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Next: 4 Acceptance Criteria Up: Flight X-ray CCD Selection Previous: 2 ACIS Detectors

3 Screening Protocol

3.1 Initial Inspection

Prior to shipment, MIT Lincoln Laboratories checks the output nodes of each device for functionality and operates the devices at -40 C to verify cosmetic quality. Once received at MIT CSR, the CCD is installed in a vacuum chamber, the chamber is evacuated to a pressure of torr, and the device is cooled to -120 C. The device is then exposed to an Fe source to ensure that it can detect X-rays and has no gross defects.

3.2 Tuning

Before the screening process begins, proper optimization of the CCD's clock voltages must occur. The CCD biases which have the greatest effect on device performance are drain bias applied to each of the four on-chip MOSFET amplifiers. Indeed, we find that excellent device performance is achieved with standard set biases for the charge transfer and reset clock voltages for all devices. Thus, only the drain biases must be adjusted for each CCD. This flexibility allows us to set charge transfer clock levels to maximal depletion depth. Bias (source-free) and X-ray data (again using an Fe source) are taken over a wide range of clock voltages. The Mn and Mn lines provide a gain conversion and allow determination of noise in electron units (see Section 3.4). During the subsequent screening and calibration measurements, the CCD's clocks are operated at the voltages that produced the lowest RMS noise.

3.3 Data Acquisition

3.3.1 CCD Readout Modes

Nominal operation of both ACIS-I and ACIS-S involves reading out each CCD through four output nodes in parallel in a timed exposure mode (time = 3.227 seconds for standard mode). During the course of the AXAF mission, however, other modes with different exposure times may be preferred for particular types of measurements. Continuous read-out modes (charge is continually clocked from the device) will be useful for observations where precision timing is important. Summation modes (blocks of pixels summed together and read out as a single pixel) exist to avoid pile up during observation of bright sources. Another mode variation allows the entire device to be readout through two output nodes (AC or BD). Reading out only a sub-array of the device reduces the integration time and permits observation of brighter sources. In total we examine eight different operational modes during the screening process. Table 1 lists each mode and briefly describes it.

3.3.2 Required Data Sets

Characterization of each readout mode requires several measurements. Using a reflection grating monochromator, the CCD is illuminated with monochromatic photons at the energy corresponding to the oxygen line (525 eV) and the aluminum line (1.49 keV). The device is also exposed to Mn and lines produced by an Fe source. These three measurements provide a determination of charge transfer inefficiency (CTI), energy scale, and spectral resolution over a broad energy range between .5 - 6 keV. Finally, dark current data are taken at a range of integration times, and a number of bias frames are taken. This procedure allows calculation of the dark current present in the CCD.

A typical measurement begins by configuring the CCD to perform in a specific mode by proper programming of the CCD clock sequencing electronics. For the three spectral resolution/CTI data sets, flux levels are checked and adjusted to maximize incident flux consistent with acceptable pile-up. We then collect enough data to ensure that photons are incident on each CCD quadrant. Bias data is taken immediately preceding the collection of X-ray data. Finally, the dark current data is taken. Table 2 shows the typical measurements made for each mode and the number of frames obtained.

Table 1: Description of Readout Modes

Table 2: Data Sets for Each Readout Mode

3.4 Data Analysis

Analysis starts with the Fe data set. The code subtracts bias levels and corrects for drift in the overclocks before using ASCA event detection and grading algorithms to find events. Event ``grades'' are assigned on the basis of the spatial charge distribution in a 3x3 pixel neighborhood surrounding each event. Grades are used to select events with well-collected charge. Overclocks are thirty additional serial transfers that occur after each row has been read out. Changes in overclock levels indicate drift in the electronics. The event list is then searched to locate hot columns (defined as a column with a bias level 50 e greater than the median level), hot pixels (defined as a pixel with a bias level 50 e greater than the median bias level), and bad columns (defined as columns with low X-ray detection efficiency). If any hot columns or hot pixels are found, events from these columns are removed from the Fe data and subsequent data sets.

With a properly corrected data set, noise, CTI, dark current, and spectral resolution can be calculated. Noise is determined by computing the pixel-by-pixel difference of two bias frames. A Gaussian is fit to the histogram of this difference frame. The standard deviation of this Gaussian is taken to be time the RMS system noise.

Histograms are made of the graded events and used for CTI and spectral resolution measurements. CTI is calculated by plotting pulse height values versus row number (parallel CTI measurements) or column number (serial CTI measurements), fitting a line of the form: to the data, and equating CTI to . Then a Gaussian is fit to the Mn k line to determine the spectral resolution and energy scale of the CCD at 5.9 keV. Each of these processing steps is repeated for the aluminum and oxygen data sets in order to provide device-level performance information over a broad energy band (0 to 5.9 keV). Section 5.2 contains sample data and plots.

Dark current is calculated for modes with exposure times that can be varied. An average bias frame is calculated from the dark current data taken at the normal exposure time (3.277 seconds for standard mode). After the data has been bias subtracted and overclock corrected, an average pixel value is determined for each dark frame from a histogram of the pixels in the image section. A linear relationship of the form: is fit to the data, where is the mean dark current in adu/pixel/sec and is the mode bias value for a frame with integration time . This value is converted from engineering units to electron/pixel/sec using the gain values determined from the Fe measurements.

