6. ACIS : Advanced CCD Imaging Spectrometer

The Advanced CCD Imaging Spectrometer (ACIS ) offers the capability to simultaneously acquire high-resolution images and moderate resolution spectra. The instrument can also be used in conjunction with the High Energy Transmission Grating (HETG ) or Low Energy Transmission Grating (LETG ) to obtain higher resolution spectra (see Chapters 8 and 9). ACIS contains 10 planar, 1024 x 1024 pixel CCDs (Figure 6.1); four arranged in a 2x2 array (ACIS-I ) used for imaging, and six arranged in a 1x6 array (ACIS-S ) used either for imaging or as a grating readout. Any combination of up to 6 CCDs may be operated simultaneously. If ACIS-I is selected in ''imaging'' mode, chips I0-I3 plus S2 and S3 are used. If ACIS-S is selected in the ''imaging'' mode, chips S0-S3 plus I2 and I3 are used (see Figure 6.2). Operating six chips enhances the chance of serendipitous science but at the price of increased total background counting rate and therefore a somewhat enhanced probability of saturating telemetry. Two CCDs are back-illuminated (BI ) and eight are front-illuminated (FI ). The response of the BI devices extends to energies below that accessible to the FI chips. The chip-average energy resolution of the BI devices is better than that of the FI devices.

**Figure 6.1:** A schematic drawing of the ACIS focal plane; insight to the terminology is given in the lower left. Note the nominal aimpoints: on S3 (the `+') and on I3 (the `x'). On S3, it has become standard practice to add an observatory Y-offset of -20 $\arcsec$ (41 pixels) from the true aimpoint (252,510) in the direction of S4 for all ACIS-S observations in order to move the source flux away from the node 0-1 boundary. Note the differences in the orientation of the I and S chips, important when using Subarrays (Section 6.11.3). Note also the (Y, Z) coordinate system and the target offset convention (see Chapter 3) as well as the SIM motion (+/-Z). The view is along the optical axis, from the source toward the detectors, (-X). The numerous ways to refer to a particular CCD are indicated: chip letter+number, chip serial number, and ACIS chip number. The node numbering scheme is illustrated lower center.
$\scalebox{0.6}{\includegraphics{../../ogplots/acis_flight_focal_plane.eps}}$

**Figure 6.2:** A schematic drawing of the ACIS focal plane, not to scale. The ACIS-I array consists of chips I0-I3. By default, chips I0-I3 plus S2 and S3 are turned on when ACIS-I is selected, as indicated in the top figure. ACIS-S consists of chips S0-S5. When operated in imaging mode chips S1-S4 plus I2 and I3 are turned on (bottom figure). When operated in spectroscopy mode S0-S6 are turned on.
$\scalebox{0.5}{\includegraphics{../../ogplots/acis_def_chips.eps}}$

ACIS is a complex instrument having many different characteristics and operating modes. Radiation damage suffered by the FI chips has had a negative impact on their energy resolution - the BI devices were not impacted - thus impacting the basic considerations as to how to make best use of the instrument (see Section 6.7.) We discuss the trade-offs in this Chapter. Software methods for improving the energy resolution of the FI CCDs are discussed in Section 6.7.1. The low energy response of ACIS has also been affected by the build-up of a contaminant on the optical blocking filter and this is discussed in Section 6.5.2.

Table 6.1: ACIS Characteristics

Focal plane arrays:
I-array	4 CCDs placed to lie tangent to the focal surface
S-array	6 CCDs in a linear array tangent to the grating Rowland circle
CCD format	1024 by 1024 pixels
Pixel size	24.0 microns (0.4920 $\pm$ 0.0001 arcsec)
Array size	16.9 by 16.9 arcmin ACIS-I
	8.3 by 50.6 arcmin ACIS-S
On-axis effective Area	$110\rm\,cm^2$ @ 0.5 keV (FI )
(integrated over the PSF	$600\rm\,cm^2$ @ 1.5 keV (FI )
to $\>$ 99% encircled energy)	$40\rm\,cm^2$ @ 8.0 keV (FI )
Quantum efficiency	$>80\%$ between 3.0 and 5.0 keV
(frontside illumination)	$>30\%$ between 0.8 and 8.0 keV
Quantum efficiency	$>80\%$ between 0.8 and 6.5 keV
(backside illumination)	$>30\%$ between 0.3 and 8.0 keV
Charge transfer inefficiency(parallel)	FI : $\sim$ 2 $\times$ 10 $^{-4}$ ; BI : $\sim$ 2 $\times$ 10 $^{-5}$
Charge transfer inefficiency(serial)	S3(BI ): $\sim$ 7 $\times$ 10 $^{-5}$ ; S1(BI ): $\sim$ 1.5 $\times$ 10 $^{-4}$ ; FI: unmeasurable
System noise	$<\sim2$ electrons (rms) per pixel
Max readout-rate per channel	$\sim100$ kpix/sec
Number of parallel signal channels	4 nodes per CCD
Pulse-height encoding	12 bits/pixel
Event threshold	FI : 38 ADU ( $\sim$ 140 eV)
	BI : 20 ADU ( $\sim$ 70 eV)
Split threshold	13 ADU
Max internal data-rate	6.4 Mbs ( kbs $\times 4 \times 16$ )
Output data-rate	24 kb per sec
Minimum row readout time	2.8 ms
Nominal frame time	3.2 sec (full frame)
Allowable frame times	0.2 to 10.0 s
Frame transfer time	40 ${\mu}$ sec (per row)
Point-source sensitivity	$4 \times 10^{-15}\rm ergs\,cm^{-2}\,s^{-1}\ in \ 10^4\, s$
	(0.4-6.0 keV)
Detector operating temperature	to $-120^\circ$ C

6.2 Basic Principles

A CCD is a solid-state electronic device composed primarily of silicon. A ``gate'' structure on one surface defines the pixel boundaries by alternating voltages on three electrodes spanning a pixel. The silicon in the active (depletion) region (the region below the gates wherein most of the absorption takes place) has an applied electric field so that charge moves quickly to the gate surface. The gates allow confined charge to be passed down a ``bucket brigade'' (the buried channel) of pixels in parallel to a serial readout at one edge by appropriately varying (``clocking'') the voltages in the gates.

The ACIS front-illuminated CCDs have the gate structures facing the incident X-ray beam. Two of the chips on the ACIS-S array (S1 and S3) have had treatments applied to the back sides of the chips, removing insensitive, undepleted, bulk silicon material and leaving the photo-sensitive depletion region exposed. These are the BI chips and are deployed with the back side facing the HRMA .

Photoelectric absorption of an X-ray in silicon results in the liberation of a proportional number of electrons (an average of one electron-hole pair for each 3.7 eV of photon energy absorbed). Immediately after the photoelectric interaction, the charge is confined by electric fields to a small volume near the interaction site. Charge in an FI device can also be liberated below the depletion region (in an inactive substrate) from where it diffuses into the depletion region. This charge may easily appear in two or more pixels.

Good spectral resolution depends upon an accurate determination of the total charge deposited by a single photon. This in turn depends upon the fraction of charge collected, the fraction of charge lost in transfer from pixel to pixel during read-out, and the ability of the readout amplifiers to measure the charge. Spectral resolution also depends on read noise and the off-chip analog processing electronics. The ACIS CCDs have readout noise less than 2 electrons RMS. Total system noise for the 40 ACIS signal chains (4 nodes/CCD ) ranges from 2 to 3 electrons (rms) and is dominated by off-chip analog processing electronics.

