ACS Instrument Science Report 99-08
CALACS Reference Files
Warren J. Hack
November 11, 1999
ABSTRACT
This report describes the reference files used by CALACS to calibrate ACS observations.
The description of each reference file includes the selection criteria used to identify which
file is appropriate for the observation and contains a detailed description of the file format
and contents.
1. Introduction
ACS uses detectors designed for STIS, therefore, it was natural to design CALACS
based on code used for STIS data. Lessons learned from the STIS calibration pipeline
software CALSTIS were applied to the creation of the ACS calibration pipeline software,
CALACS. However, the larger datasets and multiple output products imposed restrictions
on what could feasibly be done using the CALSTIS-model. As a result, CALACS evolved
into a unique pipeline for processing ACS data.
The task CALACS serves as the primary controller for the entire pipeline, although
each of the processing tasks in the package could be run on the data individually as well.
The primary function of the CALACS task itself will be to interpret any existing ACS
association table relevent to the data, and send the data to the proper tasks in the required
order for processing. In addition to working with associated data, the CALACS task processes individual exposures that are not part of any association.
Many calibration steps are performed during normal pipeline processing, and Figure 1
outlines where the individual tasks within the package are called during nominal processing by CALACS. The standard calibration processing steps performed by CALACS are
shown in Figure 2 for CCD data and Figure 3 for MAMA data. Each step relies on reference files to provide the calibrated values which need to be applied to the data. The
calibration files used for each step are listed in Figure 2 and Figure 3 on the right side of
the each step. This report describes what keywords are used to control the operation of
each step in Figure 2 and Figure 3. Section 2, “Keyword Usage” on page 4, lists those keywords which are used in the selection of the reference files and are revised during
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CALACS as they are applied to the ACS data. A complete description of the format and contents
of each reference file is given Section 3, “Reference Files” on page 5. In all, this report should
answer questions related to the generation of these reference files.
Figure 1: Flow Diagram for ACS data shown with CALACS task names.
Detector
CCD
ACSCCD
Another Image in
CR-SPLIT set?
(AtoD, DQI, BLEV, Bias)
MAMA
Another
Image in
association?
CRCORR
ACS2D
Yes
ACSREJ
DQI,Flat,Dark,...
Cosmic Ray Rejection
ACS2D
Flat, Dark,...
Another Set of CR-SPLIT Images?
RPT CORR
Yes
ACSSUM
DTH CORR
* AcsDth is a function
within the CALACS task.
Yes
AcsDth*
2
Figure 2: Flow diagram for CCD data in CALACS.
raw file
ACSCCD
doNoise
DQICORR
doDQI
BPIXTAB
ATODCORR
doAtoD
ATODTAB
BLEVCORR
doBlev
OSCNTAB
BIASCORR
doBias
BIASFILE
YES
CRREJ?
ACSREJ
NO
overscantrimmed
image
cr-combined
image
ACS2D
doNoise
Already done in ACSCCD
DQICORR SKIPPED
GLINCORR SKIPPED
doDQI
BPIXTAB
doNonLin
MLINTAB
MAMA Only
DARKCORR
doDark
DARKFILE
FLATCORR
doFlat
FLATFILE
SHADCORR
doShad
SHADFILE
PHOTCORR
doPhot
PHOTTAB
STATCORR
doStat
YES
ACSSUM
NO
RPTCORR?
summed, calibrated
image
calibrated
image
3
Figure 3: Flow diagram for MAMA data in CALACS.
raw file
ACS2D
doNoise
doDQI
BPIXTAB
GLINCORR
doNonLin
MLINTAB
DARKCORR
doDark
DARKFILE
FLATCORR
doFlat
FLATFILE
SHADCORR
doShad
SHADFILE
PHOTCORR
doPhot
PHOTTAB
STATCORR
doStat
DQICORR
YES
ACSSUM
NO
RPTCORR?
summed, calibrated
image
calibrated
image
2. Keyword Usage
Processing of the images in CALACS will be controlled by the population of keywords and
calibration switches in the input image headers, as it is in CALSTIS. Those keywords essential for
CALACS remain a subset of those used by CALSTIS, with keywords related to spectroscopy and
to binning of the images being absent. Table 1 provides a summary of those keywords which are
used by CALACS, with those that are unique to ACS marked appropriately.
