Section 2 Faint Object Camera
Calibrations that may be applied to FOC images include background subtraction, division by a flat field, correction for geometric distortion, and conversion from counts to flux density. Correction for photometric nonlinearity (ITF) cannot be done as originally planned because the response depends on the flux in neighboring pixels. For long-slit spectrographic images the flat field correction and absolute photometric correction are combined into one step. For normal images the absolute photometric correction is not actually done, but a parameter is put into the output image header giving the ratio of flux density to count rate. The geometric correction accounts for distortion from the optics (for normal images) as well as camera distortion.
Reference tables are used for the flat field correction and the absolute photometric correction. The characteristic wavelength of the bandpass and the throughput of the entire optical system, including detector integrated over the bandpass, are computed from tables of the throughput of each component. The throughput gives the count-rate-to-flux-density factor, and the wavelength is used to select the appropriate flat field.
2.1 FOC Reference Files
The FOC reference files are composed of a FITS header file and a binary data file. Although the header files follow the FITS conventions, the header listings in this section do not adhere precisely to these conventions. In the header listings contained in this section, beware of the following differences:
The single quotes required around character strings have been omitted.
Numeric values have not been right justified.
The third column in the listing denoting the variable type is additional information, not part of the FITS header itself.
2.1.1 Background (Dark Count) Files (BAC): <unique name>.R0H
The background files contain data used to subtract dark counts from FOC science images. They are used for the background calibration.
Format: Background data files contain one group of data. Every background data file has the same format as an image. It is either a 1024 sample by 1024 line image for use with normal pixel format images or a 512 sample by 1024 line image for use with zoom pixel format images. Each pixel in the background data file is a single-precision real (REAL*4) count rate value in counts per second.
Flags and Indicators: The flags and indicators used to select the background reference files are OPTCRLY (the optical relay can take a value of either F48 or F96) and PXFORMT (the pixel format is either NORMAL or ZOOM).
Restrictions: The first 16 non-comment keywords in the header for this file set are required by SOGS software. Also, the group parameters DATAMIN and DATAMAX must be set.
The SOGS software mentioned was a predecessor of the current OPUS software used by the pipeline Legacy data, however, is not recalibrated. The final calibration products are now part of the STScI static archive.
A sample image header for <unique name>.R0H follows:
SIMPLE = F / image conforms to the fits standard L1
BITPIX = 32 / bits per data value I2
DATATYPE= REAL\* 4 / data type of the array CHR
NAXIS = 2 / number of data axes I2
NAXIS1 = / samples per line I2
NAXIS2 = 1024 / lines per image I2
GROUPS = T / image is in group format L1
GCOUNT = 1 / number of groups I2
PCOUNT = 2 / number of group parameters I2
PSIZE = 64 / bits in the group parameter block I2
/ GROUP PARAMETERS: PODPS CALIBRATION
PTYPE1 = DATAMIN / minimuum data value in the group CHR
PDTYPE1 = REAL\* 4 / datatype of parameter 1 CHR
PSIZE1 = 32 / number of bits in parameter I2
PTYPE2 = DATAMAX / maximum data value in the group CHR
PDTYPE2 = REAL\*4 / datatype of parameter 1 CHR
PSIZE2 = 32 / number of bits in parameter 1 I2
/ FILE DESCRIPTOR KEYWORDS
INSTRUME= FOC / instrument id CHR
FILETYPE= BACKGROUND / type of reference file CHR
HEADNAME = / reference header file name CHR
DATANAME= / reference data file name CHR
DATE = / date this file was written CHR
/ KEYWORDS USED TO SELECT BAC REF FILES
OPTCRLY = / optical relay: F48, F96 CHR
PXFORMT = / format of the image: NORMAL, ZOOM CHR
/ DATA DESCRIPTOR KEYWORDS
SAMPBEG = / first sample number I2
LINEBEG = / first line number I2
DATAFRMT= / data format CHR
COUNT/SECOND / data source(INFLIGHT,GROUND,MODEL,DUMMY)&date CHR
PEDIGREE=
DESCRIP = / description of reference data CHR
END
[Listing Version 02 12-APR-1994]
2.1.2 Intensity Transfer Function Files (ITF): <unique name>.R1H
The intensity transfer function (ITF) files contain data used to correct FOC science images for spatial nonuniformities that depend on the image format. They are used for the format-dependent photometric correction.
Format: An ITF data file has the same form as an FOC image. The lengths of the axes are the same or larger than those of the image to be calibrated. Each entry is a REAL*4 scaling factor that is used to multiply the corresponding pixel DN count in the FOC science data file that is to be corrected.
