7. X-ray Imaging Spectrometer (XIS)

7.1 Overview of the XIS

Figure 7.1: The four XIS detectors before installation onto Astro-E2.

Astro-E2 will contain four XIS instruments, shown in Figure 7.1. These employ X-ray sensitive silicon charge-coupled devices (CCDs), which are operated in a photon-counting mode, similar to that used in the ASCA SIS, Chandra ACIS, and XMM-Newton EPIC. In general, X-ray CCDs operate by converting an incident X-ray photon into a charge cloud, with the magnitude of charge proportional to the energy of the absorbed X-ray. This charge is then shifted out onto the gate of an output transistor via an application of time-varying electrical potential. This results in a voltage level (often referred to as ``pulse height'') proportional to the energy of the X-ray photon.

The four Astro-E2 XISs are named XIS-S0, S1, S2 and S3, each located in the focal plane of an X-ray Telescope; those telescopes are known respectively as XRT-I0, XRT-I1, XRT-I2, and XRT-I3. Each CCD camera has a single CCD chip with an array of 1024 $\times$ 1024 picture elements (``pixels''), and covers an $18'\times18'$ region on the sky. Each pixel is 24 $\mu$m square, and the size of the CCD is 25 mm $\times$ 25 mm. Two of the XISs are equipped with back-side illuminated CCDs, and other two use front-side illuminated CCDs. The XIS has been partially developed at MIT (CCD sensors, analog electronics, thermo-electric coolers, and temperature control electronics), while the digital electronics and a part of the sensor housing were developed in Japan, jointly by Kyoto Univ., Osaka Univ., Rikkyo Univ., Ehime Univ., and ISAS.

Figure 7.2: One XIS instrument. Each XIS consists of a single CCD chip with $1024 \times 1024$ X-ray sensitive cells, each 24 $\mu$m square. Astro-E2 contains four CCD sensors (XIS-S0 to S3), two AE/TCUs (AE/TCE01 and AE/TCE23), two PPUs (PPU01 and PPU23), and one MPU. AE/TCU01 and PPU01 service XIS-S0 and XIS-S1, while AE/TCE23 and PPU23 service XIS-S2 and XIS-S3. Two of the XIS CCDs will be front-illuminated (FI) and two will be back-illuminated (BI).

A CCD has a gate structure on one surface to transfer the charge packets to the readout gate. The surface of the chip with the gate structure is called the ``front side''. A front-side illuminated CCD (FI CCD) detects X-ray photons that pass through its gate structures, i.e. from the front side. Because of the additional photo-electric absorption at the gate structure, the low-energy quantum detection efficiency (QDE) of the FI CCD is rather limited. Conversely, a back-side illuminated CCD (BI CCD) receives photons from ``back,'' or the side without the gate structures. For this purpose, the undepleted layer of the CCD is completely removed in the BI CCD, and a thin layer to enhance the electron collection efficiency is added in the back surface. A BI CCD retains a high QDE even in sub-keV energy band because of the absence of gate structure on the photon-detection side. However, a BI CCD tends to have a slightly thinner depletion layer, and the QDE is a little worse in the high energy band. To balance these effects, two of the XISs use BI CCDs and others use FI CCDs. As of writing this document, it is not determined which XISs will have the BI CCDs.

To minimize the thermal noise, the sensors need to be kept at $\sim -90^\circ$C during observations. This is accomplished by thermo-electric coolers (TECs), controlled by TEC Control Electronics, or TCE. The Analog Electronics (AE) drives the CCD clocks, reads and amplifies the data from the CCDs, performs the analog-to-digital conversion, and routes the signals to the Digital Electronics (DE). The AE and TCE are located in the same housing, and together, they are called the AE/TCE. Astro-E2 has two AE/TCEs; AE/TCE01 is used for XIS-S0 and S1, and AE/TCE23 is used for XIS-S2 and S3. The digital electronics system for the XISs consists of two Pixel Processing Units (PPU) and one Main Processing Unit (MPU); PPU01 is associated with AE/TCE01, and PPU23 is associated with AE/TCE23. The PPUs receive the raw data from AE, carry out event detection, and send event data to the MPU. The MPU edits and packets the event data, and sends them to the satellite's main digital processor.