The CCDs have an ``active'' or imaging Section (see Figure 6.1) which is exposed to the incident radiation and a shielded ``frame store'' region. A typical mode of the ACIS CCD operation is: (1) the active region is exposed for a fixed amount of time (full frame $\sim3.2$ s); (2) at the end of the exposure, the charge in the active region is quickly ( $\sim41$ ms) transferred into the frame store; (3) the next exposure begins; (4) simultaneously, the data in the frame store region is passed to a local processor which, after removing bias (the amount of charge in a pixel in the absence of any X-ray induced signal), identifies the position and amplitude of any ``events'' according to a number of criteria depending on the precise operating mode. These criteria always require a local maximum in the charge distribution above the event threshold (see Table 6.1). The position and the amount of charge collected, together with similar data for a limited region containing and surrounding the pixel are classified (``graded'') and then passed into the telemetry stream.

6.3 Optical Blocking Filter & Optical Contamination

Since the CCDs are sensitive to optical as well as X-ray photons, optical blocking filters (OBFs) are placed just over the CCDs between the chips and the HRMA . The filters are composed of polyimide (a polycarbonate plastic) sandwiched between two thin layers of aluminum. The nominal thicknesses of the these components for the two arrays are given in Table 6.2. Details of the calibration of these filters may be found in the ACIS calibration report at http://www.astro.psu.edu/xray/docs/cal_report/node188.html. These calibrations do not include the more recent effects of molecular contamination. This is discussed in Section 6.5.2.

**Table 6.2:** Nominal Optical Blocking Filter Composition and Thicknesses
ACIS-I	Al/Polyimide/Al	1200Å 2000Å 400Å
ACIS-S	Al/Polyimide/Al	1000Å 2000Å 300Å

The threshold for optical contamination (a 1 ADU (3.4 eV) shift in the bias level) is based on on-orbit calibrations of a number of stars with different optical spectra. The threshold for detectable visible light contamination varies according to source color and is lowest for red stars observed on ACIS-S The detection threshold for an M star on the ACIS-S array is V $\sim$ 8.1 for the nominal 3.2 second exposure or V $\sim$ 6.3 using a 0.4s frame time and a 1/8 chip subarray. The thresholds are about 5 visual magnitudes brighter for the ACIS-I array.

6.4 Calibration

Calibration of ACIS includes laboratory calibrations, a system-level ground calibration of the HRMA and ACIS at the X-Ray Calibration Facility (XRCF) at MSFC, and on-orbit calibration using celestial and on-board radioactive X-ray sources.

The on-orbit calibration of ACIS is an on-going activity. All calibration data are, or will be, described in detail, at http://cxc.harvard.edu/cal/

The user is urged to consult the WWW site and its pointers for the latest information.

6.5 Quantum Efficiency and Effective Area

The chip-averaged quantum efficiencies for the ACIS CCDs for the standard grade set, including the optical blocking filters, are shown in Figure 6.3. Note that the quantum efficiency for the FI chips varies somewhat with row number (not shown), and decreases by $\sim$ 5-15% farthest from the readout at energies above 4 keV. This is due to the migration of good grades to bad grades produced by charge transfer inefficiency which varies with the row number. The QE variation with row number for the BI chips is much smaller. A recent re-analysis of ground calibration data suggests that the of S3 may be been underestimated by about 10% below 1keV; if confirmed, Figure 6.3 (and the calibration data base - CALDB) will be revised accordingly.

**Figure 6.3:** The quantum efficiency, convolved with the transmission of the appropriate optical blocking filter, of the FI CCDs (from a row nearest the readout) and the two BI CCDs as a function of energy. S3 is somewhat thicker, hence more efficient, than S1.
$\scalebox{0.7}{\includegraphics{../../ogplots/acis_qe_allchips_obf.ps}}$

**Figure:** The **HRMA** /ACIS predicted effective area versus the energy on a log scale. The dashed line is for the FI CCD I3, and the solid line is for the BI CCD S3.
$\begin{figure}\centering \scalebox{0.6}{\rotatebox{0}{\includegraphics{../../ogplots/acis_EfA_FIBI_logx.ps}}}\end{figure}$

**Figure:** The **HRMA** /ACIS predicted effective area versus the energy on a linear scale. The dashed line is for the FI CCD I3, and the solid line is for the BI CCD S3.
$\begin{figure}\centering \scalebox{0.6}{\rotatebox{0}{\includegraphics{../../ogplots/acis_EfA_FIBI_linx.ps}}} \end{figure}$

**Figure 6.6:** Vignetting (the ratio of off-axis to on-axis effective area) as a function of energy for several off axis angles in arcminutes.
$\scalebox{0.5}{\includegraphics{../../ogplots/acis_vignetting.ps}}$

6.5.1 BI Response Matrices

The response matrices for the BI CCDs have been updated since their pre-launch versions. The new version of the S3 matrix for -120 C was released on August 7, 2001 in the CALDB 2.7 and was accompanied by an updated version of CIAO (2.1.3). Observers are encouraged to read the release notes on the CXC web page. The new version of the S3 matrix has an improved model of the spectral redistribution function, including the low-energy tail and the ``shelf'' feature of a typical pulse-height distribution, and an improved calibration of the gain at energies below 1 keV. No change (as yet) has been made to the in the for the S3 CCD . The release notes web page includes sample fits to various spectra with the old and new matrices.

6.5.2 Molecular Contamination of the OBFs

Astronomical observations and data acquired from the on-board ACIS calibration source (the External Calibration Source or ) show that there has been a slow continuous degradation in the ACIS effective area since launch. Our best interpretation is that this is due to a thin layer of molecular material on the outward-facing side (i.e. toward the HRMA ) of the Optical Blocking Filter (). The HRC shows no sign of contamination. The degradation is most severe at low energies; the effective above 1keV has changed by less than 10%. The rate at which material is deposited is still under investigation. Data from the which covers energies well above the carbon edge indicates that the rate at which material is accumulating is slowing, but measurements of the C-K edge suggests the thickness is increasing linearly with time. Additionally, the spatial distribution of the coating is non-uniform. It appears to be thicker at the edges of the where the filter is coldest, and thinner in the center where the filter is warmer. Grating observations show that the contaminant is dominated by carbon, with smaller amounts of oxygen and fluorine.

Two models of the contaminant have been developed: and . Both of these models attempt to characterize the loss of efficiency with time. is based on the ratio of the Mn L/K data from the , while is derived from studying the C-K edge using the LETG . The two models give slightly different values for the optical depth of the contamination - the disagreement being worse at 700eV, where the discrepancy is 8%. This discrepancy may arise because the data measures the optical depth over the whole array, whereas the LETG data measures the absorption at a single point on the detector. If the contaminant is non-uniform on small scales (fluffy or clumpy) the average value of the optical depth may be different from the local value. Both models can be used to correct the instrument responses used in X-ray spectral fitting programs such as Sherpa or XSPEC . All of the effective area curves in this volume (and those accessed by PIMMS ) use the model to calculate the effective for the middle of Cycle 6 (15 May 2005). Proposers should use these curves in feasibility calculations.