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Table 1. ACS Keywords Used with Reference Files in CALACS
Keyword
Task(s)
I/O
Header
APERTURE
acsccd,acs2d,acssum
Input
Primary
ATODGNA, ATODGNB,
ATODGNC, ATODGND
acs2d,acsccd,acsrej
Input/Output
SCI(Primary?)
BADINPDQ
acsrej
Output
Primary
Sample of Possible Values
WFC1,WFC2,WFC1-FIX,
WFC2-FIX,WFC,HRC,SBC,...
BINAXIS1,BINAXIS2
acsccd, acs2d,calacs
Input
Primary
CCDAMP
acsccd,acs2d,acsrej
Input
Primary
ABCD,ABC,ABD,AC,AD,ACD, BC, BD, BCD
CCDCHIP
acs2d,acsccd,acsrej,acssum
Input
SCI,DQ,ERR
1,2
CCDGAIN
acsccd,acs2d,calacs
Input
Primary
1,2,4,8
CCDOFSTA, CCDOFSTB,
CCDOFSTC, CCDOFSTD
acs2d,acsccd
Input
Primary
CRRADIUS
acsrej
Output
Primary
CRMASK
acsrej
Output
Primary
CRSIGMAS
acsrej
Output
Primary
CRTHRESH
acsrej
Output
Primary
DETECTOR
calacs,acsrej,acssum
Input
Primary
WFC,HRC,SBC
FILTER1,
FILTER2
acs2d,acsccd,acssum
Input
Primary
CLEAR1L,F555W,F775W,F625W,F550M,
F850LP,CLEAR1S,POL0UV,POL60UV,...
GLOBLIM
acs2d
Output
SCI
EXCEEDED, NOT-EXCEEDED,
NOTAPPLICABLE, UNDETERMINED
GLOBRATE
acs2d
Input
SCI
INITGUES
acsrej
Output
Primary
LTM1_1,LTM2_2
acs2d,acsccd
Input/Output
SCI,ERR,DQ
LTV1,LTV2
acs2d,acsccd
Input/Output
SCI,ERR,DQ
NPIX1,NPIX2
calacs(acsdth)
Output
SCI
READNSEA, READNSEB, acs2d,acsccd,acsrej
READNSEC, READNSED
Input/Output
SCI
REJ_RATE
acsrej
Output
Primary
SCALENSE
acsrej
Output
Primary
SDQFLAGS
acs2d,acsccd,acssum
Input
Primary
STATFLAG
calacs
Input
Primary
T,F
3. Reference Files
Each ACS observation requires calibrations suited to the particular observation mode used to
take the data. Calibration data specific to the different modes of ACS operation are prepared and
archived in the Calibration Database System (CDBS). Generic Conversion processes the raw data
from the telescope and formats it for CALACS, including querying CDBS to determine which
reference files apply to the specific configuration used for the observation. The input files for
CALACS then contain keywords for each calibration step which record the name of the reference
file that is appropriate for the observation. These keywords were listed in Figure 2 and Figure 3
along side the name of the task they control. The reference data and format for each calibration
file are given in Table 2 in the order they are applied to the data by CALACS. The following sections provide a detailed description of the format of each calibration reference file.
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Table 2. Summary of ACS Reference File Keywords
Keyword
Format
Calibration Data
ATODTAB
table
A-to-D Conversion
BPIXTAB
table
Bad Pixel positions
CCDTAB
table
CCD characteristics
OSCNTAB
table
overscan-region specification
BIASFILE
image
bias image subtraction
CRREJTAB
table
cosmic-ray rejection parameters
MLINTAB
table
MAMA linearity characteristics
DARKFILE
image
dark image subtraction
PFLTFILE
image
pixel-to-pixel flat-field
LFLTFILE
image
low-order flat-field
DFLTFILE
image
temporal change in flat-field
SHADFILE
image
shading shutter correction
PHOTTAB
table
photometry characteristics
GEOFILE
geometric distortion characteristics
ATODTAB - A-to-D Conversion Table
This reference file provides the actual number of counts for each detected count in the image.