To reduce contamination of the X-ray signal by optical and UV light, each XIS has an Optical Blocking Filter (OBF) located in front of it. The OBF is made of polyimide with a thickness of 1000 Å, coated with a total of 1200 Å of aluminum (400 Å on one side and 800 Å on the other side). To facilitate the in-flight calibration of the XISs, each CCD sensor has two $^{55}$Fe calibration sources. One is installed on the door to illuminate the whole chip, while the other is located on the side wall of the housing to illuminate two corners of the CCD. The door-mounted source will be used for initial calibration only; once the door is opened, it will not illuminate the CCD. In addition to the radio isotopes, we can utilize the charge injection capability of the CCD for some calibrations. This allows an arbitrary amount of charge to be input to the pixels at the top row of the imaging region (exposure area), i.e. the far side from the frame-store region. This charge injection capability may be used to measure the CTI (charge transfer inefficiency) of each column, or even to reduce the CTI. How the charge injection capability will be used will be determined after launch.

Fig. 7.2 provides a schematic view of the XIS system. Charge clouds produced in the CCD by the X-rays focused by the XRT are accumulated on the exposure area for a certain exposure period (typically 8 s in the ``normal'' mode), and the data are transferred to the Frame Store Area after each exposure. Data stored in the Frame Store Area are read-out sequentially by the AE, and sent to the PPU after the conversion to the digital data. The data are put into the memory in PPU named Pixel RAM. Subsequent data processing is done by accessing the Pixel RAM.

7.2 CCD Pixels and Coordinates

A single XIS CCD chip consists of four segments (marked A, B, C and D in Fig. 7.2) and correspondingly has four separate readout nodes. Pixel data collected in each segment are read out from the corresponding readout node and sent to the Pixel RAM. In the Pixel RAM, pixels are given RAWX and RAWY coordinates for each segment in the order of the readout, such that RAWX values are from 0 to 255 and RAWY values are from 0 to 1023. These physical pixels are named Active pixels.

In the same segment, pixels closer to the read-out node are read-out faster and stored in the Pixel RAM faster. Hence, the order of the pixel read-out is the same for segments A and C, and for segments B and D, but different between these two segment pairs, because of the different locations of the readout nodes. In Fig. 7.2, numbers 1, 2, 3 and 4 marked on each segment and Pixel RAM indicate the order of the pixel read-out and the storage in the Pixel RAM.

In addition to the Active pixels, the Pixel RAM stores the Copied pixels, Dummy pixels and H-Over-Clocked pixels (cf. Fig. 7.2). At the borders between two segments, two columns of pixels are copied from each segment to the other. Thus these are named Copied pixels. On both sides of the outer segments, two columns of empty Dummy Pixels are attached. In addition, 16 columns of H-Over-Clocked pixels are attached to each segment.

Actual pixel locations on the chip are calculated from the RAW XY coordinates and the segment ID during ground processing. The coordinates describing the actual pixel location on the chip are named ACT X and ACT Y coordinates (cf. Fig. 7.2). It is important to note that the RAW XY to ACT XY conversion depends on the on-board data processing mode (cf. § 7.4).

7.3 Pulse Height Determination, Residual Dark-current Distribution, and Hot Pixels

When a CCD pixel absorbs an X-ray photon, the X-ray is converted to an electric charge, which in turn produces a voltage at the analog output of the CCD. This voltage (``pulse-height'') is proportional to the energy of the incident X-ray. In order to determine the true pulse-height corresponding to the input X-ray energy, it is necessary to subtract the Dark Levels and correct possible optical Light Leaks.

Dark Levels are non-zero pixel pulse-heights caused by leakage currents in the CCD. In addition, optical and UV light may enter the sensor due to imperfect shielding (``light leak''), producing pulse heights that are not related to X-rays. In the case of the ASCA SIS, these were handled via a single mechanism: Dark Levels of $16\times16$ pixels were sampled and their (truncated) average was calculated for every exposure. Then the same average Dark Level was used to determine the pulse-height of each pixel in the sample. After the launch of ASCA, it was found that the Dark Levels of different pixels were actually different, and their distribution around the average did not necessarily follow a Gaussian. The non-Gaussian distribution evolved with time (referred to as Residual Dark-current Distribution or RDD), and resulted in a degradation of the energy resolution due to incorrect Dark Levels.