A team of scientists and engineers (including representatives from the ACIS instrument team, the CXC , Chandra Project Science at MSFC , and the spacecraft contractors at North Grumman Space Technology (NGST)) is currently evaluating a proposal to evaporate the contaminant by raising the temperature of the ACIS (``bakeout''). Simulations suggest that this might be effective at removing the contaminant from the , however the effect of bakeout on the needs to be more fully investigated. An announcement will be placed on the CXC webpage if a bakeout is scheduled during the Cycle 6 observing period.

6.6 Spatial Resolution & Encircled Energy

The spatial resolution for on-axis imaging with ACIS is limited by the physical size of the CCD pixels (24.0 ${\mu}$ m square $\sim$ 0.492 arcsec) and not the HRMA . This limitation applies regardless of whether the aimpoint is selected to be the nominal point on I3 or S3 (Figure 6.1). Approximately 90% of the encircled energy lies within 4 pixels (2 arcsec) of the center pixel at 1.49 keV and within 5 pixels (2.5 arcsec) at 6.4 keV. Figure 6.7 shows an in-flight calibration. There is no evidence for any differences in data taken with either S3 or I3 at the nominal focus. The ACIS encircled energy as a function of off-axis angle is discussed in Chapter 4 (see Section 4.2.2 and Figure 4.10).

**Figure 6.7:** The on-orbit encircled broad-band energy versus radius for an ACIS observation of point source PG1634-706. The effective energy is 1 keV.
$\begin{figure}\centering \scalebox{0.5}{\rotatebox{0}{\includegraphics{../../ogplots/encir_power_pg1634.ps}}} \centering \end{figure}$

Off-axis, the departure of the CCD layout from the ideal focal surface and the increase of the HRMA PSF with off-axis angle become dominating factors. Since the ideal focal surface depends on energy, observers, for whom such considerations may be important, are urged to make use of the MARX simulator to study the impact on their observation.

6.7 Energy Resolution

The ACIS FI CCDs originally approached the theoretical limit for the energy resolution at almost all energies, while the BI CCDs exhibited poorer resolution. The pre-launch energy resolution as a function of energy is shown in Figure 6.10. Subsequent to launch and orbital activation, the energy resolution of the FI CCDs has become a function of the row number, being near pre-launch values close to the frame store region and substantially degraded in the farthest row. An illustration of the dependence on row is shown in Figure 6.11.

**Figure 6.10:** The ACIS pre-launch energy resolution as a function of energy. (Source: *CXC* Calibration group).
$\scalebox{0.5}{\includegraphics{../../ogplots/acis_en_res_FWHM.ps}}$

**Figure 6.11:** The energy resolution of S3 and I3 as a function of row number. These data were taken at $-120^\circ$ C. Note that these curves are representative of the variation - but they do not account for the row-dependent gain variation which also increases the energy resolution by about 10-15%, especially for the larger row numbers.
$\scalebox{0.5}{\includegraphics{../../ogplots/acis_fwhm_all_120C.ps}}$

6.7.1 Correcting the Energy Resolution of the FI CCDs

The ACIS instrument team has developed a correction algorithm for the FI CCDs . The correction significantly improves the spectral resolution of the FI CCDs at all energies. Observers analyzing data from extended sources or across the ACIS-I array may find it advantageous to apply this correction. This correction for the effects of the row-dependent CTI has been incorporated in acis_process_events as of CIAO 2.3. An example of the application of the CXC -corrector is shown in Figure 6.12.

**Figure:** An example of the application of the *CXC* CTI-corrector in two energy bands. The first plot shows data for the Al-K line, and the second for Mn-K. The square data points are the FWHM of lines accumulated in 32-row bins. The triangle data points are the FWHM of CTI-corrected lines accumulated in 32-row bins. The line shows the nominal performance of the S3 chip. This data is from March 2000.

6.8 Hot Pixels and Columns

Hot columns and pixels are defined to be those which produce a high spurious or saturated pulse-height for a large number of consecutive frames of data. These depend on operating conditions such as temperature. One should always refer to the CXC web site for the most recent list. To date, S1 is the device with the largest number of such pixels and columns.

**Figure 6.13:** MARX simulations of the effect of pileup on the shape of the spectrum. The true (solid line) and the detected (dotted line) spectra are shown for four different viewing angles. The corresponding ``pileup fractions'' - see Section 6.14.2 - are 46%, 40%, 15%, and 2% as the image is moved progressively further off-axis. (Source: J. Kastner and M. Wise, *CXC* )
$\scalebox{0.9}{\includegraphics {../../ogplots/acis_offaxis_pileup.eps}}$

6.9 Aimpoints

Aimpoints are the nominal positions on the ACIS where the flux from a point source with zero commanded target offsets is placed. There are two nominal aimpoints, indicated in Figure 6.1 - one on the corner of I3 on the ACIS-I array (the ACIS-I aimpoint), and one near the boundary between nodes 0 and 1 on S3 of the ACIS-S array (the ACIS-S aimpoint). Their exact positions are given in Table 6.3. Note that the aimpoint is not the same as the on-axis position, which is defined as the position of the narrowest PSF and found approximately 20'' from the aimpoints. For zero SIM-Z offsets, the optical axis crosses ACIS-I at the I3 chip coordinates (984,995), and ACIS-S , at the S3 chip coordinates (221,532).

Approximate contours of constant encircled energy for ACIS-I and ACIS-S observations for the default aimpoints are shown in Figures 6.8 and 6.9. If required, other aimpoints can be selected along the Z-axis.

It is important to note that it has become standard practice to add an observatory Y-offset= $\Delta$ Y=-20 $\arcsec$ =41 pixels, in the direction of S4, for all ACIS-S observations in order to assure that the dithered flux from a (now almost) on-axis point source is placed entirely on a single node (node 1) of the S3 CCD . This was done both to simplify and improve the analysis of data from a point source by avoiding dealing with the response functions from two nodes. The shift has a negligible impact on image quality because the diameter of the circle, for which $\>$ 90% of the encircled energy lies within 2

, is $\sim$ 2

.4.

**Table 6.3:** Average source positions summarized in pixels (chip-x, chip-y)
ACIS-I :	(962, 964) in I3	no offsets
ACIS-S :	(252, 510) in S3	no offsets
ACIS-S :	(293, 510) in S3	if invoking the suggested -20 $\arcsec$ $\Delta$ Y offset

Note that for some observations, typically those with gratings, it is suggested to translate the SIM such that the S3 aimpoint is shifted in the negative Z direction - toward the readout node (see Table 6.4 for recommended offsets).

**Table 6.4:** Recommended SIM-Z offsets
Observation Mode	SIM-Z Offset	Source Position (w/ -20 $\arcsec$ $\Delta$ Y Offset)
ACIS-S w/ HETG TE mode:	-3mm = -1.02389 $\arcmin$	(293, 385)
ACIS-S w/ HETG CC mode:	-4mm = -1.36519 $\arcmin$	(293, 344)
ACIS-S w/ LETG TE mode:	-8mm = -2.73038 $\arcmin$	(293, 177)
ACIS-S w/ LETG CC mode:	-8mm = -2.73038 $\arcmin$	(293, 177)

These new aimpoints should be kept in mind if selecting the use of subarrays in gratings' observations, since the standard subarrays (see Section 6.11.3) will not center the zeroth order.

Lastly, it should be kept in mind that the observatory is typically dithered about the aimpoint with an 8 $\arcsec$ half-amplitude (see Section 6.10).