The conversion takes into account the gain setting, the amps used, and, typically, the exposure
time of the observation. The table contains the columns described in Table 3.
Table 3. Columns in A-to-D Reference Table
Column Name
Format
Units
Contents
CCDAMP
CH*4
CCDGAIN
S
CCDCHIP
S
CHIP to which this conversion applies
REF_KEY
CH*12
Usually EXPTIME
REF_KEY_VALUE
R
Values of REF_KEY for different A-to-D conversions
NELEM
I
Number of elements in ATOD array
ATOD
R[65536]
Array with actual values
PEDIGREE
CH*67
GROUND/DUMMY/IN-FLIGHT
DESCRIP
CH*67
How this calibration file was produced
CCDAMP keyword value
electrons/DN
Commanded gain
Each row in the table corresponds to a separate observation mode determined by the CCDGAIN, CCDAMP and CHIP keywords from the image header. The array length NELEM is read
from that row as well, and the length may differ from row to row, although the maximum length is
6
fixed. In addition, a secondary parameter could possibly affect the conversion, such as EXPTIME.
The REF_KEY column specifies the keyword to check, then the table row is selected to be the one
with the REF_KEY_VALUE closest to the keyword value from the image. This provides a mechanism for relating a secondary parameter to the A-to-D correction, for example EXPTIME or
some temperature, should such a parameter be found to affect the correction. Once the row that
best matches the observations configuration, the ATOD column contains an array of NELEM values which directly replace the pixel values in the image. The integer pixel value from the image is
the array index into the ATOD array, and that value is then written out to the output image by
CALACS.
BPIXTAB - Bad-pixel Table
This reference file, whose columns are described in Table 4, maintains a record of all known
bad pixels for each ACS CCD. These pixels may change with time as some hot pixels are
annealed and others appear over time, for example. Permanently bad pixels due to chip defects
may be flagged during Generic Conversion, and it is the job of the BPIXTAB to maintain the list
of bad pixels applicable for a given time period. The positions of the bad pixels are stored as pixel
lists using the columns described in Table 4.
Table 4. Columns in Bad Pixel Reference Table
Column Name
Format
Units
Contents
CCDAMP
CH*4
CCDGAIN
S
CCDCHIP
S
PIX1
S
pixel
X position of bad pixel list
PIX2
S
pixel
Y position of bad pixel list
LENGTH
S
pixel
Number of bad pixels in this list
VALUE
S
DQ value of bad pixels in list
AXIS
S
Bad pixels extend along this axis
CCDAMP keyword value
electrons/DN
Commanded gain
CHIP to which this conversion applies
The type of bad pixels which can be flagged are listed in Table 5. Some values will only be
marked during other processing steps (such as cosmic-ray rejection), but the VALUE column in
this table specifies how the pixel will be marked in the DQ array at the start of calibration
processing.
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Table 5. DQ Flag Values for Bad Pixels
Flag Value
Definition
0
Good Pixel
1
Reed-Solomon decoding error
2
data replaced by fill value
4
bad detector pixel or beyond aperture
8
masked by aperture feature
16
hot pixel
32
large blemish
64
reserved
128
bias level pixel
256
saturated pixel
512
bad pixel in reference file
1024
small blemish
2048
reserved
4096
reserved
8192
rejected during image combination
CCDTAB - CCD Characteristics table
Up to 4 amps can be used for any given observation, and each amp has its own read-out characteristics, however, only a single value for these characteristics can be commanded by the
observer. This table, described in Table 6, provides the conversion from the commanded values to
the calibrated values for each amp. These calibrated values are then used during processing by
CALACS to insure that a pixel read-out by an amp has been properly calibrated for that amp’s
read-out characteristics. The characteristics affected are read-out noise (READNSE), A-to-D gain
(ATODGN), and bias level (CCDOFST).