Flags and Indicators: The flags and indicators used to select the ITF files are OPTCRLY (the optical relay is either F48 or F96), PXFORMT (the pixel format is either NORMAL or ZOOM), SAMPOFF (the sample offset can be any appropriate REAL*4 value computed from the 12-bit item XCCCUHZO), LINEOFF (the line offset can be any appropriate REAL*4 value computed from the112-bit item XCCCUVRO), SAMPPLN (the samples per line will be 64, 128, 256, or 512), and LINEPFM (the lines per frame will be 64, 128, 256, 512, or 1024).
Restrictions: The first 11 non-comment keywords in the header for this file set, as well as the keywords SAMPBEG and LINEBEG, are requested by the SOGS software.
The SOGS software mentioned was a predecessor of the current OPUS software used by the pipeline Legacy data, however, is not recalibrated. The final calibration products are now part of the STScI static archive.
A sample image header for the file <unique name>.R1H follows:
SIMPLE = F / image conforms to the fits standard L1
BITPIX = 32 / bits per data value I2
DATATYPE= REAL*4 / data type of the array I2
NAXIS1 = / samples per line I2
NAXIS2 = / lines per image I2
GROUPS = T / image is in group format L1
GCOUNT = 1 / number of groups I2
PCOUNT = 0 / number of group parameters I2
PSIZE = 0 / bits in the group parameter block I2
/ FILE DESCRIPTOR KEYWORDS
INSTRUME= FOC / instrument id CHR
FILETYPE= ITF / type of reference file CHR
HEADNAME= / reference header file name CHR
DATANAME= / reference data file name CHR
DATE = / date this file was written CHR
/ KEYWORDS USED TO SELECT ITF REF FILES
OPTCRLY = / optical relay: F48, F96 CHR
PXFORMT = / format of the image: NORMAL, ZOOM CHR
SAMPOFF = / sample offset (0.0-1023.75) R4
LINEOFF = / line offset (0.0-1023.75) R4
SAMPPLN = / samples per line (64, 128, 256, or 512) I2
LINEPFM = / lines per frame (64, 128, 256, 512, or 1024) I2
/ DATA DESCRIPTOR KEYWORDS
SAMPBEG = / first sample number I2
LINEBEG = / first line number I2
PEDIGREE= / data source (INFLIGHT,GROUND,MODEL,DUMMY)&date CHR
DESCRIP = / description of the reference data CHR
END
[Listing Version 02 12-APR-1994]
2.1.3 Uniform Detector Efficiency Files (UNI): <unique name>.R2H
The uniform detector efficiency (DE) files, also known as relative detector efficiency files, contain data used to remove the FOC instrument’s spatial nonuniformities from the FOC science data. This step is known as the uniform DE calibration.
Format: A uniform DE file has the same form as an FOC image. It always has dimensions 1024 samples by 1024 lines. Each entry is a REAL*4 scaling factor used to multiply the corresponding pixel DN count in the FOC science data file that is to be corrected. For efficiency, the entries in this file are multiplied rather than divided, so the entries in this file must be the reciprocals of their usual values. All images to be corrected will find their proper position inside the chosen uniform DE data file.
Flags and Indicators: The flags and indicators used to select the uniform DE reference files are OPTCRLY (the optical relay can be either F48 or F96), CAMMODE (the coronographic apodizer mask can be either INBEAM or NOTUSED), OPTELT1, OPTELT2, OPTELT3, and OPTELT4.
Restrictions: The first 16 non-comment keywords in the header for this file set are required by the SOGS software. Also, the group parameters DATAMIN and DATAMAX must be set.
The SOGS software mentioned was a predecessor of the current OPUS software used by the pipeline Legacy data, however, is not recalibrated. The final calibration products are now part of the STScI static archive.
The following listing shows a sample image header for <uniquename>.R2H.
