Section 10. Advanced Camera For Surveys

Observations made with the Advanced Camera for Surveys (ACS) are processed by the task calacs, which makes use of the calibration files described in this chapter. More details about the files and their use in calacs may be found in Instrument Science Report ACS-99-08, “CALACS Reference Files” (Hack, 1999). The calibration files contribute to PyDrizzle which is also used to perform distortion correction on all ACS images

10.1 File Formats and Conventions

Continuing the style of STIS and NICMOS, all files are FITS files with extensions. They have names of the form <uniquename>_xyz.fits where <uniquename> is a nine character unique name, the last character of which will be a j for all ACS files. The leading eight characters are based on the date and time when the file was created. xyz represents the type of calibration file and will be one of the strings such as bia, drk, etc, the meaning of which is given in the following sections. The ACS has three detectors known as the Wide Field Channel, the High Resolution Channel, and the Solar Blind Channel, distinguished by the keyword DETECTOR, which may take the values WFC, HRC, and SBC. The WFC is composed of two CCDs, each 2048 by 4096 pixels. The data from this and the corresponding calibration files contain two imsets in a single FITS file. The extension headers contain the keyword CCDCHIP, which take the value 2 for imset 1 and the value 1 for imset 2. The HRC and SBC are each single imsets of 1024 by 1024 pixels.

10.2 ACS Reference Images

The header for an ACS reference image will be of the following form. The list is derived from ACS-99-05, “Design of the ACS Science Headers,” with observation-specific keywords removed and the calibration keywords DESCRIP, PEDIGREE, and USEAFTER added. So, a calibration file header can easily be created by editing an observation header. The keyword FILETYPE is used quite differently in calibration files and is specified below for each type of calibration file. Each string must exactly match the values specified and will be checked before being accepted by CDBS.

Not all of the listed keywords are strictly required. For instance the EXPOSURE INFORMATION set is largely meaningless for a calibration file, which may be derived from multiple observations. However, the EXPTIME must be given for such things as dark count images and should normally have the value 1 second. In general it is simplest to include the whole group because the science headers contain them. There is no problem caused by having extra keywords in the header; unneeded ones are simply ignored. The ENGINEERING PARAMETERS and CALIBRATED ENGINEERING PARAMETERS groups are only meaningful for the CCD images, namely those from the WFC and HRC.

The DESCRIP keyword contains a brief description of the data. More extensive information should be supplied in the load files used when the calibration files are installed in CDBS. It may be convenient to keep the same information as history lines in the file header. PEDIGREE can have the values DUMMY, GROUND, or INFLIGHT. If the value DUMMY is present, OPUS

will skip the related calibration step even if the switch is set to PERFORM. The USEAFTER keyword contains the date from which the file is to be applied, superseding any other files with the same selection parameter values and earlier USEAFTER dates.

ACS File Header

SIMPLE  = T data conform to FITS standard                               L1
BITPIX  =   bits per data value                                         I2
NAXIS   = 0     number of data axes                                     I2
EXTEND  = T     file may contain standard extensions                    L1
NEXTEND =   number of standard extensions                               I2
GROUPS  = F     image is in group format                                L1
DATE        =   date this file was written (yyyy-mm-dd)                 C10
FILENAME    =   name of file                                            C18
FILETYPE    =   calibration file type                                   C67

TELESCOP    = HST   telescope used to acquire data                      C03
INSTRUME    =   identifier for instrument used to acquire data          C06
EQUINOX     = 2000.0    equinox of celestial coord. System              R4

          / CDBS KEYWORDS
PEDIGREE    =   data source is DUMMY, GROUND, or INFLIGHT               C8
DESCRIP =   a brief description of the data                             C67
USEAFTER    = 25 Mar 1999   date following which this file applies      C12

         / DATA DESCRIPTION KEYWORDS

ROOTNAME    =   rootname of the observation set                         C09
IMAGETYP    =   type of exposure identifier                             C18
PRIMESI =   instrument designated as prime                              C06