6.10 Dither

Unless specially requested, the spacecraft is dithered during all observations. The dither pattern is a Lissajous figure. For observations with ACIS , the dither pattern spans 16 arcsec peak to peak. The dither serves two purposes: (1) to provide some exposure in the gaps between the CCDs , and; (2) to smooth out pixel-to-pixel variations in the response. The dither is removed during high-level ground processing of the data. The exposure time in the gaps between chips (and at the outside edges) will be less than that for the remainder of the field.

6.10.1 Gaps Between the CCDs

6.11 Operating Modes

Note that the selected operating mode (TE/CC) discussed below for the ACIS CCDs applies to all selected CCDs . It is not possible to simultaneously operate individual CCDs in different modes.

6.11.1 Timed Exposure (TE) Mode

A timed exposure refers to the mode of operation wherein a CCD collects data (integrates) for a preselected amount of time - the Frame Time. Once this time interval has passed, the charge from the 1024 x 1024 active region is quickly ( $\sim41$ ms) transferred to the framestore region and subsequently read out through (nominally) 1024 serial registers.

6.11.2 Frame Times - Full Frames

Frame times are selectable within a range of values spanning the time interval from 0.2 to 10.0 seconds. If the data from the entire CCD are utilized (full frame) then the nominal (and optimal!) frame time is 3.2s. Selecting a frame time shorter than the nominal value (e.g. to decrease the probability of pileup - Section 6.14) has the consequence that there will be a time during which no data are taken, as 3.2s are required for the readout process regardless of the frame time. The fraction of time during which data are taken is simply the ratio of the selected frame time to the sum of this and the nominal frame time - e.g. for a new frame time of n (

3.2) secs, the fraction of time during which data are taken is n/(3.2+n). We note, strictly speaking, the full-frame time depends on how many CCD s are on - see the equation in Section 6.11.3 - but the differences are very small. Finally, we note that selecting a frame time longer than the optimum increases the probability of pileup occurring and is not recommended.

6.11.3 Frame Times & Subarrays

It is also possible for one to select a subarray - a restricted region of the CCD in which data will be taken. A subarray is fully determined by specifying the number of rows separating the subarray from the framestore region (q) and the number of rows in the subarray (n). Examples of subarrays are shown in Figure 6.14. The nominal frame time for a subarray depends on (q), (n), and the total number of CCDs that are activated (m) - see Table 6.5. The nominal frame time is given by:

**Table 6.5:** CCD Frame Time (seconds) for Standard Subarrays
Subarray	ACIS-I (no. of chips)	ACIS-S (no. of chips)
	1	6	1	6
1	3.0	3.2	3.0	3.2
1/2	1.5	1.8	1.5	1.8
1/4	0.8	1.1	0.8	1.1
1/8	0.5	0.8	0.4	0.7

**Figure 6.14:** Examples of various subarrays. The heavy dot in the lower left indicates the origin
$\scalebox{0.6}{\includegraphics{../../ogplots/acis_def_subarr.xfig.eps}}$

6.11.4 Trailed Images

It takes 40 ${\mu}$ sec to transfer the charge from one row to another during the process of moving the charge from the active region to the framestore region. This has the interesting consequence that each CCD pixel is exposed, not only to the region of the sky at which the observatory was pointing during the long (frame time) integration, but also, for 40 ${\mu}$ sec each, to every other region in the sky along the column in which the pixel in question resides. Figure 6.15 is an example where there are bright features present, so intense, that the tiny contribution of the flux due to trailing is stronger than the direct exposure - hence the trailed image is clearly visible. Trailed images are also referred to as ``read out artifact'' and ``out-of-time images''. The user needs to be aware of this phenomenon as it has implications for the data analysis - e.g. estimates of the background. In some cases, the trailed image can be used to measure an unpiled spectrum and can also be used to perform 40 microsecond timing analysis (of really bright sources).

**Figure 6.15:** Trailed image of a strong X-ray source. The core of the image is faint due to pileup. Most events here are rejected because of bad grades. The readout direction is parallel to the trail.
$\begin{figure}\centering \scalebox{0.5}{\includegraphics{../../ogplots/acis_trail_image.ps}} \end{figure}$

6.11.5 Continuous Clocking (CC) Mode

The continuous clocking mode is provided to allow 3 msec timing at the expense of one dimension of spatial resolution. In this mode one obtains 1 pixel x 1024 pixel images, each with an integration time of 3 msec. Details as to the spatial distribution in the columns are lost - other than that the event originated in the sky along the line determined by the length of the column.

In the continuous clocking mode, data is continuously clocked through the CCD and framestore. The instrument software accumulates data into a buffer until a virtual detector of size 1024 columns by 512 rows is filled. The event finding algorithm is applied to the data in this virtual detector and 3 x 3 event islands are located and recorded to telemetry in the usual manner. This procedure has the advantage that the event islands are functionally equivalent to data accumulated in TE mode, hence differences in the calibration are minimal. The row coordinate (called CHIPY in the FITS file) maps into time in that a new row is read from the framestore to the buffer every 2.85 msec. This does have some minor impacts on the data. For example, since the event-finding algorithm is looking for a local maximum, it cannot find events on the edges of the virtual detector. Hence CHIPX cannot be 1 or 1024 (as in TE mode). Moreover, CHIPY cannot be 1 or 512. In other words, events cannot occur at certain times separated by 512*2.85 msec or 1.4592 sec. Likewise, it is impossible for two events to occur in the same column in adjacent time bins.

At present, the TIMEs in continuous-clocking mode event data files are the read-out times, not the times of arrival. The differences between the read-out times and the times of arrival are in the range 2.9 to 5.8 s. The differences depend on the nominal location of the source on the CCD and the dither of the telescope. Code to compute the times of arrival at the spacecraft from the read-out times has been developed and will be released as part of the tool acis_process_events.

6.12 Bias Maps and Telemetry Formats

6.12.1 Bias Maps

In general the CCD bias, the amplitude of the charge in each pixel in the absence of external radiation, is determined at various times - every change of mode when ACIS is in place at the focus of the telescope. These bias maps have proven to be remarkably stable and are automatically applied in routine data processing.

The bias maps for continuous-clocking mode observations can be corrupted by cosmic rays. If a cosmic ray deposits a lot of charge in most of the pixels in one or more adjacent columns, the bias values assigned to these columns will be too large. As a result, some low-energy events that would have been telemetered will not be telemetered because they do not satisfy the minimum pulse height criterion and the spectrum of a source in the affected columns will be skewed to lower energies. The BI CCDs are relatively insensitive to the problem. A new bias algorithm has been developed (but is not yet implemented) to mitigate the problem.

6.13 Event Grades

During the first step in the algorithm for detecting X-ray events, the on-board processing examines every pixel in the full CCD image (even in the continuous clocking mode (Section 6.11.5)) and selects as events regions with bias-subtracted pixel values that both exceed the event threshold and are greater than all of the touching or neighboring pixels (i.e., a local maximum). The surrounding 3x3 neighboring pixels are then compared to the bias-subtracted split-event threshold; those that are above the threshold establish the pixel pattern. On the basis of this pattern, the event is assigned a grade. Depending on the grade, the data are then included in the telemetry. On-board suppression of certain grades is used to limit the telemetry bandwidth devoted to background events (see Section 6.15.1).