SIMPLE = F / image conforms to the fits standard L1
BITPIX = 32 / bits per data value I2
DATATYPE= REAL*4 / data type of the array CHR
NAXIS = 2 / number of data axes I2
NAXIS1 = 1024 / samples per line I2
NAXIS2 = 1024 / lines per image I2
GROUPS = T / image is in group format L1
GCOUNT = 1 / number of groups I2
PCOUNT = 2 / number of group parameters I2
PSIZE = 64 / bits in the group parameter block I2
/ GROUP PARAMETERS: PODPS CALIBRATION
PTYPE1 = DATAMIN / minimum data value in the group CHR
PDTYPE1 = REAL*4 / datatype of parameter 1 CHR
PSIZE1 = 32 / number of bits in parameter 1 I2
PTYPE2 = DATAMAX / maximum data value in the group CHR
PDTYPE2 = REAL*4 / datatype of parameter 2 CHR
PSIZE2 = 32 / number of bits in parameter 2 I2
/ FILE DESCRIPTOR KEYWORDS
INSTRUME= FOC / instrument id CHR
FILETYPE= UNIFORM DE / type of reference file CHR
HEADNAME= / reference header file name CHR
DATANAME= / reference data file name CHR
DATE = / date this file was written CHR
/ KEYWORDS USED TO SELECT UNI REF FILES
OPTCRLY = / optical relay: F48, F96 CHR
CAMMODE = / coronographic apodizer mask: INBEAM, NOTUSED CHR
OPTELT1 = / filter element, wheel 1 I2
OPTELT2 = / filter element, wheel 2 I2
OPTELT3 = / filter element, wheel 3 I2
OPTELT4 = / filter element, wheel 4 I2
WAVELEN = / pivot wavelength in Angstroms R4
/ DATA DESCRIPTOR KEYWORDS
SAMPBEG = / first sample number I2
LINEBEG = / first line number I2
PEDIGREE= / data source(INFLIGHT,GROUND,MODEL,DUMMY)&date CHR
DESCRIP = / description of reference data CHR
END
[Listing Version 02 12-APR-1994]
2.1.4 Spectrographic Detector Efficiency Files (SDE): <unique name>.R3H
The spectrographic detector efficiency data files are used for the spectrographic DE calibration step.
Format: A spectrographic DE data file has the same form as an FOC image. It always has dimensions 1024 samples by 1024 lines. Each entry is a REAL*4 scaling factor that is used to multiply the corresponding pixel DN count in the FOC science data file that is to be corrected. All images to be corrected will find their proper position inside the chosen spectrographic DE data file.
Flags and Indicators: The flags and indicators used to select the spectrographic DE reference files are PXFORMT (the pixel format can be either NORMAL or ZOOM), OPTELT1, and OPTELT2.
Restrictions: The first 16 non-comment keywords in the header for this file set are required by the SOGS software. Also, the group parameters DATAMIN and DATAMAX must be set.
The SOGS software mentioned was a predecessor of the current OPUS software used by the pipeline Legacy data, however, is not recalibrated. The final calibration products are now part of the STScI static archive.
A sample image header for the SDE file (<unique name>.R3H) is shown below.
SIMPLE = F / image conforms to the fits standard L1
BITPIX = 32 / bits per data value I2
DATATYPE= REAL*4 / datatype of the group array CHR
NAXIS = 2 / number of data axes I2
NAXIS1 = 1024 / samples per line I2
GROUPS = T / image is in group format L1
GCOUNT = 1 / number of groups I2
GCOUNT = 2 / number of parameters I2
PSIZE = 64 / bits in the group parameter block I2
/ GROUP PARAMETERS: PODPS CALIBRATION
PTYPE1 = DATAMIN / minimum data value in the group CHR
PDTYPE1 = REAL*4 / datatype of parameter 1 CHR
PSIZE1 = 32 / number of bits in parameter 1 I2
PTYPE2 = DATAMAX / maximum data value in the group CHR
PDTYPE2 = REAL*4 / datatype of parameter 2 CHR
PSIZE2 = 32 / number of bits in parameter 2 I2
/ FILE DESCRIPTOR KEYWORDS
INSTRUME= FOC / instrument id CHR
FILETYPE= SPECTROGRAPHIC DE / type of reference file CHR
HEADNAME= / reference header file name CHR
DATANAME= / reference data file name CHR
DATE = / date this file was writt CHR
/ KEYWORDS USED TO SELECT SDE REF FILES
PXFORMT = / format of the image: NORMAL, ZOOM CHR
OPTELT1 = / optical element of filter wheel 1 (0-7) I2
OPTELT2 = / optical element of filter wheel 2 (0-7) I2
/ DATA DESCRIPTOR KEYWORDS
SAMPBEG = / first sample number I2
LINEBEG = / first line number I2
WAVELEN = / wavelength in angstroms to which data corresponds R4
PEDIGREE= / data source (INFLIGHT,GROUND,MODEL,DUMMY) & date CHR
DESCRIP = / description of the reference data CHR
END
[Listing Version 01 12-APR-1994]
2.1.5 Geometric Distortion Files (GEO): <unique name>.R5H
The geometric distortion correction data files contain data used to correct FOC science images for the geometric distortion caused by the FOC optics and the Photon Detector Assembly (PDA). They are used for the geometric calibration.