         / EXPOSURE INFORMATION

DATE-OBS    =   UT date of start of observation (yyyy-mm-dd)            C10
TIME-OBS    =   UT time of start of observation (hh-mm-ss)              C08
EXPSTART    =   exposure start time (Modified Julian Date)              R8
EXPEND  =   exposure end time (Modified Julian Date)                    R8
EXPTIME     =   exposure duration (seconds)--calculated                 R4
EXPFLAG     = NORMAL    exposure interruption indicator                 C13


         / SCIENCE INSTRUMENT CONFIGURATION

OBSTYPE     =   observation type - imaging or spectroscopic             C14
OBSMODE     =   operating mode                                          C08
SCLAMP  =   lamp status, NONE, or name of lamp which is on              C09
NRPTEXP     =   number of repeat exposures in set: default 1            I2
SUBARRAY    =   data from a subarray (T) or full frame (F)              L1
DETECTOR    =   detector in use: WFC, HRC, or SBC                       C03
FILTER1     =   element selected from filter wheel 1                    C18
FILTER2     =   element selected from filter wheel 2                    C18
LRFWAVE     =   proposed linear ramp filter wavelength                  R4
APERTURE    =   aperture name                                           C16
CRSPLIT     =   number of cosmic ray split exposures                    I2



/ ENGINEERING PARAMETERS

CCDAMP  =   CCD amplifier readout configuration                         C04
CCDGAIN     =   commanded gain of CCD                                   I2
CCDOFSTA    =   commanded CCD bias offset for amplifier A               I4
CCDOFSTB    =   commanded CCD bias offset for amplifier B               I4
CCDOFSTC    =   commanded CCD bias offset for amplifier C               I4
CCDOFSTD    =   commanded CCD bias offset for amplifier D               I4

         / CALIBRATED ENGINEERING PARAMETERS

ATODGNA     =   calibrated gain for CCD amplifier A                     R4
ATODGNB =   calibrated gain for CCD amplifier B                         R4
ATODGNC     =   calibrated gain for CCD amplifier C                     R4
ATODGND =   calibrated gain for CCD amplifier D                         R4
READNSEA    =   calibrated read noise for amplifier A                   R4
READNSEB    =   calibrated read noise for amplifier B                   R4
READNSEC    =   calibrated read noise for amplifier C                   R4
READNSED    =   calibrated read noise for amplifier D                   R4

END

XTENSION    =   extension type                                          C08
BITPIX  =   bits per data value                                         I2
NAXIS       = 2     number of data axes                                 I2
NAXIS1  =   length of first data axis                                   I4
NAXIS2  =   length of second data axis                                  I4
PCOUNT  = 0     number of group parameters                              I2
GCOUNT  = 1     number of groups                                        I2
TFIELDS     =   number of fields in each table row                      I2
INHERIT     = T     inherit the primary header                          L1
EXTNAME     =   extension name                                          C06
EXTVER  =   extension version number                                    I2
ROOTNAME    =   rootname of the observation set                         C09
EXPNAME     =   9 character exposure identifier                         C09
DATAMIN     =   the minimum value of the data                           R8
DATAMAX     =   the maximum value of the data                           R8
BUNIT       =   brightness units                                        C18
BSCALE  = 1.0   scale factor for array value to physical value          R8
BZERO       = 32768.    physical value for an array value of zero       R8

         / WFC CCD CHIP IDENTIFICATION

CCDCHIP =   CCD chip (1 or 2)                                           I2

         / World Coordinate System and Related Parameters

LTV1        = 0     offset in X to subsection start                     R4
LTV2        = 0     offset in Y to subsection start                     R4
LTM1_1  = 1     reciprocal of sampling rate in X                        R4
LTM2_2  = 1     reciprocal of sampling rate in Y                        R4