The table contains one row for each amp configuration used in the readout. This configuration
is uniquely identified by the list of amps used(CCDAMP), the particular chip being read out
(CCDCHIP), the commanded gain (CCDGAIN), the commanded bias level (CCDBIAS), and the
bin sizes of the pixels read out (BINAXIS). Each amp can be used to read out a section of the chip
or the entire chip depending on how many amps are used to read out the observation. As a result,
the values AMPX and AMPY specify the boundaries between amp read-out sections when used
in concert to read out a chip. For multi-amp read out of a chip, AMPX specifies the first column
affected by the second amp used to read out that row, and is set to zero when only one amp is used
to read out each row. Similarly, AMPY specifies the first row affected by the second set of amps
used to read out the chip, and is set to zero if only 1 amp or set of amps is used to read out the
chip. AMPY is always set to zero for WFC observations since each chip only has one set of amps
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to read them out. These values are also used throughout CALACS to determine which pixels were
read out by which amp and apply the corresponding read noise, gain and bias level to them.
Table 6. Columns in CCD Parameters Table
Column Name
Format
Units
Contents
CCDAMP
CH*4
CCDAMP keyword value
CCDCHIP
S
CHIP to which this conversion applies
CCDGAIN
S
Commanded gain
CCDBIAS
R
Commanded bias level
CCDOFSTA
S
actual bias for amp 1 of CCD
CCDOFSTB
S
actual bias for amp 2of CCD
CCDOFSTC
I
actual bias for amp 3of CCD
CCDOFSTD
I
actual bias for amp 4 of CCD
BINAXIS1
S
commanded bin size for axis 1
BINAXIS2
S
commanded bin size for axis 2
ATODGNA
R
actual gain for amp1 used for readout
ATODGNB
R
actual gain for amp2 used for readout
ATODGNC
R
actual gain for amp3 used for readout
ATODGND
R
actual gain for amp4 used for readout
READNSEA
R
electrons
calibrated value of readout noise for amp 1
READNSEB
R
electrons
calibrated value of readout noise for amp 2
READNSEC
R
electrons
calibrated value of readout noise for amp 3
READNSED
R
electrons
calibrated value of readout noise for amp 4
AMPX
S
first column affected by second amp
AMPY
S
first row affected by second set of amps
SATURATE
R
PEDIGREE
CH*67
How this row was created (DUMMY, GROUND)
DESCRIP
CH*67
When this row was created
DN
CCD saturation threshold
OSCNTAB - Overscan region table
This table, described in Table 7, has no counterpart in any previous calibration pipelines, as it
describes the overscan regions for each chip along with the regions to be used for determining the
actual bias level of the observation. Each row corresponds to a specific configuration as given by
the amps used (CCDAMP), the chip (CCDCHIP), and the size of the image with overscan regions
(NX, NY).
The columns TRIMX* give the number of columns to trim off the beginning and end of each
line, while the TRIMY* columns give the number of rows to trim off the top and bottom of each
9
column. These completely specify the physical overscan regions for each chip and these columns
and rows will be trimmed off the image during processing. The result of trimming (TRIMX1 +
TRIMX2) columns from the image (NX) should result in the desired calibrated image sizes, 4096
for a full WFC image and 1024 for a full HRC image. The same can be said for NY - (TRIMY1 +
TRIMY2), which should be 2048 for a full WFC image and 1024 for a full HRC image.
The columns BIASSECTA* give the range of columns to be used for determining the bias
level in the leading overscan region, while the BIASSECTB* columns give the range of columns
to be used to determine the bias level in the trailing overscan region. Finally, the virtual overscan
starts at pixel (VX1, VY1) and extends to (VX2, VY2).
All coordinates and column numbers are specified in terms of the untrimmed image.