The grade of an event is thus a code that identifies which pixels, within the three pixel-by-three pixel island centered on the local charge maximum, are above certain amplitude thresholds. The thresholds are listed in Table 6.1. Note that the local maximum threshold differs for the FI and the BI CCDs . A Rosetta Stone to help one understand the ACIS grade assignments is shown in Figure 6.16, and the relationship to the ASCA grading scheme is given in Table 6.6.

**Figure 6.16:** Schematic for determining the grade of an event. The grade is determined by summing the numbers for those pixels that are above their thresholds. For example, an event that caused all pixels to exceed their threshold is grade 255. A single pixel event is grade 0.
$\scalebox{0.6}{\includegraphics{../../ogplots/acis_grad_dip.xfig.eps}}$

**Table:** ACIS and *ASCA* Grades
ACIS Grades	ASCA Grade	Description
0	0	Single pixel events
64 65 68 69	2	Vertical Split Up
2 34 130 162	2	Vertical Split Down
16 17 48 49	4	Horizontal Split Right
8 12 136 140	3	Horizontal Split Left
72 76 104 108	6	``L'' & Quad, upper left
10 11 138 139	6	``L'' & Quad, down left
18 22 50 54	6	``L'' & Quad, down right
80 81 208 209	6	``L'' & Quad, up right
1 4 5 32 128	1	Diagonal Split
33 36 37 129	1
132 133 160 161	1
164 165	1
3 6 9 20 40	5	``L''-shaped split with corners
96 144 192 13 21	5
35 38 44 52 53	5
97 100 101 131	5
134 137 141 145	5
163 166 168 172	5
176 177 193 196	5
197	5
24	7	3-pixel horizontal split
66	7	3-pixel vertical split
255	7	All pixels
All other grades	7

It is important to understand that most, if not all, calibrations of ACIS are based on a specific subset of ACIS grades. This ``standard'' set comprises ASCA grades 0,2,3,4, and 6 - G(02346). In the absence of pileup, this particular grade selection appears to optimize the signal-to-background ratio, but this conclusion depends on the detailed spectral properties of the source. Further, most of the scientifically important characteristics of ACIS (effective area, sensitivity, point spread function, energy resolution, etc.) are grade- and energy-dependent.

6.13.1 Telemetry Formats

There are a number of telemetry formats available. Specifying a format determines the type of information that is included in the telemetry stream. The number of bits per event depends on which mode and which format is selected. The number of bits per event, in turn, determines the event rate at which the telemetry will saturate and data will be lost until the on-board buffer empties. The formats available depend on which mode (Timed Exposure or Continuous Clocking) is used. The modes, associated formats, and approximate event rates at which the telemetry saturates and one begins to limit the return of data, are listed in Table 6.7. The formats are described in the following paragraphs. Event ``arrival time'' is given relative to the beginning of the exposure in TE mode, or relative to read out in CC mode.

**Table 6.7:** Telemetry Saturation Limits
Mode	Format	Bits/event	Events/sec	Number of Events
				in full buffer
CC	Graded	58	375.0	128,000
CC	Faint	128	170.2	58,099
TE	Graded	58	375.0	128,000
TE	Faint	128	170.2	58,099
TE	Very Faint	320	68.8	23,273

6.13.1.0.1 Faint

Faint format provides the event position in detector coordinates, an arrival time, an event amplitude, and the contents of the 3 x 3 island that determines the event grade. The bias map is telemetered separately. Note that certain grades may be not be included in the data stream (Section 6.15.1).

6.13.1.0.2 Graded

Graded format provides event position in detector coordinates, an event amplitude, the arrival time, and the event grade. Note that certain grades may be not be included in the data stream (Section 6.15.1).

6.13.1.0.3 Very Faint

Very Faint format provides the event position in detector coordinates, the event amplitude, an arrival time, and the pixel values in a 5 x 5 island. As noted in Table 6.7, this format is only available with the Timed Exposure mode. Events are still graded by the contents of the central 3 x 3 island. Note that certain grades may be not be included in the data stream (Section 6.15.1). This offers the advantage of reduced background after ground processing (see Section 6.15.2) but only for sources with low counting rates that avoid both telemetry saturation and pulse pileup.

6.14 Pileup

Pileup results when two or more photons are detected as a single event. The fundamental impacts of pileup are: (1) a distortion of the energy spectrum - the apparent energy is approximately the sum of two (or more) energies; and (2) an underestimate as to the correct counting rate - two or more events are counted as one. A simple illustration of the effects of pileup is given in Figure 6.17. There are other, somewhat more subtle impacts discussed below (6.14.1).

**Figure 6.17:** The effects of pileup at 1.49 keV (Al K $\alpha$ ) as a function of source intensity. Data were taken during **HRMA** -ACIS system level calibration at the XRCF. Single-photon events are concentrated near the pulse height corresponding to the Al K $\alpha$ line ( $\sim380$ ADU), and events with 2 or more photons appear at integral multiples of the line energy.
$\scalebox{0.7}{\includegraphics{../../ogplots/acis_XRCF_pileup.eps}}$

6.14.1 Other Consequences of Pileup

There are other consequences of pileup in addition to the two principal features of spurious spectral hardening and underestimating the true counting rate by under counting multiple events. These additional effects are grade migration and pulse saturation, both of which can cause distortion of the apparent PSF .

6.14.1.0.1 Grade migration

Possibly the most troubling effect of pileup is that the nominal grade distribution that one expects for X-ray events changes. The change of grade introduced by pileup has become to be referred to as ``grade migration''. Table 6.8 shows an example of grade migration due to pileup as the incident flux is increased. In this simple test, which involved only mono-energetic photons, the largest effect is the depletion of G0 events and the increase of G7 events. In general, as the incident flux rate increases, the fraction of the total number of events occupying a particular event grade changes as photon-induced charge clouds merge and the resulting detected events ``migrate'' to other grades which are not at all necessarily included in the standard (G02346) set. If one then applies the standard calibration to such data, the true flux will be under-estimated.

**Table:** *ASCA* Grade Distributions for different Incident Fluxes at 1.49 keV (Al-K ${\alpha}$ , based on data taken at the XRCF during ground calibration using chip I3; *CXC* Calibration Group)
Incident
Flux	G0	G1	G2	G3	G4	G5	G6	G7
9	0.710	0.022	0.122	0.053	0.026	0.009	0.024	0.035
30	0.581	0.057	0.132	0.045	0.043	0.039	0.029	0.073
98	0.416	0.097	0.127	0.052	0.050	0.085	0.064	0.108
184	0.333	0.091	0.105	0.040	0.032	0.099	0.077	0.224

arbitrary units

6.14.1.0.2 Pulse Saturation

One consequence of severe instances of pileup is the creation of a region with no events! In this case the pileup is severe enough that the total amplitude of the event is larger than the on-board threshold (typically 15 keV) and is rejected. Holes in the image can also be created by grade migration of events into ACIS grades (e.g. 255) that are filtered on-board.

**Figure:** The effects of pileup on the radial distribution of the *PSF* are illustrated. These data were taken during ground calibration at the XRCF. The specific ``OBSIDs'', the counting rate per CCD frame (``c/f''), and the ``pileup fraction'' as defined in Section 6.14.2 are given in the inset.
$\begin{figure}\centering \scalebox{0.65}{\includegraphics{../../ogplots/acis_xrcf_psf_depres.ps}}\end{figure}$

6.14.1.0.3 PSF distortion

Obviously the effects of pileup are severest when the flux is highly concentrated on the detector. Thus, the core of the PSF suffers more from pileup induced effects than the wings. Figure 6.18 illustrates this point.

