Spatial Variations in Quantum Efficiency

As discussed in section 4.7.1 above, we have measured quantum efficiency (relative to a reference detector) of each ACIS flight device as a function of position. For purposes of presentation, we have binned the relative quantum efficiency into 32-by-32-pixel ``superpixels''; we expect that in typical AXAF observations the celestial image will be dithered over an area of about this size ($\sim15$ arcsec) on the ACIS focal plane.

Maps showing the spatial uniformity of the relative quantum efficiency at each measurment energy are presented in Figures 4.77-4.86. Alongside each map is a histogram of the superpixel relative efficiencies. The efficiencies shown in these figures have not been corrected for pileup. The reference CCD count rates used to generate each superpixel relative efficiency have been corrected for cosmetic defects, including the effects of shadowing by the framestore covers used during subassembly calibration at MIT. The flight CCD count rates have not been corrected for cosmetic defects, but have been corrected for shadowing by the subassembly calibration framestore cover. In addition, each uniformity map has been adjusted to reproduce the effect of shadowing by the flight framestore cover. This procedure is intended to produce spatial uniformity maps which replicate the quantum efficiency patterns that will be present in data obtained with the flight CCDs on orbit. The number of rows in each device shadowed by the subassembly and flight framestore covers are listed in Table 4.57.

Histogram legends in Figures 4.77-4.86 indicate the centroid and standard deviation of a Gaussian fit to each histogram. The fitted standard deviation of the histogram provides a good measure of the residual spatial variation in relative quantum efficiency, excluding the effects of the flight framestore shield.

Spatial variations in relative QE are comparable to expected statistical fluctuations for the FI CCDs, except the ratios which are affected by bad or hot columns or pixels and border effects (especially the shadows of frame store covers). These superpixel ratios are generally outliers in the QE ratio histograms, and only weakly affect the Gaussian fit. These effects may be seen in Figures 4.77-4.86. Columns darker than their surrounding regions correspond to either bad columns in the flight CCD or hot columns in the reference CCD, while lighter columns typically correspond to bad columns in the reference CCD. The dark row at the bottom of each relative uniformity map is due to shadowing by the flight framestore cover.

Larger spatial variations are observed in the BI CCDs, and are believed to be caused in part by relatively high charge transfer inefficiency (CTI), exceeding 25% at higher energies. Some provision will need to be developed to characterize this spatial nonuniformity in the two backside flight CCDs (S1 and S3). These variations affect the quality of the Gaussian fit to the histogram of QE ratios.
 
 

Table 4.57: Number of CCD Rows Shadowed by the Framestore Covers

 

Flight	Position	Framestore Cover
Device		Subassembly	Flight
w203c4r	I0	4	7
w193c2	I1	6	8
w158c4r	I2	7	7
w215c2r	I3	6	7
w168c4r	S0	6	6
w182c4r	S2	7	6
w457c4	S4	1	5
w201c3r	S5	4	5
w140c4r	S1	6	6
w134c4r	S3	8	5
w190c3	Ref.	10
w203c2	Ref.	9
w103c4	Ref.	7

 
  Figure 4.77: Uniformity maps and histograms showing the spatial uniformity of the quantum efficiency of ACIS flight device I0 (w203c4r) relative to reference device w190c3 at seven energies. Each pixel value is the ratio of the raw count rate (i.e. uncorrected for pileup) in a 32 x 32 pixel subarray of the flight device to the count rate in the corresponding subarray of the reference device.

 
  Figure 4.78: Uniformity maps and histograms showing the spatial uniformity of the quantum efficiency of ACIS flight device I1 (w193c2) relative to reference device w103c4 at seven energies. Each pixel value is the ratio of the raw count rate (i.e. uncorrected for pileup) in a 32 x 32 pixel subarray of the flight device to the count rate in the corresponding subarray of the reference device.

 
  Figure 4.79: Uniformity maps and histograms showing the spatial uniformity of the quantum efficiency of ACIS flight device I2 (w158c4r) relative to reference device w103c4 at seven energies. Each pixel value is the ratio of the raw count rate (i.e. uncorrected for pileup) in a 32 x 32 pixel subarray of the flight device to the count rate in the corresponding subarray of the reference device.

 
  Figure 4.80: Uniformity maps and histograms showing the spatial uniformity of the quantum efficiency of ACIS flight device I3 (w215c2r) relative to reference device w190c3 at seven energies. Each pixel value is the ratio of the raw count rate (i.e. uncorrected for pileup) in a 32 x 32 pixel subarray of the flight device to the count rate in the corresponding subarray of the reference device.

 
  Figure 4.81: Uniformity maps and histograms showing the spatial uniformity of the quantum efficiency of ACIS flight device S0 (w168c4r) relative to reference device w190c3 at seven energies. Each pixel value is the ratio of the raw count rate (i.e. uncorrected for pileup) in a 32 x 32 pixel subarray of the flight device to the count rate in the corresponding subarray of the reference device.

 
  Figure 4.82: Uniformity maps and histograms showing the spatial uniformity of the quantum efficiency of ACIS flight device S2 (w182c4r) relative to reference device w103c4 at seven energies. Each pixel value is the ratio of the raw count rate (i.e. uncorrected for pileup) in a 32 x 32 pixel subarray of the flight device to the count rate in the corresponding subarray of the reference device.

 
  Figure 4.83: Uniformity maps and histograms showing the spatial uniformity of the quantum efficiency of ACIS flight device S4 (w457c4) relative to reference device w190c3 at seven energies. Each pixel value is the ratio of the raw count rate (i.e. uncorrected for pileup) in a 32 x 32 pixel subarray of the flight device to the count rate in the corresponding subarray of the reference device.

 
  Figure 4.84: Uniformity maps and histograms showing the spatial uniformity of the quantum efficiency of ACIS flight device S5 (w201c3r) relative to reference device w190c3 at seven energies. Each pixel value is the ratio of the raw count rate (i.e. uncorrected for pileup) in a 32 x 32 pixel subarray of the flight device to the count rate in the corresponding subarray of the reference device.

 
  Figure 4.85: Uniformity maps and histograms showing the spatial uniformity of the quantum efficiency of ACIS flight device S1 (w140c4r) relative to reference device w103c4 at seven energies. Each pixel value is the ratio of the raw count rate (i.e. uncorrected for pileup) in a 32 x 32 pixel subarray of the flight device to the count rate in the corresponding subarray of the reference device.

 
  Figure 4.86: Uniformity maps and histograms showing the spatial uniformity of the quantum efficiency of ACIS flight device S3 (w134c4r) relative to reference device w203c2 at seven energies. Each pixel value is the ratio of the raw count rate (i.e. uncorrected for pileup) in a 32 x 32 pixel subarray of the flight device to the count rate in the corresponding subarray of the reference device.