/ READOUT DEFINITION PARAMETERS

CENTERA1    =   subarray axis1 center pt in unbinned detector pixels    I4
CENTERA2    =   subarray axis2 center pt in unbinned detector pixels    I4
SIZAXIS1    =   subarray axis1 size in unbinned detector pixels         I4
SIZAXIS2    =   subarray axis2 size in unbinned detector pixels         I4
BINAXIS1    = 1 axis1 data bin size in unbinned detector pixels         I2
BINAXIS2    = 1 axis2 data bin size in unbinned detector pixels         I2

/SBC-SPECIFIC PARAMETERS

MOFFSET1    =   axis 1 MAMA offset                                      I2
MOFFSET2    =   axis 2 MAMA offset                                      I2

END


XTENSION    =   extension type                                          C08
BITPIX  =   bits per data value                                         I2
NAXIS       = 2     number of data axes                                 I2
NAXIS1  =   length of first data axis                                   I4
NAXIS2  =   length of second data axis                                  I4
PCOUNT  = 0     number of group parameters                              I2
GCOUNT  = 1     number of groups                                        I2
TFIELDS     =   number of fields in each table row                      I2
INHERIT     = T     inherit the primary header                          L1
EXTNAME     =   extension name                                          C06
EXTVER  =   extension version number                                    I2
ROOTNAME    =   rootname of the observation set                         C09
EXPNAME     =   9 character exposure identifier                         C09
DATAMIN     =   the minimum value of the data                           R8
DATAMAX     =   the maximum value of the data                           R8
BUNIT       =   brightness units                                        C18
NPIX1       =   length of constant array axis 1                         I4
NPIX2       =   length of constant array axis 2                         I4
PIXVALUE    =   values of pixels in constant array                      R4

         / World Coordinate System and Related Parameters

LTV1        = 0     offset in X to subsection start                     R4
LTV2        = 0     offset in Y to subsection start                     R4
LTM1_1  = 1     reciprocal of sampling rate in X                        R4
LTM2_2  = 1     reciprocal of sampling rate in Y                        R4

END


XTENSION    =   extension type                                          C08
BITPIX  =   bits per data value                                         I2
NAXIS       = 2     number of data axes                                 I2
NAXIS1  =   length of first data axis                                   I4
NAXIS2  =   length of second data axis                                  I4
PCOUNT  = 0     number of group parameters                              I2
GCOUNT  = 1     number of groups                                        I2
TFIELDS     =   number of fields in each table row                      I2
INHERIT     = T     inherit the primary header                          L1
EXTNAME     =   extension name                                          C06
EXTVER  =   extension version number                                    I2
ROOTNAME    =   rootname of the observation set                         C09
EXPNAME     =   9 character exposure identifier                         C09
DATAMIN     =   the minimum value of the data                           R8
DATAMAX     =   the maximum value of the data                           R8
BUNIT       =   brightness units                                        C18
NPIX1       =   length of constant array axis 1                         I4
NPIX2       =   length of constant array axis 2                         I4
PIXVALUE    =   values of pixels in constant array                      I4


         / World Coordinate System and Related Parameters

LTV1        = 0     offset in X to subsection start                     R4
LTV2        = 0     offset in Y to subsection start                     R4
LTM1_1  = 1     reciprocal of sampling rate in X                        R4
LTM2_2  = 1     reciprocal of sampling rate in Y                        R4

END

10.2.1 Bias Reference Image (BIA): <unique name>_bia.fits

The FILETYPE keyword must be the string “BIAS”.

Selection of the appropriate reference image will depend on the DETECTOR, CCDAMP, CCDGAIN, NAXIS1, NAXIS2, LTV1 and LTV2 keywords. They are only applicable to the CCD detectors (DETECTOR = WFC or HRC).

For ACS CCD science data, it is normally the case that bias correction will be performed before the data is combined for cosmic-ray rejection. The reference image, therefore, must contain the same keywords, including the NAXIS1, NAXIS2 information in the SCI array.