When the analysis has finished, the code processes the results and creates a summary sheet that details the CCD's performance and determines if it meets the acceptance criteria. This report presents all the relevant information in a readily digestible form and provides the ACIS team with the information needed to decide whether to fully calibrate a particular device. Figure 2 is the screening report for device c17-185-3. Besides displaying the measured quantities in tables, the summary sheet also lists the acceptance criteria and then decides if the device passed (P) or failed (F). The entire data acquisition and analysis process can be completed in about four hours.


Screening Report (FI) for c17-185-3 [T=-120.0 C]
(data_dir = /nfs/rico/d7/w185c3/23Jan96)
(output_dir = /ohno/da/Screen/c17-185-3/23Jan96/screen)
Fri Apr  5 14:56:38 EST 1996

Event Threshold = 100 ADU
Split Threshold: 
     Node0: 13
     Node1: 13
     Node2: 12
     Node3: 13
Grade: cti(0-7); spec_resol(0,2-4); gain(0,2-4)

          <Readout Noise (noise[im]+d < 5 e-)>
Node(im)  Noise(electron)    Node(oc)  Noise(electron)     Test
c0im    2.133 +/- 0.002     c0oc     2.084 +/- 0.007      P
c1im    2.735 +/- 0.003     c1oc     2.672 +/- 0.009      P
c2im    1.885 +/- 0.002     c2oc     1.812 +/- 0.006      P
c3im    1.904 +/- 0.002     c3oc     1.761 +/- 0.006      P

          <Parallel CTI [Fe55 at 5.9 keV] (< 5e-6)>
Node    CTI                                       Test
c0      1.06836e-07 +/- 3.37265e-07               P
c1      6.86257e-08 +/- 3.38151e-07               P
c2     -2.11046e-07 +/- 3.28082e-07               P
c3     -1.13295e-07 +/- 3.49976e-07               P

          <Parallel CTI [O_k at 0.525 keV] (< 5e-5)>
Node    CTI                                       Test
c0      8.01778e-06 +/- 1.87914e-06               P
c1      1.90632e-06 +/- 2.00573e-06               P
c2      7.50403e-06 +/- 1.80480e-06               P
c3      5.86082e-06 +/- 2.05938e-06               P

          <Serial CTI [Fe55 at 5.9 keV] (< 1e-5)>
Node    CTI                                       Test
c0     -3.91665e-06 +/- 1.33823e-06               P
c1     -1.53416e-06 +/- 1.33965e-06               P
c2     -3.74164e-07 +/- 1.30727e-06               P
c3      1.37959e-06 +/- 1.35999e-06               P

          <Mean Dark Current (< 0.3 e/pix/s)>
Node    Dark Current (e/pix/s)     (95% uplimit)  Test
c0    0.039285    +/- 0.000205    (0.220000   )   P
c1    0.034888    +/- 0.000254    (0.260000   )   P
c2    0.029906    +/- 0.000211    (0.220000   )   P
c3    0.070693    +/- 0.000202    (0.260000   )   P

          <Spectral Resolution [O_k at 0.525 keV] (FI:< 70eV)>
Node    FWHM(eV)                                  Test
c0      44.3276     +/- 0.412155                  P
c1      48.4973     +/- 0.455946                  P
c2      43.8841     +/- 0.412187                  P
c3      42.9952     +/- 0.463458                  P

          <Spectral Resolution [Al_k at 1.487 keV] (FI:< 100eV)>
Node    FWHM(eV)                                  Test
c0      67.4721     +/- 0.344109                  P
c1      69.5684     +/- 0.367777                  P
c2      67.093      +/- 0.363496                  P
c3      66.5883     +/- 0.385113                  P

          <Spectral Resolution [Fe55 at 5.898 keV] (< 165eV)>
Node    FWHM(eV)                                  Test
c0      128.472     +/- 0.716822                  P
c1      129.882     +/- 0.726135                  P
c2      127.188     +/- 0.706473                  P
c3      127.231     +/- 0.729545                  P

Node O_Gain(eV/ADU)           Al_Gain(eV/ADU)            Fe_Gain(eV/ADU)
c0 4.07735 +/- 0.00187521    4.04416 +/- 0.000548207   4.05378 +/- 0.000287081
c1 4.14169 +/- 0.00204711    4.10557 +/- 0.000597824   4.11283 +/- 0.000293099
c2 4.53748 +/- 0.00206868    4.50461 +/- 0.000644549   4.508   +/- 0.00031659
c3 4.22243 +/- 0.00215265    4.1942  +/- 0.000632717   4.21641 +/- 0.000302656

<Hot Pixels>
     0 hotpixel
     0 hotcol
     0 hotpixel not in hotcol

<Bad Columns>
     0 badcol

Figure 2: Screening Report for device c17-185-3

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Next: 4 Acceptance Criteria Up: Flight X-ray CCD Selection Previous: 2 ACIS Detectors