Table 7. Columns in Overscan Region Table
Column Name
Format
Units
Contents
CCDAMP
CH*4
CCDAMP keyword value
CCDCHIP
S
CHIP to which this conversion applies
BINX
S
commanded bin size for axis 1
BINY
S
commanded bin size for axis 2
NX
S
pixel
number of columns in image with overscan regions
NY
S
pixel
number of rows in image with overscan regions
TRIMX1
S
pixel
Number of columns to trim off beginning of each line
TRIMX2
S
pixel
Number of columns to trim off end of each line
TRIMY1
S
pixel
Number of lines to trim off beginning of each column
TRIMY2
S
pixel
Number of line to trim off end of each column
BIASSECTA1
S
beginning column for leading bias sectio
BIASSECTA2
S
ending column for leading bias sectio
BIASSECTB1
S
beginning column for trailing bias section
BIASSECTB2
S
ending column for trailing bias section
VX1
S
pixels
X coordinate of virtual overscan origin
VX2
S
pixels
Y coordinate of virtual overscan origin
VY1
S
pixels
X coordinate of top corner of virtual overscan region
VY2
S
pixels
Y coordinate of top corner of virtual overscan region
DESCRIPTION
CH*67
Source and quality of specified overscan regions
BIASFILE - Bias reference image
For ACS science data, it will normally be the case that this correction will be performed
before the data has been combined for cosmic-ray rejection. Selection of the appropriate reference
image will depend on the DETECTOR, CCDAMP, and CCDGAIN keywords, as well as the
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dimensions of the science image. The reference image, therefore, must contain the same keywords, along with the NAXIS1,NAXIS2 information in the SCI array.
The reference image for this step, the BIASFILE, will have the same dimensions as a full-size
science image complete with overscan regions, which would be 1062x1044 for HRC and
4144x2068 for WFC. They are not 1024x1024 and 4096x2048 since this calibration step occurs
prior to trimming off the overscan regions. CALACS also assumes that CCD data will not be
read-out in binned mode, and any image which does not have the same size as the full size image
will be a sub-array read-out. Finally, the bias image is assumed to have already been scaled by the
gain and had the overscan values subtracted from the bias image. This should result in a bias
image with average pixel values less than one. The exposure time is not used by CALACS, so the
EXPTIME keywords do not need to be present. However, if they are present, they should be set to
0.0.
CRREJTAB - Cosmic-ray Rejection Parameter Table
This table, described in Table 8, contains all the basic parameters necessary for performing
cosmic-ray rejection. The appropriate row gets selected for use in the CALACS task ACSREJ first
based the chip being processed (CCDCHIP), then on the number of images the original exposure
was split into (CRSPLIT) and the exposure time of each CR-SPLIT image. The exposure time for
each CR-SPLIT image will be compared to MEANEXP from this table and the row with the lowest MEANEXP without being less than the input image’s exposure time will be selected. Once the
appropriate row has been selected, then the rest of the columns will serve as the input parameters
to control the detection algorithms.
The cosmic-ray rejection process requires a number of input parameters to control how the
cosmic-rays are detected. The process starts by creating a first guess for the CR-combined image
either by median combining or minimum value combining the input CR-SPLIT exposures, as
specified by INITGUES. Determination of the sky and noise values are controlled by SKYSUB
and SCALENSE values respectively. Actual detection of the cosmic-rays requires the specification of a threshold above which a pixel value is considered a cosmic-ray (CRSIGMAS and
CRTHRESH) and the distance from the detected pixel which the cosmic-ray can affect other pixels (CRRADIUS). Once a pixel is determined to be affected by a cosmic-ray, the value in
BADINPDQ specifies the DQ value to use to mark that pixel in the exposures DQ array, if
CRMASK was set to yes.