The reference image for this step, the BIASFILE, will normally have the dimensions of a full- size science image, complete with overscan regions, which would be 1062x1044 for HRC and 4144x2068 for WFC. The bias calibration step occurs prior to trimming off the overscan regions. Calacs also assumes that CCD data is not being read out in binned mode, and any image which does not have the same size as the full size image is a subarray readout. Only for subarrays will the LTV1 and LTV2 keywords be non-zero. Finally, the bias image is assumed to have already been scaled by the gain. Although the exposure time is not used by calacs , the EXPTIME keywords should be set to 0.0.

10.2.2 Dark Image (DRK): <unique name>_drk.fits

The FILETYPE keyword must be the string “DARK”.

Dark images are selected by DETECTOR and, except when DETECTOR=SBC, further selection is by CCDAMP and CCDGAIN. SBC science headers will not have entries for CCDAMP or CCDGAIN and SBC dark calibration image headers will have values of “N/A” and –999.

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 that is smaller than the full detector size is assumed to be a subarray 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.

10.2.3 Flat-Field Images (PFL, LFL): <unique name>_pfl.fits, and <unique name>_lfl.fits

The FILETYPE keyword must be one of the strings “PIXEL-TO-PIXEL FLAT,” or “LARGE SCALE FLAT.”

Flat fields are selected by DETECTOR, CCDAMP, CCDGAIN, FILTER1, and FILTER2.

Additionally the PFL files are selected by OBSTYPE which may take the values IMAGING, INTERNAL or CORONAGRAPHIC. For the SBC, the string variables CCDAMP, CCDGAIN and FILTER2 are given the value “N/A” and the integer CCDGAIN is given the value -999. ACS can utilize two different flat-field images during calibration: the pixel-to-pixel file (PFLTFILE), 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 overscan-trimmed image (2048x4096 for each WFC chip and 1024x1024 for the HRC and SBC). The low-order flat accounts for any large-scale flat-field variations across each detector.

These flat field images should all be scaled to an exposure time of 1 second, and the EXPTIME keyword should be updated to reflect this. 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.

Subarray science images will use the same reference file as a full-size image; however, calacs will extract the appropriate region from the reference file and apply it to the subarray input image.

10.2.4 Shutter-Shading Correction Image (SHD): <unique name>_shd.fits

The FILETYPE keyword must be the string “SHUTTER SHADING”. Shutter shading images are selected by DETECTOR, but do not apply to the SBC. This reference file is designed to correct the image for the differential exposure time across the detector that results from the shutter travel time as it opens to start the exposure. These files can be applied either during cosmic-ray rejection, should that be done for the observation, or during the basic processing in ACS2D for single or REPEAT-OBS exposures. 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 that can be applied to the data, rather than images, but the baseline version of calacs was designed to work with images. At present (October 2002) no need for SHD files has been identified.

10.2.5 Post flash image (FLS): <unique name>_fls.fits

The FILETYPE is “POSTFLASH”. It will be selected by DETECTOR, CCDAMP, CCDGAIN, FLASHCUR AND SHUTRPOS. This reference file will be used to support a post flash image designed to mitigate CTE effects. It is a short exposure to an LED which adds some background counts to the image. The image is of the same nature as a dark image and is normalized to a 1 second exposure. The science header

has a keyword FLASHDUR, being the duration of the applied flash, hence allowing a scaled value of the flash image to be subtracted from the science image by calacs .

10.2.6 Geometric Delta Image (DXY): <unique name>_dxy.fits

The FILETYPE keyword must be the string “DELTA XY”.

Selection of the appropriate reference image will depend on the DETECTOR, FILTER1, and FILTER2 keywords.

These images contain refinements to the geometric correction beyond that defined by the polynomials described in the IDC tables. There are corrections in the x and y directions.

So for the WFC there are four images in the file, two for each chip, while for the HRC and SBC there will be two image sets. The images will have the dimensions of a trimmed (no overscan) science image.

