See the poster by Jones et al. and the poster by Nousek et al. for a discussion on modelling and the results of calibration of the effect of pileup on ACIS data.
The ACIS team also conducted several miscellaneous tests for specific purposes listed below.
The Spatial Linearity tests consist of moving the PRF across chips to determine that the desired motion converts properly from the spatial domain to the CCD pixel domain. Also look for PRF variation indicating that the CCD chips are tilted with respect to the HRMA axis.
In the Flight Contamination Monitor tests, we position the ACIS or ACIS-2C at infinite conjugate focus and observe the Forward Contamination Cover sources (Mn K and Ag L) reflecting off the mirrors over both BI and FI chips. This allowed the facility to monitor the HRMA for contamination buildup throughout XRCF testing and baselines the radiation source spectra for on-orbit contamination monitoring.