For the Astro-E2 XIS, Dark Levels and Light Leaks are calculated separately in normal mode. Dark Levels are defined for each pixel; those are expected to be constant for a given observation. The PPU calculates the Dark Levels in the Dark Initial mode (one of the special diagnostic modes of the XIS); those are stored in the Dark Level RAM. The average Dark Level is determined for each pixel, and if the dark level is higher than the hot-pixel threshold, this pixel is labeled as a hot pixel. Dark Levels can be updated by the Dark Update mode, and sent to the telemetry by the Dark Frame mode. Unlike the case of ASCA, Dark Levels are not determined for every exposure, but the same Dark Levels are used for many exposures unless they are initialized or updated. Analysis of the ASCA data showed that Dark Levels tend to change mostly during the SAA passage of the satellite. Dark Update mode may be employed several times a day after the SAA passage.

Hot pixels are pixels which always output over threshold pulse-heights even without input signals. Hot pixels are not usable for observation, and their output has to be disregarded during scientific analysis. The ASCA SIS did not identify hot pixels on-board, and all the hot pixel data were telemetered and removed during the data analysis procedure. The number of hot pixels increased with time, and eventually occupied significant parts of the telemetry. In the case of XIS, hot pixels are detected on-board by the Dark Initial mode, and their positions and pulse-heights are stored in the Hot-pixel RAM and sent to the telemetry. Thus, hot pixels can be recognized on-board, and they are excluded from the event detection processes. It is also possible to specify the hot pixels manually.

The Light Leaks are calculated on board with the pulse height data after the subtraction of the Dark Levels. A truncated average is calculated for $64\times64$ pixels in every exposure and its running average produces the Light Leak. Thus, the Light Leak is basically same as the Dark Level in ASCA SIS.

The Dark Levels and the Light Leaks are merged in the parallel-sum (P-Sum) mode, so Dark Update mode is not available in P-Sum mode. The Dark Levels, which are defined for each pixel as the case of the normal mode, are updated every exposure. It may be considered that the Light Leak is defined for each pixel in P-Sum mode.

7.4 On-board Event Analysis

The main purpose of the on-board processing of the CCD data is to reduce the total amount transmitted to ground. For this purpose, the PPU searches for a characteristic pattern of charge distribution (called an event) in the pre-processed (post- Dark Levels and Light Leaks subtraction) frame data. When an X-ray photon is absorbed in a pixel, the photoionized electrons can spread into at most 4 adjacent pixels. An event is recognized when a valid pulse-height (one between the Event Lower and Upper Thresholds) is found that exceeds the pulse-heights in the eight adjacent pixels (e.g. it is the peak value in the $3 \times 3$ pixel grid). In P-Sum mode, only the horizontally adjacent pixels are considered. The Copied and Dummy pixels ensure that the event search is enabled on the pixels at the edges of each segment. Again, in the case of P-Sum mode only the inner one of the two columns of Copied or Dummy pixels on each side of the Segment is necessary and used. The RAW XY coordinates of the central pixel are considered the location of the event. Pulse-height data for the adjacent $5 \times 5$ square pixels (or in P-Sum mode 3 horizontal pixels) are sent to the Event RAM as well as the pixel location.

The MPU reads the Event RAM and edits the data to the telemetry format. The amount of information sent to telemetry depends on the editing mode of the XIS. All the editing modes (in normal mode; see §7.5) are designed to send the pulse heights of at least 4 central pixels of an event to the telemetry, because the charge cloud produced by an X-ray photon can spread into at most 4 pixels. Information of the surrounding pixels may or may not output to the telemetry depending on the editing mode. The $5 \times 5$ mode outputs the most detailed information to the telemetry, i.e. all 25 pulse-heights from the $5 \times 5$ pixels containing the event. The size of the telemetry data per event is reduced by a factor of 2 in $3 \times 3$ mode, and another factor of 2 in $2 \times 2$ mode. Details of the pulse height information sent to the telemetry are described in the next section.

7.5 Data Processing Modes

There are two different kinds of on-board data processing modes. The Clock modes describe how the CCD clocks are driven, and determine the exposure time, exposure region, and time resolution. The Clock modes are determined by a kind of program loaded to the AE. The Editing modes specify how detected events are edited, and determine the formats of the XIS data telemetry. Editing modes are determined by the digital electronics.

It is possible to select different mode combinations for the four XISs independently. However, we expect that most observations will use all four in Normal 5 $\times$ 5 or 3 $\times$ 3 Mode (without Burst or Window options). Other modes are useful for bright sources (when pile-up or telemetry limitations are a concern) or if a higher time resolution ($<$8 s) is required.