Use of these images is only for the most stringent astrometric applications and provides corrections of order 0.1 pixels. In most cases this calibration step can be omitted with no visible impact on the images or degradation of the analysis. Images have only been derived for a few filters and when a DXY image is not available, this will not be considered an error. Processing will continue omitting this step.

10.3 ACS Tables

Tables are treated slightly differently from images by CDBS. For the ACS each type of table is supplied independently for each detector. Then, for each detector, the associated table must have rows for all calibrated modes. When a new table is delivered, it must contain rows matching all previously delivered rows, even if some of them are unchanged, because the previous table will not be referenced for observations later than the USEAFTER date. The new table may contain rows that did not appear in previous tables. Table headers are quite simple, only needing the keywords shown in the tables below. The PEDIGREE and DESCRIP keywords appear in the table rows, since each row may have a different pedigree.

ACS Table Header

SIMPLE  =T  Fits standard
BITPIX  =16 Bits per pixel
NAXIS   =0  Number of axes
ORIGIN  =   FITS file originator
IRAF-TLM    =   Time of last modification
EXTEND  =T  There may be standard extensions
DATE    =24/02/99
FILENAME    =
TELESCOP    =HST
INSTRUME    =ACS
DETECTOR    =   WFC, HRC, or SBC
DATE    =   Date tape was written
FILETYPE    =   Calibration file type
USEAFTER    =   Date from which table is applicable

10.3.1 Analog-to-Digital Table (A2D): <unique name>_a2d.fits

The FILETYPE keyword must be the string “ANALOG-TO-DIGITAL”. Table selection is by DETECTOR and row selection by CCDCHIP, CCDAMP, CCDGAIN. This type is only applicable to CCDs This table provides the actual number of counts for each detected count in the image and allows for possible irregularities that might occur in the conversion such as were seen on the original WFPC. The conversion takes into account the gain setting, the amps used, and, typically, the exposure time of the observation. Table 10-1 defines the A2D table columns.

Table 10-1. Analog-to-Digital Table (A2D), <unique name>_a2d.fits

Column Name Data Type Units Description
CCDAMP CH*4 CCDAMP keyword value
CCDGAIN S Electrons/DN Commanded gain
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/INFLIGHT
DESCRIP CH*67 Short note describing this row

As of October 2002, no need for this table has been identified.

10.3.2 Bad Pixel Table (BPX): <unique name>_bpx.fits

The FILETYPE is “BAD PIXELS”.

Bad Pixel Tables are selected by DETECTOR (WFC, HRC or SBC) and table rows by CCDCHIP, CCDAMP, and CCDGAIN except that if DETECTOR=SBC there is no row selection. The science header for the SBC does not have entries for CCDCHIP, CCDAMP, or CCDGAIN. As for the image keywords, the not applicable table entries show “N/A” or –999 according to the data type. This reference file maintains a record of all known bad pixels for each ACS CCD. These pixels change with time as some hot pixels are annealed and others appear. 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 defined in Table 10-2:

Table 10-2.Bad Pixel Table (BPX)), <unique name>_bpx.fits

Column Name Data Type Units Description
CCDAMP DESCRIP CH*67
CCDGAIN CH*4 CH*67
CCDCHIP S
PIX1 S Electrons/DN CCDAMP keyword value
PIX2 S Commanded gain
LENGTH S Pixel CHIP to which this conversion applies
VALUE S Pixel X position of bad pixel list
AXIS S Pixel Y position of bad pixel list
PEDIGREE S Number of bad pixels in this list

The type of bad pixels which can be flagged are listed below. Some values are only marked during other processing steps (such as cosmic-ray rejection), but the VALUE column in this table specifies how the pixel is marked in the DQ array at the start of calibration processing.