6.14.2 Pileup Estimation

It is clearly important in preparing a Chandra observing proposal to determine if the observation will be impacted by pileup, and if so, decide what to do about it (or convince the peer review why the specific objective can be accomplished without doing anything). There are two approaches to estimating the impact of pileup on the investigation. The most sophisticated uses the pileup models in XSPEC , Sherpa , and ISIS to create a simulated data set which can be analyzed in the same way as real data. A less sophisticated, but very useful, approach is to use the web version of PIMMS to estimate pileup or to use the figures in this chapter.

6.14.2.0.1 Simple Pileup Estimates

**Figure 6.19:** The pileup fraction as a function of the the counting rate (in the absence of pileup in units of photons/frame). The solid line is for on-orbit, the dashed line and the data points are for, and from, ground-based data respectively. The difference between the ground and flight functions are a consequence of the improved *PSF* on-orbit, where gravitational effects are negligible - see Chapter 4. Note that when pileup occurs there are two or more photons for each event, so the fraction of events with pileup is always less than the fraction of photons with pileup.
$\scalebox{0.6}{\includegraphics{../../ogplots/acis_pileup_model.ps}}$

The PIMMS tool provides the pileup fraction using the algorithm described here, both for direct observation with ACIS and also for the zeroth-order image when a grating is inserted.

6.14.2.0.2 Simulating Pileup

John Davis at MIT has developed an algorithm for modeling the effects of pileup on ACIS spectral data. The algorithm has been implemented as of XSPEC V11.1 and Sherpa V2.2. The algorithm can be used to attempt to recover the underlying spectrum from a source, or to simulate the effects of pileup for proposal purposes.

The algorithm has been tested by comparing CCD spectra with grating spectra of the same sources. Care should be taken in applying the algorithm - for example, using the appropriate regions for extracting source photons and avoiding line-dominated sources. A description of the algorithm can be found in Davis 2001 (Davis, J.E. 2001, ApJ, 562, 575). Details on using the algorithm in Sherpa are given in a Sherpa ``thread'' as of CIAO V2.2 on the CXC CIAO web page: http://cxc.harvard.edu/ciao/.

6.14.3 Reducing Pileup

6.15 On-Orbit Background

There are three components to the on-orbit background. The first is that due to the diffuse X-ray background (most of which may resolve into discrete sources during an observation with Chandra). The second component is commonly referred to as the charged particle background. This latter arises both from charged particles, photon, and other neutral particle interactions that ultimately deposit energy in the instrument. The third component depends on the flux from the very objects under observation and is a consequence of the "trailing" of the image discussed in Section 6.11.4. Strictly speaking, this last component is only ``background'' to the extent that one doesn't recognize that it is signal.

The background rates differ between the BI and the FI chips, in part because of differences in the efficiency for identifying charged particle interactions. Figure 6.20 illustrates why.

**Figure 6.20:** Enlarged view of an area of a FI chip I3 (left) and a BI chip (right) after being struck by a charged particle. There is far more ``blooming'' in the FI image since the chip is thicker. The overlaid 3x3 detection cells indicate that the particle impact on the FI chip produced a number of events, most of which end up as *ASCA* Grade 7, and are thus rejected with high efficiency. The equivalent event in the BI chip, is much more difficult to distinguish from an ordinary x-ray interaction, and hence the rejection efficiency is lower.
$\scalebox{0.6}{\includegraphics[70mm,90mm][155mm,160mm]{../../ogplots/acis_cosm_fi.ps}}$ $\scalebox{0.6}{\includegraphics[70mm,90mm][155mm,160mm]{../../ogplots/acis_cosm_bi.ps}}$

6.15.1 The Non-X-ray Background

In September 2002 a 53 ksec 'observation' was carried out with the ACIS in the stowed position but collecting data in normal TE VF mode at -120C. The SIM position was chosen so that the on-board calibration source did not illuminate the ACIS chips. This allowed us to characterize the non-celestial contribution to X-ray background (ie. from charged particles). The resulting spectra is shown in in Figure 6.21. Standard grade filtering has been applied; additional VF mode cleaning was not applied. Chip S2 is similar to I023 and not shown for clarity.

**Figure 6.21:** Energy spectra of the charged particle ACIS background (standard grade filtering) with ACIS in the stowed position taken in 2002, September. Line features are due to fluorescence of material in the telescope and focal plane.
$\scalebox{0.5}{\includegraphics{../../ogplots/ax_bg_s_i_novf_ucover62850.ps}}$

6.15.2 The total background

Once the HRMA doors were opened, two more components to the background came into play. The first is the cosmic X-ray background which, for moderately long ( $\sim100$ ks) observations will be mostly resolved into discrete sources, but, nevertheless, contributes to the overall counting rate. The second is due to any charged particles that may reflect from the telescope and have sufficient momentum so as not to be diverted from the focal plane by the magnets included in the observatory for that purpose, or from secondary particles. Figure 6.23 shows a representative ``quiescent'' background spectra for both types of CCDs taken after the doors were opened and with no bright sources in the field. The total background counting rates in various energy bands and for the standard grades are given in Tables 6.9 and 6.10. Insertion of the gratings makes little measurable difference in the background rates. Although these rates are slowly changing on the timescale of months, Tables 6.9 and 6.10 can be used for rough sensitivity estimates. Note that Table 6.11 includes all grades which are telemetered (see Section 6.13 and 6.15.1).

To aid in estimating the probability of telemetry saturation, Table 6.11 gives total background count rates for each type of chip, including all grades that are telemetered (see Section 6.13). These rates have been declining until Summer 2000, then flat until Summer 2001, and may be starting to increase, apparently anti-correlating with the solar cycle.

For aid in planning background-critical observations, the CXC has combined a number of deep, source-free exposures (including all components of the background) into experimental quiescent background event files for different time periods. These datasets can be found on the web off the calibration page (http://cxc.harvard.edu/cal/Acis/Cal_prods/bkgrnd/current/background.html).

For data from low counting rate sources taken using the Very Faint (VF) telemetry format (Section 6.13.1), the background can be further reduced in data processing by screening out events with significant flux in border pixels of the 5 $\times$ 5 event islands. This screening leaves the data from faint sources essentially the same while reducing the FI background at different energies: a factor of $\sim$ 1.4 (E

keV); $\sim$ 1.1(1-5 keV); and $\sim$ 2 (near $\sim$ 0.5 keV). For the BI chips the reductions are: 1.25 (E

keV); $\sim$ 1.1(1-5 keV); and $\sim$ 3 (near $\sim$ 0.3 keV). This screening also eliminates almost all spurious ``afterglow'' events, caused by the slow leakage of charge deposited from particularly bright cosmic ray hits in subsequent frames. The screening algorithm has been incorporated into the CIAO tool ``acis_process_events''. Further discussion may be found at

Proposers should be aware that telemetry saturation is more probable for observations using the VF format, and that they may need minimize the number of CCDs in operation to avoid problems. Proposers should also be aware that if there are bright point sources in the field of view, that the flux is more likely to be piled up in the VF format if the above VF mode screening is applied. However, there is no intrinsic increase of pileup in VF data, and the screening software can be selectively applied to regions, excluding bright point-like sources. The screening criterion discussed above is then more likely to remove source events (albeit piled up) if the source is bright. Point sources should have count rates significantly less than 1 count/sec to be unaffected.