7.5.1 Clock Modes

The following two kinds of Clock Modes are available. Furthermore, two options (Window and Burst options) may be used in combination with the Normal Mode.

• Normal Mode: If neither Window nor Burst option (see below) is specified, the exposure time is 8 seconds, and all the pixels on the CCD are read out every 8 seconds. This can be combined with either of the 5 $\times$ 5, 3 $\times$ 3, and 2 $\times$ 2 Editing modes.

• Parallel Sum Mode: The pixel data from multiple rows are summed in the Y-direction on the CCD, and the sum is put in the Pixel RAM as a single row. The number of rows to add is commandable. Parallel Sum mode can be used only with the Timing Editing mode, and the Y coordinate is used to determine the event arrival time. As a result, no spatial resolution is available in the Y-direction. The time resolution of the Parallel Sum Mode is 8 s/1024 $\sim$ 7.8 ms.

7.5.2 Window and Burst Options

The following table indicates how the effective area and exposure time are modified by the Burst and Window options.

Option	Effective area	Exposure time
	(nominal: $1024 \times 1024$ pixels)	(in 8 s period)
None	$1024 \times 1024$ pixels	8 s
Burst	$1024 \times 1024$ pixels	$(n/256)\times8$ s $\times$ 1 exposure
Window	$256\times1024$ pixels	2 s $\times$ 4 exposures
	$128\times1024$ pixels	1 s $\times$ 8 exposures
	$64\times1024$ pixels	0.5 s $\times$ 16 exposures
Burst & Window	$256\times1024$ pixels	$(n/64)\times2$ s $\times$ 4 exposure
	$128\times1024$ pixels	$(n/32)\times1$ s $\times$ 8 exposure
	$64\times1024$ pixels	$(n/16)\times0.5$ s $\times$ 16 exposure
Note: $n$ is an integer.

In the Normal Clock mode, the Window and Burst options can modify the effective area and exposure time, respectively. The two options are independent, and may be used simultaneously. These options cannot be used with the Parallel Sum Clock mode.

• Burst Option: All the pixels are read out every 8 seconds (if the Window option is not specified), but the exposure time can be chosen arbitrarily (with a 1/32 second step) within the read-out interval. This option may be used to avoid event pile-up when observing a bright source. However, a dead time is introduced in the exposure. If the exposure is $t$ s, there is a $8-t$ s dead-time every 8 s.

• Window Option: This option allows shorter exposure times by reading out more frequently only a portion of the CCD. Only the parts of the chip within the Y-direction range specified by the commandable Window is used for exposure (cf. Fig. 7.2). The Window width in the Y-direction is either 256, 128 or 64 pixels, and its position is arbitrary. When the Window width is 256 pixels, the exposure time becomes a quarter of that without the Window option (i.e. 2 s), and the Pixel RAM is filled with the data from four successive exposures. Similarly, when the Window width is 128 or 64 pixels, the exposure time becomes 1/8 or 1/16 of that without the Window option respectively, and the Pixel RAM is filled with the data from 8 or 16 successive exposures.

We show in Fig. 7.3 the time sequence of exposure, frame-store transfer, CCD readout, and storage to the pixel RAM (in PPU) in normal mode with or without Burst/Window option. Note that a dead time is introduced when the Burst option is used.

Figure 7.3: Time sequence of the exposure, frame-store transfer, CCD readout, and data transfer to the pixel RAM in PPU is shown (1) in normal mode without options, (2) in normal mode with Burst option, and (3) in normal mode with Window option. In this example, the 1/4 Window option is assumed.

7.5.3 Editing Modes

We explain only the observation modes here. Three modes ($5 \times 5$, $3 \times 3$, and $2 \times 2$) are usable in normal modes, and only the timing mode in the P-Sum mode.

Observation Modes

• $5 \times 5$ mode: All the pulse heights of the 25 pixels centered at the event center are sent to the telemetry. This is used with the Normal Clock mode.

• $3 \times 3$ mode: Pulse heights of the 9 pixels centered at the event center are sent to the telemetry with the 1-bit information (pulse height larger than the Split Threshold Outer or not) for the surrounding 16 pixels. This is used with the Normal Clock mode.

