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Last modified: 26 October 2016

URL: http://cxc.harvard.edu/ciao/data_products_guide/events.html

Event Files

Data Products Guide

Filename Description Content ICD SDP Pipeline Next Pipeline
acis_evt1.fits ACIS L1 event file EVT1 v2.3 ACIS1 TG1.5 or ACIS2
hrc_evt1.fits HRC L1 event file EVT1 v1.7 HRC1 TG1.5 or HRC2
acis_evt2.fits ACIS L2 imaging event file EVT2 v1.2 ACIS2 none
hrc_evt2.fits HRC L2 imaging event file EVT2 v1.2 HRC2 none
acis_evt1a.fits ACIS L1.5 grating event file TGEVT1 v1.6 TG1.5 TG2
hrc_evt1a.fits HRC L1.5 grating event file TGEVT1 v1.6 TG1.5 TG2
acis_evt2.fits ACIS L2 grating event file EVT2   TG2 none
hrc_evt2.fits HRC L2 grating event file EVT2   TG2 none

Overview

The reference document that defines FITS files standards for the CXC data products is the ASC FITS Designer's Guide (Version 2.0; postscript).

The event file contains photon event data stored as a table. The EVENT list is the primary component of the file and CIAO will accept this file in all tasks that accept EVENT files or IMAGE files. Additionally, event files contain the good time interval (GTI) data stored as start and stop time records. The sum of the GTI times represents the acceptable science time (EXPTIME in the header) as determined by pipeline processing.

Time description:

Time is derived by applying the Space Craft Clock Correlation to the VCDU readouts in the telemetry stream.

Column name Description
TIME S/C TT corresponding to mid-exposure

Coordinate description:

Calibration corrections are applied to the events, and the instrument readouts (CCD_ID, NODE_ID, CHIP coordinates) are transformed into sky pixel coordinates (X,Y). The sky coordinates are computed by applying the aspect solution correction that accounts for spacecraft motion as a function of time to the raw coordinates via a transformation library (pixlib). The intermediate transforms (TDET and DET) are saved and are sometimes useful to use as filters to view the image as it was before aspect correction. The instrument dither and spacecraft motion can often be traced in a point source.

The reference document that defines the coordinates convention for the CXC is the Coordinate Systems for Analysis of On-Orbit Chandra Data (Paper I: Imaging).

Column name Description
CHIPX,CHIPY position of center pixel of event; HRC: degapping is applied
TDETX,TDETY position of event in tiled detector coordinates
DETX,DETY position of event in ACIS detector detector coordinates
X, Y position of event in sky coordinates

L1 Event File (Imaging and Grating)

Filename Description Content ICD SDP Pipeline Next Pipeline
acis_evt1.fits ACIS L1 event file EVT1 v2.3 ACIS1 TG1.5 or ACIS2
hrc_evt1.fits HRC L1 event file EVT1 v1.7 HRC1 TG1.5 or HRC2

Creator pipeline: acis_L1, hrc_L1

The L1 event list covers the time range of an Observation Interval (OBI). Most Chandra Observations are single OBI, but in some cases the observations are split into observing intervals.

The pipeline performs grading and coordinate transformations on the data. During the coordinate transformations, the aspect solution is applied to compensate for spacecraft dither which occurred while the data was being collected. In addition pulse invarience is calculated and charge transfer ineffiency is compensated for. Bad pixel corrections and graded data corner mean checks are done and the appropriate status bits are set.

The Standard GTIs for the OBI are computed in L1 but not applied to the events until L2. The output L1 event list contains all input events and the GTI extension reflects the OBI start and stop time intersected with telemetry dropouts; this accounts for any gaps in the data. A standard Chandra FITS header is attached.

Creator tools:

acis_format_events - receives faint TE, faint CC, faint w/ bias, very faint, graded CC, and graded TE data and performs several functions on them. The tool is responsible for ensuring that a consistent data format is output so that acis_process_events does not have to handle multiple formats of data. This is primarily achieved by standardizing data column names. In addition, the routine produces auxiliary data products such as exposure stats and dropped exposure tables. Bias correction and overclock correction of data is performed on faint mode data.

acis_process_events - receives an event file that has been processed by acis_format_events and performs grading and coordinate transformations on the data. During the coordinate transformations, the aspect solution is applied to compensate for spacecraft dither which occurred while the data was being collected. In addition, pulse invarience is calculated and charge transfer ineffiency is compensated for. Bad pixel corrections and graded data corner mean checks are done and appropriate status bits are set.

hrc_process_events - receives an event file and performs grading and coordinate transformations on the data. During the coordinate transformations, the aspect solution is applied to compensate for spacecraft dither which occurred while the data was being collected. In addition, pulse invarience is calculated. Bad pixel corrections are done and appropriate status bits are set.