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 New hot pixel
32 Large blemish
64 Pre-existing hot pixel
128 Bias level pixel
256 Saturation (full well or A-to-D)
512 Bad pixel in reference file
1024 Weak trap
2048 A-to-D saturation
4096 Reserved
8192 Cosmic ray – rejected during image combination

10.3.3 CCD Characteristics Table (CCD): <unique name>_ccd.fits

The FILETYPE keyword must be the string “CCD PARAMETERS”.

The CCD table is selected by DETECTOR. There are tables only for WFC and HRC. The database then tracks at the row level for CCDCHIP, CCDAMP, and CCDGAIN. Up to four amplifiers can be used for any given observation, and each amplifier has its own read-out characteristics. However, only a single value for these characteristics can be commanded by the observer. This table (with columns as defined in Table 10-3) 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 readout characteristics. The characteristics affected are readout noise (READNSE), A-to-D gain (ATODGN), bias offset level (CCDOFST), and bias level (CCDBIAS).

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), 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 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 10-3. CCD Characteristics Table (CCD), <unique name>_ccd.fits

Column Name Data Type Units Description
CCDAMP CH*4 CCDAMP keyword value
CCDCHIP S Chip to which this conversion applies
CCDGAIN S Commanded gain
CCDBIASA R Commanded bias level for amp A
CCDBIASB R Commanded bias level for amp B
CCDBIASC R Commanded bias level for amp C
CCDBIASD R Commanded bias level for amp D
CCDOFSTA S Actual bias for amp 1 of CCD
CCDOFSTB S Actual bias for amp 2 of CCD
CCDOFSTC I Actual bias for amp 3 of 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 amp 1 used for readout
ATODGNB R Actual gain for amp 2 used for readout
ATODGNC R Actual gain for amp 3 used for readout
ATODGND R Actual gain for amp 4 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 DN CCD saturation threshold
PEDIGREE CH*67 How this row was created (DUMMY, GROUND)
DESCRIP CH*67 Short note describing this row

10.3.4 Overscan Region Table (OSC): <unique name>_osc.fits

The FILETYPE keyword must be the string “OVERSCAN”.

The OSC table is selected by DETECTOR and is only used for the CCDs. CDBS tracks only the CCDCHIP and CCDAMP rows within the table. This table (with columns defined in Table 10-4) 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 column. These completely specify the physical overscan regions for each chip, and these columns and rows are 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 BIASSECTA1, BIASSECTA2 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 pixel (VX2, VY2). All coordinates and column numbers are specified in terms of the untrimmed image.

Table 10-4. Overscan Region Table (OSC), <unique name>_osc.fits

Column Name Data Type Units Description
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 section
BIASSECTA2 S Ending column for leading bias section
BIASSECTB1 S Beginning column for trailing bias section
BIASSECTB2 S Ending column for trailing bias section
VX1 S Pixel X coordinate of virtual overscan origin
VX2 S Pixel Y coordinate of virtual overscan origin
VY1 S Pixel X coordinate of top corner of virtual overscan region
VY2 S Pixel Y coordinate of top corner of virtual overscan region
PEDIGREE CH*67 Pedigree of this row was created (DUMMY, GROUND, or INFLIGHT)
DESCRIP CH*67 Source and quality of specified overscan regions

10.3.5 Cosmic-Ray Rejection Parameter Table (CRR): <unique name>_crr.fits

The FILETYPE keyword must be the string “COSMIC RAY REJECTION”.

The Cosmic Ray Rejection table is selected by DETECTOR (WFC or HRC) and CCDCHIP. The SBC does not use this method as it rejects cosmic rays on input by pulse height discrimination. This table (see Table 10-5) 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 on 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 is compared to MEANEXP from this table, and the row with the lowest MEANEXP without being less than the input image’s exposure time is selected. Once the appropriate row is selected, then the rest of the columns 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 is controlled by the 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, 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.