• $2 \times 2$ mode: Pulse heights of the $2 \times 2$ square pixels are sent to the telemetry. The $2 \times 2$ pixels are selected to include the event center, second highest pixel in the cross centered at the event center, and the 3rd (or 4th) highest pixel in the cross. The 1-bit information (pulse height larger than the Split Threshold Outer or not) of the 8-attached pixels is also output to the telemetry. This is used with the Normal Clock mode.

• Timing mode: Total pulse height and the Grades of the event are output to the telemetry. Pulse heights of at most three pixels in the X-direction are summed to give the total pulse height if they are over the Inner Split Threshold. Position and number of pixels exceeding the threshold determines the Grades. This is used only with the P-Sum Clock mode. Window and Burst Options are not available in the Timing mode.

We show in Fig. 7.4 the pixel pattern whose pulse height or 1-bit information is sent to the telemetry. We do not assign grades to an event on board in the Normal Clock mode. This means that a dark frame error, if present, can be corrected accurately during the ground processing even in 2 $\times$ 2 mode. The definition of the grades in P-Sum mode is shown in Fig. 7.5.

The current expectation is that there will be no differences between the $5 \times 5$, $3 \times 3$ and $2 \times 2$ modes. However, we recommend the use of the $5 \times 5$ mode, or $3 \times 3$ mode, whenever this is possible within the telemetry limit (which is the case for all but the brightest sources), just in case the extra information might make a difference in reducing some unforeseen systematic errors. With the Chandra CCDs, for example, $5 \times 5$ mode has been used to aid in identifying cosmic ray events.

Besides the observation modes given above, the XIS instrument has several diagnostic modes, used primarily in determining the dark current levels. It is unlikely that those would be used by guest observers.

Figure 7.4: Information sent to the telemetry is shown for $5 \times 5$, $3 \times 3$, and $2 \times 2$ modes. 1-bit information means whether or not the PH of the pixel exceeds the outer split threshold. In $2 \times 2$ mode, the central 4 pixels are selected to include the second and the third (or fourth) highest pixels among the 5 pixels in a cross centered at the event center.

Figure 7.5: Definition of the grades in the P-Sum/timing mode. Total pulse height and the grade of the event are output to the telemetry. Note that the grades are defined referring to the direction of the serial transfer, so the central pixel of a grade 1 event has the larger RAWX value, while the opposite is true for a grade 2 event.

7.5.4 Discriminators

Two kinds of discriminators, area and grade discriminators, can be applied during the on-board processing. The grade discriminator is available only in the timing mode.

The area discriminator is used when we want to reject some (or most) of the frame data from the event extraction. The discriminator works on the Pixel RAM. When the discriminator is set, a part of the Pixel RAM is not used for the event extraction. This may be useful when a bright source is present in the XIS field of view other than the target source. If we set the discrimination area to include only the bright source, we can avoid outputting unnecessary events to the telemetry. Only a single, rectangular area can be specified in a segment for discrimination. Either inside or outside of the area can be rejected from the event extraction. The area discriminator works on the Pixel RAM, not for the physical area of the CCD. This is important when we apply the discriminator with the window option.

The Grade discriminator is used only in the timing mode. Any combination of the 4 grades can be selected to discriminate the grade for telemetry output.

We do not have the level discriminator which was used in ASCA SIS. The same function can be realized, however, by changing the event threshold.

7.6 Photon pile-up

The XIS is essentially a position-sensitive integrating instrument, with the nominal interval between readouts of 8 s. If during the integration time one or more photons strike the same CCD pixel, or one of its immediate neighbors, these cannot be correctly detected as independent photons: this is the phenomenon of photon pile-up. Here, the modest angular resolution of the Astro-E2 XRT is an advantage: the central $3 \times 3$ pixel area receives 2% of the total counts of a point source, and $\sim$10% of the counts fall within $\sim$0.15 arcmin of the image center. We calculated the count rate at which 50% of the events within the central $3 \times 3$ pixels are piled-up (the pile-up fraction goes down as we move out of the image center; this fraction is $<$5% for the 0.15 arcmin radius) -- although we offer no formal justification for this particular limit, this is compatible with our ASCA SIS experience (i.e., at this level, the pile-up effects do not dominate the systematic uncertainties). This limit corresponds to $\sim$10 cps for a point source observed in the normal mode (full window). For somewhat brighter sources, window options can be used to reduce the exposure time per frame (the count rate limit is inversely proportional to the exposure time -- 1/8 window option reduces the exposure time from 8 s to 1 s, and raises the limit to $\sim$80 cps). For even brighter sources, timing mode is recommended: because of the extremely short effective exposure time (8 s/1024 $\sim$ 7.8 ms), the pile-up limit is several thousand cps (despite the on-board summing of rows and the one dimensional nature of the event detection algorithm).