ACIS EVENT extension columns:

Column Name Description
TIME S/C TT corresponding to mid-exposure
CCD_ID ccd reporting event
NODE_ID ccd serial readout amplifier node
EXPNO exposure number of ccd frame reporting event
CHIPX X position of center pixel of event
CHIPY Y position of center pixel of event
TDETX X position of event in tiled detector coordinates
TDETY Y position of event in tiled detector coordinates
DETX X position of event in ACIS detector coordinates
DETY Y position of event in ACIS detector coordinates
X X position of event in sky coordinates
Y Y position of event in sky coordinates
SKY_1D 1D spatial coordinate (cc-mode data)
PHAS[9] 3x3 array of bias-corrected pixel pulse heights
PHA Total pulse height of event
PHA_RO Total readout pulse height of event
ENERGY nominal energy of event
PI Pulse invariant energy of event
FLTGRADE event grade in flight system
GRADE 'binned' event grade in ACIS/ASCA/USER system
STATUS bit description

HRC EVENT extension columns:

Column Name Description
TIME S/C TT corresponding to mid-exposure
CRSV Coarse position V axis
CRSU Coarse position U axis
AMP_SF Amplitude scale factor
AV1 V axis ADC 1
AV2 V axis ADC 2
AV3 V axis ADC 3
AU1 U axis ADC 1
AU2 U axis ADC 2
AU3 U axis ADC 3
RAWX Raw X (no degapping)
RAWY Raw Y (no degapping)
CHIPX X position of center pixel of event
CHIPY Y position of center pixel of event
TDETX X position of event in tiled detector coordinates
TDETY Y position of event in tiled detector coordinates
DETX X position of event in ACIS detector coordinates
DETY Y position of event in ACIS detector coordinates
X X position of event in sky coordinates
Y Y position of event in sky coordinates
PHA Total pulse height of event
PI Pulse invariant energy of event
SUMAMPS Sum of all amp readouts
CHIP_ID Chip ID
STATUS event status bits (memo)

GTI extension columns:

Column Name Description
START Good time interval start
STOP Good time interval stop

L2 Imaging Event File

Filename Description Content ICD SDP Pipeline Next Pipeline
acis_evt2.fits ACIS L2 imaging event file EVT2 v1.2 ACIS2 none
hrc_evt2.fits HRC L2 imaging event file EVT2 v1.2 HRC2 none

Creator pipeline: acis_L2, hrc_L2

The L2 pipeline is run on the output of L1 for imaging observations. If the Chandra observation is a multi-OBI observation, then the first step of processing is to merge the stack of L1 event files for the observation with dmmerge.

The standard GTI time filter computed in L1 are applied to the events. The events are also filtered by dmcopy based on the following attributes: status=0 (i.e. no bit filters set) and a standard ACIS grade set (grade=0,2,3,4,6). The L1 PHAS and CORN_PHA columns are filtered out. The output L2 event list includes all "good" events events for an observation with a standard Chandra FITS header attached.

Simple data analysis in L2 includes running celldetect to create a source list. Full-resolution and centered images of the data are also created.

ACIS EVENT extension columns:

Column Name Description
TIME S/C TT corresponding to mid-exposure
CCD_ID ccd reporting event
NODE_ID ccd serial readout amplifier node
EXPNO exposure number of ccd frame reporting event
CHIPX X position of center pixel of event
CHIPY Y position of center pixel of event
TDETX X position of event in tiled detector coordinates
TDETY Y position of event in tiled detector coordinates
DETX X position of event in ACIS detector coordinates
DETY Y position of event in ACIS detector coordinates
X X position of event in sky coordinates
Y Y position of event in sky coordinates
SKY_1D 1D spatial coordinate (CC-mode data)
PHA Total pulse height of event
PHA_RO Total readout pulse height of event
ENERGY nominal energy of event
PI Pulse invariant energy of event
FLTGRADE event grade in flight system
GRADE 'binned' event grade in ACIS/ASCA/USER system
STATUS bit description

HRC EVENT extension columns:

Column Name Description
TIME S/C TT corresponding to mid-exposure
CHIPX Chip X (degapping applied)
CHIPY Chip Y (degapping applied)
TDETX Tiled X
TDETY Tiled Y
DETX Focal plane X
DETY Focal plane Y
X Sky X
Y Sky Y
PHA Total pulse height of event
PI Pulse invariant energy of event
CHIP_ID Chip ID
STATUS event status bits (memo)

GTI extension columns:

Column Name Description
START Good time interval start
STOP Good time interval stop

L1.5 Grating Event File

Filename Description Content ICD SDP Pipeline Next Pipeline
acis_evt1a.fits ACIS L1.5 grating event file TGEVT1 v1.6 TG1.5 TG2
hrc_evt1a.fits HRC L1.5 grating event file TGEVT1 v1.6 TG1.5 TG2

Creator pipeline: acis_L15, hrc_L15

The L1.5 pipeline is run on the output of L1 for grating observations. The event list covers the time range of an Observation Interval (OBI).