**Figure 6.23:** ACIS quiescent background spectrum for an FI CCD (S2) (top) and a BI CCD (S3)(bottom). The lower curve shows the spectrum before the **HRMA** doors were opened; the upper curve after. Both curves have G02346 filtering.
$\rotatebox{-90}{\scalebox{0.4}{\includegraphics{../../ogplots/s2_back_spec_quies.ps}}}$ $\rotatebox{-90}{\scalebox{0.4}{\includegraphics{../../ogplots/s3_back_spec_quies.ps}}}$

**Table 6.9:** Approximate on-orbit standard grade background counting rates with ACIS positioned at the ACIS-I aimpoint, T=-120C. The background rate are cts/s/chip, using only *ASCA* grades 02346, excluding background flares, bad pixels/columns and celestial sources identifiable by eye, Feb 2000 - Oct 2000 without gratings. These values should be used for sensitivity calculations.
	Bkgrd rates (cts/sec)
Energy
Band (keV)	I0	I1	I2	I3	S2	S3	I0123 avg
0.3-10	0.27	0.28	0.27	0.28	0.29	0.74	0.27
0.5-2	0.06	0.06	0.07	0.06	0.07	0.14	0.06
0.5-7	0.16	0.16	0.16	0.17	0.17	0.32	0.16
5.0-10	0.14	0.14	0.13	0.14	0.14	0.42	0.14
10-12	0.08	0.08	0.08	0.08	0.08	0.51	0.08

**Table 6.10:** Approximate on-orbit standard grade background counting rates with ACIS positioned at the ACIS-S aimpoint, T=-120C. These values should be used for sensitivity calculations.
	Bkgrd rates (cts/sec) $^{\ast}$
Energy
Band (keV)	I1	I2	I3	S1	S2	S3	S4
0.3-10	0.29	0.29	0.29	1.41	0.33	0.79	0.34
0.5-2	0.07	0.08	0.07	0.19	0.09	0.16	0.10
0.5-7	0.17	0.17	0.17	0.44	0.20	0.35	0.21
5.0-10	0.15	0.14	0.14	0.96	0.16	0.44	0.15
10-12	0.08	0.08	0.08	0.72	0.09	0.53	0.09

**Table 6.11:** Typical total quiescent background rates (cts/s/chip), including *all* grades that are telemetered (not just standard *ASCA* grades), by chip type and upper energy cutoff. These values to be used to estimate the probability of telemetry saturation.
Period	Aug 1999	Fall 2000 - Summer 2001
Upper cutoff	15 keV	15 keV	13 keV	10 keV
Chip S2 (FI)	10	6.3	5.8	5.0
Chip S3 (BI)	11	7.7	5.0	2.5

6.15.3 Background variability

In general the background counting rates are stable during an observation. Occasionally, however, there are significant variations (flares), as illustrated in Figure 6.24. Figure 6.25 shows the frequency of such variations when compared to the quiescent background. Several types of flare have been identified, including flares that occur only in the BI chips, and flares that occur in both the FI and BI chips. Figure 6.26 shows the spectra of two of the most common flare species. Both flares have spectra significantly different from the quiescent background.

Users should note that the counting rate in the BI CCDs can significantly increase during a flare. It is possible to saturate telemetry, especially if all 4 ACIS-I chips are turned on. In these circumstances users might consider turning off the BI chips. However, the CXC recommends that both BI chips be turned on if ACIS-S is used in imaging mode. The advantage is that for most types of flares S1 can be used to create a flare template, which can then be subtracted from S3.

**Figure 6.24:** An example of the ACIS background counting rate versus time - BI chip (S3; top curve) and an FI chip (I2; bottom curve). These are for the standard grades and the band from 0.3 - 10 keV.
$\rotatebox{0}{\scalebox{0.55}{\includegraphics{../../ogplots/obs_affect_hi_bkg.ps}}}$

**Figure 6.25:** An estimate of the cumulative probability that the ratio of the background counting rate to the quiescent background counting rate is larger than a given value. Upper plot for a representative FI chip - S2, and the lower curve for a representative BI chip - S3. The vertical dotted line is a limiting factor 1.2 used in creating the background data sets.
$\scalebox{0.5}{\includegraphics{../../ogplots/flare_freq.ps}}$

**Figure 6.26:** Spectra of different background flares in chip S3. Thick crosses show a common flare species that affects only the BI chips. Thin crosses show one of the several less common flare species that affect both the BI and FI chips. Note how both these spectra are different from the quiescent spectrum (see Figure 6.23).
$\rotatebox{-90}{\scalebox{0.5}{\includegraphics{../../ogplots/s3_bi_Fe_flares_ED.ps}}}$

6.15.4 Background in Continuous Clocking Mode

Apart from compressing the data into one dimension (Section 6.11.5), there is essentially no difference in the total background in CC mode and that encountered in the timed exposure mode. The background per-sky-pixel, however, will be 1024 times larger, since the sky-pixel is now 1 x 1024 ACIS pixels.

6.16 Sensitivity

The ACIS sensitivity for detecting a point source, on axis, during times of quiescent background is approximately 4x10 $^{-15}$ ergs cm $^{-2}$ s $^{-1}$ in a 10 ks exposure in the 0.4-6.0 keV band. Necessarily, the sensitivity is a function of energy and depends on which CCD FI(I3) or BI (S3) is selected. Figure 6.27 shows the minimum detectable flux, for a point source and a 20''-diameter extended source for different choices of spectral parameters.

**Figure 6.27:** The minimum detectable flux as a function of exposure time for an on-axis observation of a point source using an FI chip (top) and a BI chip (bottom). The calculation assumes a $3 \sigma$ 2-10 keV absorbed *flux* detection criterion in a 5x5 pixel detection cell and no pileup. The power law index is a photon index. The extended source spectrum assumed the -1.4 index power law and corresponding column.
$\scalebox{0.5}{\includegraphics[0.3in, 2.3in][7.9in, 9.8in]{../../ogplots/acis_lim_flux.eps}}$

6.17 Bright Source X-ray Photon Dose Limitations

Pre-Flight radiation tests have shown that $\sim$ 200 krads of X-ray photon dose can positively damage the CCDs . The mechanism for the damage is the trapped ionization in the dielectric silicon oxide and nitride separating the gates from the depletion region. Since the charge is trapped, the damage is cumulative. Because the structure of the BI's differs significantly from that of the FI's, the two types of chips have different photon dose limitations. Specifically, the BI's are more than 25 times as tolerant to a dose of X-ray photons as compared to the FI's since the former have 40 $\mu$ m of bulk Si 'protecting' the gate layer.

Simulations of astrophysical sources have yielded a very conservative, spectrally-averaged, correspondence of 100 counts/pix = 1 rad. (By 'counts' in this context we mean all photons that impinged on the detector, whether or not they were piled-up and discarded.)