From the standpoint of Astro-E2 proposals, prior to launch, the most appropriate mode can only be estimated; again, we currently believe that a source yielding a total XIS counting rate of 10 counts/s/XIS or less can be observed using the ``normal'' mode. We emphasize that the Astro-E2 personnel at ISAS/JAXA or the NASA Astro-E2 GOF will work with the observers to assure the optimum yield of every observation via selection of the best XIS mode for a given target.

7.7 Expected background rate and the telemetry limit

In the case of the XIS, most of the background originates from charged particles (the non-X-ray background, or NXB). The cosmic X-ray background (CXB) rate is much smaller and the flickering pixels are negligible. However, the NXB rate may be much higher compared to ASCA SIS, because XIS has larger CCD area and thicker CCD substrate. We estimated the NXB rate of XIS by referring to the ASCA SIS and Chandra ACIS data.

Telemetry limits of FI CCD (counts/XIS/8s)
Data rate	5 $\times$ 5	3 $\times$ 3	2 $\times$ 2
Superhigh	1950 (2100)	4040 (4190)	8240 (8380)
High	850 (1000)	1850 (2000)	3860 (4010)
Medium	310 (460)	770 (910)	1680 (1830)
Low	-- (46)	-- (92)	40 (180)
Note: the values in the parenthesis are telemetry limits when no background is present. An event threshold of 100 adu (0.36 keV) is assumed.

The above table shows the estimated telemetry limits of the FI CCD in various editing modes and the telemetry data rates. The NXB rate of the BI CCD may be higher than the FI CCD. The estimated NXB rate in the BI CCD is not available as of the writing of this document; see the Astro-E2 websites for updated information (see App. C). The low telemetry data rate will not be usable with a BI CCD. When we calculate the telemetry limits, we assumed a nominal telemetry allocation ratio among XRS, XIS, and HXD. The ratio depends on the data rate and may be changed after the launch. The telemetry limits also depend on the event threshold and the data compression efficiency. We apply a simple data compression algorithm to the event data, whose efficiency may depend on the energy spectrum of the source. Thus the telemetry limits listed above should be regarded as only approximate values.

7.7.1 Out-of-time events

X-ray photons detected during the frame-store transfer do not correspond to the true image, but instead appear as a streak or blur in the readout direction. These events are called out-of-time events., and they are an intrinsic feature of CCD detectors. Similar streaks are seen from bright sources observed with Chandra and XMM-Newton. Out-of-time events produce a tail in the image, which can be an obstacle to detecting a low surface brightness feature in an image around a bright source. Thus the out-of-time events reduce the dynamic range of the detector. Since XIS spends 25 ms in the frame-store transfer, about 0.3% ( $=0.025/8\times100$) of all events will be out-of-time events. However, because the orientation of the CCD chip is different among the sensors, one can in principle distinguish a true feature of low surface brightness and the artifact due to the out-of-time events by comparing the images from two or more XISs.

7.8 Radiation Damage and On-board Calibration of the XIS

The performance of X-ray CCDs gradually degrades in the space environment due to the radiation damage. This generally causes an increase in the dark current and a decrease of the charge transfer efficiency (CTE). In the case of XIS, the increase of the dark current is expected to be small due to the low ($-\!90^\circ$C) operating temperature of the CCD. However, a decrease in CTE is unavoidable. Thus, continuous calibration of CCD on orbit is essential to the good performance of the XIS. For this purpose, we use a radio isotope source and charge injection as explained below:

(i) Each XIS carries $^{55}$Fe calibration sources near the two corners of the chip, which will be used to monitor the instrument gain.

(ii) Each XIS CCD is equipped with charge injection capability, which may be useful to measure and even suppress CTI.

It is difficult to predict before launch how well we can calibrate the long-term performance change of XIS on orbit. However, the XIS may be assumed to perform at the nominal pre-launch level for the purpose of Cycle 1 proposals.