The first step is to detect the zero order source(s) about the pointing position. Then a mask region is created and used to separate the grating events from the background. Grating coordinates are assigned to all identified regions and the wavelength and order of the photons is calculated. Finally the fits region file is appended to the output event file, as a REGION extension and a standard Chandra FITS header is attached.

Creator tools:

tgdetect - detects zero order source(s) using a wrapper script which sets up and executes several DM tools including celldetect. The steps are as follows: A box region about the center pointing position (nom_ra/dec in pixel coordinates) is created; celldetect is run over the section of the event list designated in the filter above and outputs a source list of all sources detected in that region; Several filtering steps are applied to the source list to find the zeroth order in the event that more than one source was detected.

tg_create_mask - creates a region file to define spectrum sky boundaries. Before wavelengths can be computed from diffracted event positions, each event is assigned to a part of the spectrum according to its spatial location. The parts are zero-order or diffracted order. If diffracted order, HETG has two diffracted parts, one for HEG and the other MEG. If LETG, there is only one diffracted part. The output is an ASCII region file which enumerates the parts and specifies the region shape, size, and orientation in sky pixel-plane coordinates.

The spatial region is determined via error-budget calculations incorporating the affects of the mirror point-spread function vs. off-axis angle, defocus, and Rowland spectrograph astigmatism. It is by default of generous dimensions compared to the one-sigma characteristic size so that sub-selection can be applied later into source and background regions when binning a spectrum (see tgextract). Prior to binning, rigorous conversion of event coordinates to diffraction coordinates is done from chip coordinates and the aspect solution (see tg_resolve_events); sky coordinates are only used for spatial filtering. Regions for up to 10 sources may be computed at a time and it is this default that are used in the pipeline.

tg_resolve_events - assign grating events to spectral orders using PHA separation if available. This step compares event positions with 3-D locations (including PHA) to which dispersed photons can appear, given the grating equation and 0-order position, and assign them a wavelength and an order. First grating diffraction coordinates (primarily wavelength and cross-diffraction angle) are computed for each event in spatial regions assigned by the input mask (regionfile parameter) by decomposing the event coordinates into components parallel and perpendicular to the diffraction direction, as defined by the instantaneous zero-order centroid location, the event location, the grating node, and the grating bar rotation angle. If the detector has intrinsic energy resolution, an energy response table is used to assign each photon an order (cf., the one-dimensional grating equation: m*lambda = P*sin(theta), where m is the order, lambda the wavelength, P the grating period, and theta the diffraction angle).

ACIS EVENT extension columns:

Column Name Description
TIME S/C TT corresponding to mid-exposure
EXPNO exposure number of ccd frame reporting event
TG_R diffraction angle; default form is 1D.
TG_D cross-dispersion angle; default form is 1D.
CHIPX X position of center pixel of event
CHIPY Y position of center pixel of event
TDETX X position of event in tiled detector coordinates
TDETY Y position of event in tiled detector coordinates
DETX X position of event in ACIS detector coordinates
DETY Y position of event in ACIS detector coordinates
X X position of event in sky coordinates
Y Y position of event in sky coordinates
SKY_1D 1D spatial coordinate (CC-mode data)
CCD_ID ccd reporting event
PHA Total pulse height of event
PI Pulse invariant energy of event
ENERGY nominal energy of event
GRADE 'binned' event grade in ACIS/ASCA/USER system
FLTGRADE event grade in flight system
NODE_ID ccd serial readout amplifier node
TG_M order (m)
TG_LAM wavelength. (lambda) Default form is 1D.
TG_MLAM m x lambda. Default form is 1D.
TG_SRCID source ID
TG_PART spatial component (0=zero order, 1=HEG, 2=MEG, 3=LEG, 99=background)
TG_SMAP source map
STATUS bit description
GDPX grating x diffraction pixel, dispersion direction (optional)
GDPY grating y diffraction pixel, cross-dispersion (optional)

HRC EVENT extension columns:

Column Name Description
TIME S/C TT corresponding to mid-exposure
TG_R diffraction angle; default form is 1D.
TG_D cross-dispersion angle; default form is 1D.
CHIPX Chip X (degapping applied)
CHIPY Chip Y (degapping applied)
TDETX Tiled X
TDETY Tiled Y
DETX Focal plane X
DETY Focal plane Y
X Sky X
Y Sky Y
CHIP_ID Chip ID
PHA Total pulse height of event
PI Pulse invariant energy of event
TG_M order (m)
TG_LAM wavelength. (lambda) Default form is 1D.
TG_MLAM m x lambda. Default form is 1D.
TG_SRCID source ID
TG_PART spatial component (0=zero order, 1=HEG, 2=MEG, 3=LEG, 99=background)
TG_SMAP source map
STATUS event status bits
GDPX grating x diffraction pixel, dispersion direction (optional)
GDPY grating y diffraction pixel, cross-dispersion (optional)