In consultation with the IPI team the CXC has adopted the following mission allowances, per pixel of the two types of chips:
FI chips: 25 krads 2,500,000 cts/pix
BI chips: 625 krads 62,500,000 cts/px

If your observation calls for observing a bright point-like source close to on-axis, we suggest you use the MARX simulator (with the parameter DetIdeal=yes & dither, typically, on) to calculate whether your observation may reach 1% of the above mission limits in any one pixel. If so, please contact the CXC HelpDesk in order to custom design an observational strategy which may accommodate your science aims, while maintaining the health & safety of the instrument.

6.18 Observing Planetary and Solar System Objects with ACIS

Chandra has successfully observed several solar system objects, including Venus, the Moon, Mars, Jupiter and several comets. Observation of planets and other solar system objects is complicated because these objects move across the celestial sphere during an observation and the optical light from the source can produce a significant amount of charge on the detectors (this is primarily an issue for ACIS-S observations). Some information regarding observation planning and data processing is given here. Users are encouraged to contact the CXC for more detailed help.

6.18.1 Observations with ACIS-I

Any solar system object can be observed with ACIS-I . Previous solar system observations with ACIS-I have not revealed significant contamination from optical light. However, proposers are encouraged to work with the CXC when planning the specifics of a given observation. Since the source moves across the celestial sphere in time, an image of the event data will exhibit a "streak" associated with the source. The CIAO tool sso_freeze can be used to produce an event data file with a pair of coordinates that have the motion of the source removed.

6.18.2 Observations with ACIS-S

The ACIS-S array can be used with or without a grating. The back-illuminated CCD s in the S array (chips S1 and S3) are more sensitive to soft X-rays than the I array CCD s, but the entire S array suffers from the disadvantage that its optical blocking filter is thinner than for ACIS-S and may transmit a non-negligible flux of visible light onto the CCD s. It is thus necessary to estimate the amount of charge produced in the CCD s due to the optical light. More detailed information can be found at http://www.astro.psu.edu/xray/docs/cal_report/ and from the CXC via Helpdesk.

If the optical light leak is small enough, it can be mitigated by simply shortening the frame time. This leads to a linear drop in the number of ADU due to optical light. If possible, VF mode should be used, since in this mode the outer 16 pixels of the 5x5 region allows a "local" bias to be subtracted from the event to correct for any possible light leakage.

The optical light also invalidates the bias taken at the beginning of the observation if a bright planet is in the field. It is therefore desirable to take a bias frame with the source out of the field of view. This bias map is useful even when processing 5x5 pixels in VF mode since it can be employed as a correction to the local average "bias" computed from the 16 outer pixels, thereby correcting for hot pixels, cosmetic defects etc.

A more sophisticated approach to dealing with excess charge due to optical light is to make an adjustment to the event and split thresholds. Event grades are described in more detail in Section 6.13. Excess charge (in adu) due to optical light will be added to the event and split counters on-board. Without an adjustment to the thresholds (or a large enough one), many of the X-ray events may have all nine pixels of a 3 pixel x 3 pixel event detection cell above the split threshold, in which case the event will not be telemetered to the ground. If the adjustment is too large, X-ray events may not be detected because they may not exceed the event threshold.

Users should be aware that if the detection thresholds are adjusted, standard CXC processing of planetary data will give inaccurate estimates of event pulse heights and GRADEs. A thorough understanding of the energy calibration process and manual massaging of the data will be required

6.19 Observing with ACIS - the Input Parameters

This section describes the various inputs that either must be, or can be, specified in order to perform observations with ACIS . The sub-sections are organized to match the RPS form. We have added some discussion as to some of the implications of the possible choices. As emphasized at the beginning of the Chapter, ACIS is moderately complex and the specific characteristics of the CCDs and their configuration in the instrument lead to a number of alternatives for accomplishing a specific objective - detailed trade-offs are the responsibility of the observer. Thus, e.g. it might seem obvious that observations of a faint point source may be best accomplished by selecting the ACIS-S array with the aim point on S3, the BI device that can be placed at the best focus of the telescope, and the CCD with the best average energy resolution. On the other hand, perhaps the science is better served by offset pointing (by a few arcminutes) the target onto S2, very near to the framestore, where the FI energy resolution is better than that of S3. On the other hand, if the object is very faint, so that the number total number of photons expected is just a handful - not enough to perform any significant spectroscopy - the advantage of S2 nor S3 may not be so obvious considering the smaller field of view, and perhaps the ACIS-I array, which would optimize the angular resolution over a larger field, may be more attractive.

6.19.1 Required Parameters

There are certain ACIS input parameters that must be specified: the number and identity of the CCDs to be used, the Exposure Mode, and the Event Telemetry Format. If pileup and telemetry saturation are not considered to be a problem for the observation, then these are the only parameters that need to be specified.

6.19.1.1 $\bullet$ Number and Choice of CCD

Up to six CCDs can be operated at once. Specifying ACIS-S turns on S0-S5 and sets the aimpoint. Specifying ACIS-I turns on I0-I3, S2 and S3, and sets the aimpoint. For any other combination, the identity of the CCDs and the desired aimpoint will have to be specified.

6.19.1.2 $\bullet$ Exposure Mode

There are only two choices: Timed Exposure (Section 6.11.1) or Continuous Clocking (Section 6.11.5).

6.19.1.2.1 Timed Exposure Mode

The timed exposure mode with the default nominal (and optimal) frame time of 3.2s is the typical mode for ACIS observations. Note that the option of selecting frame times shorter than nominal reduces observing efficiency, and hence the number of photons collected for a given observation time.

6.19.1.2.2 Continuous Clocking Mode

The Continuous Clocking mode is useful when timing data are so critical and/or pileup is such a problem that the sacrifice of one dimension of spatial data is warranted. The use of continuous clocking may also lead one to consider specifying a particular satellite roll orientation (see Chapter 3) in order to avoid having two different sources produce events in the same CCD column.

6.19.2 Optional Parameters

6.19.2.0.1 Alternating Exposures

This option applies only to Timed Exposures. The parameters specifying an Alternating Exposure are:

Frame times and efficiencies in TE mode are discussed in Sections 6.11.2 and 6.11.3.

6.19.2.1 Energy Filtering

It is possible to remove events from the telemetry stream, and thus avoid telemetry saturation, by specifying an energy acceptance filter within which detected events will be telemetered. The default discards events above 3750 ADU (nominally 15 keV). The total per-chip background rates for different upper energy cut-offs are in Table 6.11.

6.19.2.2 Spatial Windows

A more sophisticated approach to removing data from the telemetry stream, and thus avoiding telemetry saturation, is by the use of a Spatial Window. This option offers a good deal of flexibility. One may define up to 6 Spatial Windows per CCD . Each window can be placed anywhere on the chip. Note there is a significant difference between a Spatial Window and a Subarray (Section 6.11.3): Subarrays affect the transmission of CCD data to the on-board ACIS processors; Spatial Windows select events detected by the processors and only impact the telemetry rate. The user may also specify the window energy threshold and energy range.

Spatial windows can be inclusive or exclusive. A Spatial Window could be used to eliminate a bright, off-axis source that would otherwise overwhelm the telemetry stream. The order in which the spatial windows are specified is important if they overlap.

6.19.3 Non-ACIS Parameters Relevant to an Observation with ACIS

There are a small number of additional parameters that need to be considered in specifying an observation with ACIS : (1) the off-axis pointing (if required), which reduces the flux, and spreads out the image; (2) the roll angle (Chapter 4); (3) time constraints (if any); and (4) time monitoring intervals (if any).