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Table 8. Columns in Cosmic-Ray Rejection Parameters Table
Column Name
Format
Units
Contents
CRSPLIT
S
Number of exposures observation was split into
CCDCHIP
S
CHIP to which this conversion applies
MEANEXP
R
SCALENSE
CH*8
multiplicative noise in percents
INITGUES
CH*8
Scheme of computing initial-guess image
SKYSUB
CH*4
Sky levels subtraction scheme
CRSIGMAS
CH*20
Rejection thresholds
CRRADIUS
R
CRTHRESH
R
Propagation factor
BADINPDQ
S
Data quality pset
CRMASK
B
flag CR-rejected pixels in input files?
sec
average exposure time for each image
pixels
Radius (in pixels) to propagate the cosmic ray
MLINTAB - MAMA Linearity Table
This table, with the columns listed in Table 9, provides the basic parameters for determining
linearity in MAMA images. Although there is only one MAMA detector within ACS, the first column specifies the detector name. The global limit given in the GLOBAL_LIMIT column refers to
the total counts/sec for the entire image at which the data is affected by greater than 10% non-linearity. CALACS will attempt to correct for non-linearity up to this limit using the non-linearity
constant given in the column TAU. Linearity for each pixel can be much higher, as given in the
column LOCAL_LIMIT. Each pixel found to exceed this limit will be marked as non-linear in the
DQ file out to a radius from the pixel given in the EXPAND column. The PEDIGREE and
DESCRIPTION columns are simply used to keep track of how the parameters were derived and
whether they are test parameters or whether they came from a calibration program.
Table 9. Columns in MAMA Linearity Reference Table
Column Name
Format
Units
Contents
DETECTOR
CH*10
GLOBAL_LIMIT
D
c/s
count rate resulting in 10% global nonlinearity
LOCAL_LIMIT
D
c/s
count rate resulting in 10% local nonlinearity
TAU
D
s
time constant in global nonlinearity expression
EXPAND
R
pixel
radius in high-res pixels
PEDIGREE
CH*67
How this row was created (DUMMY, GROUND)
DESCRIP
CH*67
When this row was created
Name of MAMA detector used
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DARKFILE - Reference Dark Image
The reference file for dark subtraction, DARKFILE, gets selected based on the values of
DETECTOR, CCDAMP, and CCDGAIN keywords in the input science image. This file gets
applied after the overscan regions are trimmed from the input science image and therefore must
have its overscan regions trimmed off as well. As with the BIASFILE, it is assumed that CCD
images will not be binned, so any input image smaller than the full detector size will be assumed
to be a sub-array image.
For CCD data, the dark image is multiplied by the exposure time and divided by the gain
before subtracting. This requires the dark image to already be scaled to an exposure time of 1 second and a gain of 1. The dark time is just the exposure time and does not include the idle time
since the last flushing of the chip or the readout time. For MAMA data, the dark image is just multiplied by the exposure time before subtracting, again implying that the reference file be scaled to
an exposure time of 1 second. This exposure time should be reflected in the EXPTIME keyword
of the SCI array of the reference image.
PFLTFILE, LFLTFILE, DFLTFILE - Flat-field images
ACS can utilize up to three different flat-field images during calibration: the pixel-to-pixel file
(PFLTFILE), the delta flat (DFLTFILE), and the low-order flat (LFLTFILE). The PFLTFILE represents the flat-field response for every pixel on the detector and has the same size as an overscantrimmed image (2048x4096 for each WFC chip and 1024x1024 for the HRC and SBC). A sample
PFLTFILE is shown in Figure 4 for the WFC Chip 2. Similarly, the delta flat tracks time-dependent changes of the PFLTFILE should any be found during operation. This image, if ever used,
will also have the same size as the PFLTFILE. Finally, the low-order flat accounts for any largescale flat-field variations across each detector. As such, this reference image usually gets stored in
as a sub-sampled image and expanded when being applied by CALACS. It is unclear, however, if
the DFLTFILE or LFLTFILE will ever be used for ACS, but CALACS provides the mechanism
for applying them to ACS data should they be needed.