Format: The geometric distortion correction data files contain the distorted location of each corner of each pixel of the undistorted (i.e., output, geometrically corrected) file. Keywords NPIX1 and NPIX2 specify the size of the undistorted file. It is not required, but normally the geometric reference file would have a size of NAXIS1 = 2 * (NPIX1 + 1) by NAXIS2 = (NPIX2 + 1). The left half of the image contains the X coordinates, and the right half contains the Y coordinates. Pixel (i,j) of the geometric reference file (i < NAXIS / 2) is the distorted X coordinate of the lower left corner of pixel (i, j) in the undistorted file, and pixel (i + NAXIS1 / 2, j) is the distorted Y coordinate of the lower left corner of pixel (i, j) in the undistorted file.
Flags and Indicators: The flags and indicators used to select the geometric distortion correction files are OPTCRLY (the optical relay will be either F48 or F96), CAMMODE (the coronographic apodizer mask is either INBEAM or NOTUSED), PXFORMT (the pixel format is either NORMAL or ZOOM), SMMMODE (the spectrographic mirror mechanism is either INBEAM or NOTUSED), OPTELT1, OPTELT2, SAMPOFF (the sample offset can be any appropriate REAL*4 value computed from the 12-bit item XCCCUHZO), LINEOFF (the line offset can be any appropriate REAL*4 value computed from the 12-bit item XCCCUVRO), SAMPPLN (the samples per line can be 64, 128, 256, or 512), and LINEPFM (the lines per frame can be 64, 128, 256, 512, or 1024).
Restrictions: The first 10 keywords in the header for this file set are required by the SOGS software.
The SOGS software mentioned was a predecessor of the current OPUS software used by the pipeline Legacy data, however, is not recalibrated. The final calibration products are now part of the STScI static archive.
The following listing shows a sample geometric reference file <unique_name>.R5H.
SIMPLE = F / image conforms to the fits standard L1
BITPIX = 32 / bits per data value I2
DATATYPE= REAL*4 / datatype of the group array CHR
NAXIS = 2 / number of data axes I2
NAXIS1 = / I2
NAXIS2 = / I2
GROUPS = T / image is in group format L1
GCOUNT = 1 / number of groups I2
PCOUNT = 0 / number of group parameters I2
PSIZE = 0 / bits in the group parameter block I2
/ FILE DESCRIPTOR KEYWORDS
INSTRUME= FOC / instrument id CHR
FILETYPE= GEOMETRIC / type of reference file CHR
HEADNAME= / reference header file name CHR
DATANAME= / reference data file name CHR
DATE = / date this file was written CHR
/ KEYWORDS USED TO SELECT GEO REF FILES
OPTCRLY = / optical relay: F48, F96 CHR
CAMMODE = / coronographic apodizer mask: INBEAM, NOTUSED CHR
PXFORMT = / format of the image: NORMAL, ZOOM CHR
SMMMODE = / spectrographic mirror: INBEAM, NOTUSED CHR
OPTELT1 = / filter element, wheel 1 I2
OPTELT2 = / filter element, wheel 2
SAMPOFF = / sample offset (0.0-1023.75) R4
LINEOFF = / line offset (0.0-1023.75) R4
SAMPPLN = / samples per line (64, 128, 256, or 512) I2
LINEPFM = / lines per frame (64, 128, 256, 512, or 1024) I2
/ DATA DESCRIPTOR KEYWORDS
NPIX1 = / X size of output image I2
NPIX2 = / Y size of output image I2
IMSCALE = / image scale in arcseconds per pixel R4
CRVAL1 = 0. / position along slit R8
CRVAL2 = / wavelength (Angstroms) at reference pixel R8
CRPIX1 = / reference pixel in X direction R4
CRPIX2 = / reference pixel in Y direction R4
CD1_1 = / degrees /pixel along slit R4
CD1_2 = 0. / R4
CD2_1 = 0. / R4
CD2_2 = / wavelength increment (Angstroms) per pixel R4
CTYPE1 = ANGLE / type of coordinate for X axis CHR
CYTPE2 = LAMBDA / type of coordinate for Y axis CHR
PEDIGREE= / data source (INFLIGHT,GROUND,MODEL,DUMMY) & date CHR
DESCRIP = / description of reference data CHR
END
[Listing Version 03 12-APR-1994]
2.1.6 Data Quality Initialization Files (BLM): <unique name>.R7H
The FOC data quality initialization files contain a priori information about the effect of the FOC’s photocathodes on the quality of output data values. The data quality initialization file is used in conjunction with relation CGQ1_FIL_MSK to construct the FOC data quality mask.