Table 10-5. Cosmic-Ray Rejection Parameter Table (CRR), <unique name>_crr.fits

Column Name Data Type Units Description
CRSPLIT S Number of exposures observation was split into
CCDCHIP S CHIP to which this conversion applies
MEANEXP R Sec Average exposure time for each image
SCALENSE CH*8 Multiplicative noise in percents
INITGUES CH*8 Scheme of competing initial-guess image
SKYSUB CH*4 Sky levels subtraction scheme
CRSIGMAS CH*20 Rejection thresholds
CRRADIUS R Pixel Radius (in pixels) to propagate the cosmic ray
CRTHRESH R Propagation factor
BADINPDQ S Data quality pset
CRMASK B Flag CR-rejected pixels in input files?
PEDIGREE CH*67 Pedigree of this row (DUMMY, GROUND, or INFLIGHT)
DESCRIP CH*67 Source and quality of specified overscan regions

10.3.6 MAMA Linearity Table (LIN): <unique name>_lin.fits

The FILETYPE keyword must be the string “LINEARITY”.

For a given time, there is a single MAMA linearity table selected by DETECTOR, which can only have the value SBC. Although this barely constitutes a selection, the method is consistent with the rest of CDBS and the table still must be chosen according to the USEAFTER date. This table (see Table 10-6) 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. Local non-linearity for some pixels can occur even when the global limit is not exceeded. Each pixel found to exceed the limit given in the column LOCAL_LIMIT is marked as nonlinear in the DQ file out to a radius from the pixel given in the EXPAND column.

Table 10-6. MAMA Linearity Table (LIN), <unique name>_lin.fits

Column Name Data Type Units Description
DETECTOR CH*10 Name of MAMA detector used
GLOBAL_LIMIT D C/s Count rate resulting in 10% global nonlinearity
LOCAL_LIMIT D C/s/pixel Count rate resulting in 10% local nonlinearity
TAU D S Time constant in global nonlinearity expression
EXPAND R Pixel Radius in pixels
PEDIGREE CH*67 How this row was created (DUMMY, GROUND)
DESCRIP CH*67 Short note describing this row

10.3.7 Photometry and Throughput Table (PHT): <unique name>_pht.fits

The FILETYPE keyword must be the string “PHOTOMETRY”. The photometry table is selected by DETECTOR, with rows selected by FILTER1 and FILTER2.

Photometry on ACS observations requires a transformation from DN measured in the image to flux units. The STSDAS package synphot has a task called `**bandpar**’ which can provide the parameters for that transformation; namely, PHOTFLAM, PHOTZPT, PHTOBW, and PHOTPLAM. The PHOTTAB table, with the columns listed in Table 10-7, provides the calibrated throughput for every filter combination used for ACS observations. This table is selected by 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 that has the same combination as used in the observation is 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 10-7. Photometry and Throughput Table (PHT), <unique name>_pht.fits

Column Name Data Type Units Description
FILTER1 DESCRIP CH*67
FILTER2 CH*24 CH*67
NELEM CH*24 Name of Filter1 used for observation
WAVELENGTH I Name of Filter2 used for observation
THROUGHPUT R[NELEM] Angstroms Number of wavelengths in throughput array
PEDIGREE R[NELEM] Wavelength at which throughput is measured

The PHT table is no longer used. The keywords GRAPHTAB and COMPTAB point to files used by synphot This method replaces the earlier method of calculating photometry and is now applied across instruments.

10.3.8 Image Distortion CoefficientsTable (IDC): <unique name>_idc.fits

The FILETYPE is “DISTORTION COEFFICIENTS”.

Tables are selected by DETECTOR, FILTER1, FILTER2.

This table is principally designed to support the Drizzle package. It contains the distortion solution in the form of polynomials plus reference positions and size information. Since the distortion is somewhat affected by the filters there can be a separate table for each filter combination. In practice only a few tables will be supplied each referring to a number of filter combinations. CDBS maintains a structure which allows multiple instrument configurations to point to a single reference file or table. Because of this the value of FILTER1 or FILTER2 in the header will sometimes take the value ANY. The table header must contain in addition to the normal keywords, NORDER, an integer giving the order of the polynomial fit, and PARITY which will always be –1 for the ACS. The number of coefficients in the table row depends on NORDER and is equal to (NORDER+1)(NORDER+2)/2. There can be sets of coefficients for DIRECTION = FORWARD which converts the distorted image to an undistorted version and for DIRECTION=INVERSE which transforms from the corrected image to the distorted view.