GTI extension columns:

Column Name Description
START Good time interval start
STOP Good time interval stop

REGION extension:

Column Name Description
SOURCE Source Number
SHAPE Shape of region
X x coordinate
Y y coordinate
R[2] Radius Vector for SHAPE
ROTANG[2] Rotation angle vector for SHAPE
GRATING Applicable grating: HETG or LETG
TG_PART spatial component (0=zero order, 1=HEG, 2=MEG, 3=LEG, 99=background)
COMPONENT Component number of SHAPE

L2 Grating Event File

Filename Description Content ICD SDP Pipeline Next Pipeline
acis_evt2.fits ACIS L2 grating event file EVT2   TG2 none
hrc_evt2.fits HRC L2 grating event file EVT2   TG2 none

Creator pipelines: TG2

The L2 pipeline is run on the output of L1 for grating observations. If the Chandra observation is a multi-OBI observation, then the first step of processing is to merge the stack of L1 event files for the observation with dmmerge. The L2 event file retains the REGION extension that was created in the L1.5 pipeline.

ACIS EVENT extension columns:

Column Name Description
TIME S/C TT corresponding to mid-exposure
EXPNO exposure number of ccd frame reporting event
TG_R diffraction angle; default form is 1D.
TG_D cross-dispersion angle; default form is 1D.
CHIPX X position of center pixel of event
CHIPY Y position of center pixel of event
TDETX X position of event in tiled detector coordinates
TDETY Y position of event in tiled detector coordinates
DETX X position of event in ACIS detector coordinates
DETY Y position of event in ACIS detector coordinates
X X position of event in sky coordinates
Y Y position of event in sky coordinates
SKY_1D 1D spatial coordinate (CC-mode data)
CCD_ID ccd reporting event
PHA Total pulse height of event
PI Pulse invariant energy of event
ENERGY nominal energy of event
GRADE 'binned' event grade in ACIS/ASCA/USER system
FLTGRADE event grade in flight system
NODE_ID ccd serial readout amplifier node
TG_M order (m)
TG_LAM wavelength. (lambda) Default form is 1D.
TG_MLAM m x lambda. Default form is 1D.
TG_SRCID source ID
TG_PART spatial component (0=zero order, 1=HEG, 2=MEG, 3=LEG, 99=background)
TG_SMAP source map
GDPX grating x diffraction pixel, dispersion direction (optional)
GDPY grating y diffraction pixel, cross-dispersion (optional)
STATUS bit description

HRC EVENT extension columns:

Column Name Description
TIME S/C TT corresponding to mid-exposure
TG_R diffraction angle; default form is 1D.
TG_D cross-dispersion angle; default form is 1D.
CHIPX Chip X (degapping applied)
CHIPY Chip Y (degapping applied)
TDETX Tiled X
TDETY Tiled Y
DETX Focal plane X
DETY Focal plane Y
X Sky X
Y Sky Y
CHIP_ID Chip ID
PHA Total pulse height of event
PI Pulse invariant energy of event
TG_M order (m)
TG_LAM wavelength. (lambda) Default form is 1D.
TG_MLAM m x lambda. Default form is 1D.
TG_SRCID source ID
TG_PART spatial component (0=zero order, 1=HEG, 2=MEG, 3=LEG, 99=background)
TG_SMAP source map
STATUS event status bits (memo)
GDPX grating x diffraction pixel, dispersion direction (optional)
GDPY grating y diffraction pixel, cross-dispersion (optional)

GTI extension columns:

Column Name Description
START Good time interval start
STOP Good time interval stop

REGION extension:

Column Name Description
SOURCE Source Number
SHAPE Shape of region
X x coordinate
Y y coordinate
R[2] Radius Vector for SHAPE
ROTANG[2] Rotation angle vector for SHAPE
GRATING Applicable grating: HETG or LETG
TG_PART spatial component (0=zero order, 1=HEG, 2=MEG, 3=LEG, 99=background)
COMPONENT Component number of SHAPE

Last modified: 26 October 2016
Smithsonian Institute Smithsonian Institute

The Chandra X-Ray Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory. 60 Garden Street, Cambridge, MA 02138 USA.   Email:   cxchelp@head.cfa.harvard.edu Smithsonian Institution, Copyright © 1998-2017. All rights reserved.