Long Term Gain Stability

Checking the long term gain stability of the ACIS chips is an important step in the pre-launch calibration. During the course of sub-assembly calibration, the stability in chip gain was firmly established and it is completely expected that ACIS CCD's will share this characteristic. While the earliest gain measurements of the flight instrument date back to March 1997 (LLTV), the first high accuracy (i.e. several characteristic K$\alpha$ lines used) determination occurred at the XRCF Flat Field (XRCF-FF) calibration in May 1997. The next opportunity to measure the gain came during the first thermal vacuum test at Ball (ISIM-TV1) in October 1997. During this test a significant amount of data was acquired from the External Calibration Source (ExtCalSrc), allowing an accurate gain determination of all ACIS chips. Below, we compare the results of these two calibrations.

Table 4.67 lists the mean gains (all four quadrants of a chip) measured at XRCF-FF and ISIM-TV1. Taking the XRCF-FF values as a baseline, the I-Array chips (imaging CCD's and S2 and S3) have higher gains at ISIM-TV1, while the outer S-Array chips (S0,S1,S4,S5) have approximately the same gains. The gain is known to be dependent on DEA temperature and focal plane temperature. As the effect of the focal plane temperature on the gain is an order of magnitude smaller than that of the electronics temperature, we will focus on the later as a possible explanation of the difference in gains between the two measurements. The mean temperature dependence relationship is: d(ln Gain)/dT=$4 \times$10^-4 (see Section 4.9.2 for details). The thermal conditions for both calibrations were nominally the same: focal plane temperature of -110$\mbox{$^{\circ}$ }$ C, and DEA -Z plane temperature of +13$\mbox{$^{\circ}$ }$ C. Allowing for a drift in the DEA temperature of $\sim1\mbox{$^{\circ}$ }$ C , we would expect a gain variation on order $\sim$ 0.002, much higher than that observed for the I-array chips. Figure 4.98 plots d(ln Gain) for each quadrant of all ten chips.
 
 

Table 4.67: Mean Chip Gain determined at XRCF-FF and ISIM-TV1

 

Chip	XRCF-FF	ISIM-TV1	Chip	XRCF-FF	ISIM-TV1
i0	3.827	3.857	s1	4.413	4.398
i1	3.849	3.892	s2	3.757	3.795
i2	3.592	3.632	s3	4.507	4.560
i3	3.882	3.911	s4	4.306	4.308
s0	3.797	3.791	s5	4.101	4.095

Systematic variations, order of a few degrees, are expected in the DEA temperature; the dashed lines indicate the values of d(ln Gain) expected for $\Delta$T=$\pm$5$\mbox{$^{\circ}$ }$ C. The outer S-Array chips are mostly distributed between this range, indicating that the gains measured with the ExtCalSrc for these chips are stable, within systematic uncertainties. Another dashed line indicates the value of d(ln Gain) for $\Delta$T=25$\mbox{$^{\circ}$ }$ C; the I-Array chips measured with the ExtCalSrc are tightly grouped abou this line. At this time, the source of the gain shift is not known. However, given the uncertainties in the operating conditions and the thermal environment during the ISIM-TV tests, there exists the distinct possibility that a strong thermal gradient existed inside the DEA unit. This condition can explain the apparantly paradoxical behavior that some of the gains indicate that the DEA -Z plane was in fact $\sim$13$\mbox{$^{\circ}$ }$ C, while the other chips indicate that the the electronics were $\sim$25$\mbox{$^{\circ}$ }$ C hotter. This scenario requires that the boards used to read the I-Array chips were in the hottest part of the gradient and furthest from the DEA -Z plane temperature sensor. Further evidence for this hypothesis comes from Internal Calibration Monitor (ICM) data. The ICM can also be used to determine gains, but only for four quadrants. The ICM data was also taken as part of the XRCF-FF tests, as well as during the second ISIM thermal vacuum test, ISIM-TV2^4.8. The XRCF-ICM gains (blue symbols in the plot) are also distributed near the $\pm$5$\mbox{$^{\circ}$ }$ C lines, establishing that the ICM source can provide an accurate gain measurement. During ISIM-TV2, however, the ICM gains (orange symbols in the plot) are also grouped about the 25$\mbox{$^{\circ}$ }$ C line. The fact that the two ISIM thermal vacuum tests were separated by more than two months indicates that whatever condition caused the shift in the ExtCalSrc gains was also present during the subsequent testing. If the mechanism was in fact a thermal gradient, it must have have been stable.
 

Figure 4.98: The difference in gain measured at XRCF-FF and ISIM-TV1. The figure plots d(ln Gain)=Delta Gain/Gain, where the gain from XRCF-FF is taken as a base line. The red points compare the gains calculated from the ExtCalSrc data at ISIM-TV1, the blue points compare the gains calculated fro m the ICM source at XRCF-FF, and the orange points compare the gains calculated from t he ICM source at ISIM-TV2. The dashed lines and labels indicate the change in DEA temp erature required to shift the gain by that amount. The ICM source only allows gain meas urements for S2 Quad D and S3 Quads A-C.