Table 10-8. Image Distortion Table (IDC), <unique name>_idc.fits

Column Name Data Type Units Description
DETCHIP I Chip number, 1 or 2
DIRECTION CH*8 FORWARD or INVERSE
XSIZE I Size in x direction of final output image
YSIZE I Size in y direction of final output image
XREF R X pixel position of reference point
YREF R Y pixel position of reference point
V2REF R Matching V2 position of reference point
V3REF R Matching V3 position of reference point
SCALE R Linear dimension in arcsec of square output image pixel
CX10 R First distortion coefficient
... ... Further coeffcicents
CXnn Last coefficient in X direction
CY10 First coefficient in Y direction
... R Further coefficients
Cynn R Last coefficient in Y direction
PEDIGREE CH*67 How this row was created (DUMMY, GROUND)
DESCRIP CH*67 Short note describing this row

10.3.9 Synphot Tables (SYN): <ins_component_version>_syn.fits

SYNPHOT tables are used by CALACS to populate the photometric keyword values during ACS pipeline processing. The SYNPHOT tables are the quantum efficiency curves, with throughput as a function of wavelength. They are selected in the comptab and graphtab tables by name, rather than by any keywords.

10.3.10 COMPTAB (<uniqname>_tmc.fits) and GRAPHTAB (<uniqname>_tmg.fits)

The COMPTAB contains a listing of each synphot throughput file used for all of HST’s instruments. Any updates to a throughput file will require the same version number update to the COMPTAB. For example acs_f330w_003_syn.fits updates to acs_f330w_004_syn.fits. to (NORDER+1)(NORDER+2)/2. There can be sets of coefficients for DIRECTION = FORWARD which converts the distorted image to an undistorted version and for DIRECTION=INVERSE which transforms from the corrected image to the distorted view.

Table 10-8. Image Distortion Table (IDC), <unique name>_idc.fits

Column Name Data Type Units Description
DETCHIP I Chip number, 1 or 2
DIRECTION CH*8 FORWARD or INVERSE
XSIZE I Size in x direction of final output image
YSIZE I Size in y direction of final output image
XREF R X pixel position of reference point
YREF R Y pixel position of reference point
V2REF R Matching V2 position of reference point
V3REF R Matching V3 position of reference point
SCALE R Linear dimension in arcsec of square output image pixel
CX10 R First distortion coefficient
... ... Further coeffcicents
CXnn Last coefficient in X direction
CY10 First coefficient in Y direction
... R Further coefficients
Cynn R Last coefficient in Y direction
PEDIGREE CH*67 How this row was created (DUMMY, GROUND)
DESCRIP CH*67 Short note describing this row

10.3.9 Synphot Tables (SYN): <ins_component_version>_syn.fits

SYNPHOT tables are used by CALACS to populate the photometric keyword values during ACS pipeline processing. The SYNPHOT tables are the quantum efficiency curves, with throughput as a function of wavelength. They are selected in the comptab and graphtab tables by name, rather than by any keywords.

10.3.10 COMPTAB (<uniqname>_tmc.fits) and GRAPHTAB (<uniqname>_tmg.fits)

The COMPTAB contains a listing of each synphot throughput file used for all of HST’s instruments. Any updates to a throughput file will require the same version number update to the COMPTAB. For example acs_f330w_003_syn.fits updates to acs_f330w_004_syn.fits. The GRAPHTAB instructs synphot on exactly which of the synphot throughout files listed in the COMPTAB are to be used in calculations for a given detector and filter combination. Both files are selected only by USEAFTER for all instruments.

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