All flat-field reference images will be chosen based on the detector, amp, gain, and filters used
for the observation. Any sub-array science images will use the same reference file as a full-size
image would, however, CALACS will extract the appropriate region from the reference file and
apply it to the sub-array input image. These flat field images should all be scaled to a mean pixel
value of 1. All the flat-field files which are to be applied to the science data will be multiplied
together to form a single flat-field, then divided into the science data. In order to avoid any floating-point errors in this operation, none of the flat-field images should contain any pixels with a
value of zero.
13
Figure 4: Test PFLAT file used for WFC Chip 2
SHADFILE - Shading-shutter correction image
This reference file corrects the image for the differential exposure time across the detector that
results from the shutter travel time as it opens to start the exposure. It is selected based on the values of the DETECTOR keyword in the input science image. This file can either be applied during
cosmic-ray rejection, should that be done for the observation, or during the basic processing in
ACS2D for single or REPEAT-OBS exposures.
At present, this reference image is a sub-sampled, overscanned-trimmed image representing
the change in exposure time across the detector due to the shutter opening. A sample shutter-shading correction image for the WFC Chip 2 is shown in Figure 5 with pixel values from 0 to 0.01.
This reference image was not intended to illustrate the actual correction applicable to the WFC
detector, but should be representative of the type of correction that will be applied. Currently,
these reference files are sub-sampled by a factor of 8 for each dimension, resulting in a 128x128
HRC SHADFILE and a 256x512 WFC file.
This reference image will be divided by the exposure time, have 1 added to it, then divided
into the input science exposure during processing. The image header for this reference file should
be the same as a science image taken with the same detector, with the keywords populated to
reflect how the shutter-shading data was obtained. Future versions of these files may be delivered
as mathematical models which can be applied to the data, rather than images, but the baseline version of CALACS was designed to work with images.
14
Figure 5: Test Shutter Shading Reference Image for WFC Chip 2.
PHOTTAB - Photometry and Throughput Table
Photometry on ACS observations requires a transformation from DN measured in the image
to flux units. The STSDAS package synphot has a task called ‘phopar’ which can provide the
parameters for that transformation; namely, PHOTFLAM, PHOTZPT, PHTOBW, and PHOTPLAM. The PHOTTAB table, with the columns listed in Table 10, provides the calibrated
throughput for every filter combination used for ACS observations. This table gets selected by the
DETECTOR, so every row in the table should be applicable to the observation.
The columns FILTER1 and FILTER2 give the filter combinations used for that detector, and
the row which has the same combination as used in the observation will be selected for use in
CALACS. The calibrated throughputs are stored as arrays of NELEM elements in the WAVELENGTH and THROUGHPUT columns, forming what is commonly referred to as a 3-D table.
Table 10. Columns in Photometry Reference Table
Column Name
Format
Units
Contents
FILTER1
CH*24
Name of Filter1 used for observation
FILTER2
CH*24
Name of Filter2 used for observation
NELEM
I
Number of wavelengths in throughput array
WAVELENGTH
R[NELEM]
THROUGHPUT
R[NELEM]
PEDIGREE
CH*67
““
How this row was created (DUMMY, GROUND)
DESCRIP
CH*67
““
When this row was created
Angstroms
Wavelength which throughput is measured at
Throughput for this filter combination
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GEOFILE - Geometric Distortion Characteristics
No geometric correction will be performed by CALACS, therefore this reference file will not
be needed for operation of CALACS. However, off-line tasks will require some information
regarding the geometric distortion of the detector used for the observation. This keyword will
point to a calibration product which contains some description of the geometric distortion applicable to the detector for use by separate tasks. The format of this file has not been determined yet,
and no tasks for correcting for this distortion exist yet. This keyword, therefore, serves as simply a
place-holder for these future developments.
4. Summary
Many parameters are required in order to calibrate ACS data, and the reference tables and
images serve as the inputs which contain those parameters. These reference files are selected by
the CDBS system and the keywords in the science image is populated with their filenames.
CALACS then relies expects them to have the formats described here in order to operate properly.
The descriptions of the reference files in this report should serve as the basis their creation during
instrument calibration program as the camera becomes operational.
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