Format: Data quality initialization files contain one group of data. Every data quality initialization file has the same format as an image. It is either a 1024 sample by 1024 line image for use with normal pixel format images or a 512 sample by 1024 line image for use with zoom pixel format images. Each pixel in the data quality initialization file is a REAL*4 count rate value.
Flags and Indicators: The flags and indicators used to select the data quality initialization reference files are OPTCRLY (the optical relay can be either F48 or F96) and PXFORMT (the pixel format can be either NORMAL or ZOOM).
Restrictions: The first 16 non-comment keywords in the header for this file set (as listed in the text below) are required by the SOGS software. Also, the group parameters DATAMIN and DATAMAX must be set.
The SOGS software mentioned was a predecessor of the current OPUS software used by the pipeline Legacy data, however, is not recalibrated. The final calibration products are now part of the STScI static archive.
The following listing shows a sample data quality initialization file header.
SIMPLE = F / image conforms to the fits standard L1
BITPIX = 32 / bits per data value I2
DATATYPE= REAL*4 / datatype of the group array CHR
NAXIS = 2 / number of data axes I2
NAXIS1 = / samples per line I2
NAXIS2 = 1024 / lines per image I2
GROUPS = T / image is in group format L1
GCOUNT = 1 / number of groups I2
PCOUNT = 2 / number of group parameters I2
PSIZE = 64 / bits in the group parameter block I2
/ GROUP PARAMETERS: DATA QUALITY
PTYPE1 = DATAMIN / the minimum value of the data CHR
PDTYPE1 = REAL*4 / datatype of parameter 1 CHR
PSIZE1 = 32 / number of bits in parameter 1 I2
PTYPE2 = DATAMAX / the maximum value of the data CHR
PDTYPE2 = REAL*4 / datatype of parameter 2 CHR
PSIZE2 = 32 / number of bits in parameter 2 I2
/ FOC DATA DESCRIPTOR KEYWORDS
INSTRUME= FOC / instrument CHR
FILETYPE= DQ INIT / data quality initialization file CHR
HEADNAME= / reference header file name CHR
DATANAME= / reference data file name CHR
DATE = / date this file was written CHR
/ KEYWORDS USED TO SELECT QIN REF FILES
OPTCRLY = / optical relay: F48, F96 CHR
PXFORMT = / format of the image: NORMAL, ZOOM CHR
SAMPOFF = / sample offset (0.0-1023.75) R4
LINEOFF = / line offset (0.0-1023.75) R4
SAMPPLN = / samples per line (64, 128, 256, or 512) I2
LINEPFM = / lines per frame (64, 128, 256, 512, or 1024) I2
/ DATA DESCRIPTOR KEYWORDS
SAMPBEG = / first sample number I2
LINEBEG = / first line number I2
PEDIGREE= / data source (INFLIGHT,GROUND,MODEL,DUMMY) & data CHR
DESCRIP = / description of the reference data CHR
END
[Listing Version 02 12-APR-1994]
2.2 FOC Reference Relations and STSDAS Tables
2.2.1 Flat Fields Table: < unique name>.CXU
The flat fields table contains the names of the UNI files (inverse flat fields) that are used to correct for nonuniform detector sensitivity.
Format: The relay name OPTCRLY may be either F48 or F96. CAMMODE specifies whether the high resolution apodizer is used in the F96 relay, resulting in a focal ratio of f / 288; CAMMODE may have a value of either INBEAM or NOTUSED. WAVELENGTH is the characteristic wavelength for the observation that was used to create the UNI file (e.g., the LED color). HEADER_FILE and DATA_FILE are the names of the header and data files for the UNI image.
Selection Scheme: The value to be obtained is HEADER_FILE, the name of the UNI header file. DATA_FILE is ignored. The pivot wavelength of the observation to be calibrated is obtained from the observing mode; the pivot wavelength depends on the throughput of the optics, the transmission of the filters, and the sensitivity of the detector. This is compared with the values in the WAVELENGTH column for rows that have the same OPTCRLY and CAMMODE as the observation, and the row is selected that has a value of pivot wavelength divided by WAVELENGTH that is closest in absolute value to unity.
Restrictions: There should be at least one entry for each value of OPTCRLY. CAMMODE is irrelevant if OPTCRLY = F48.
A sample flat fields table (<unique name>.CXU) appears below.
Table 2-1. FOC Flat Fields Table, <unique name>.CXU
Column Name |
Data Type |
Units |
Description |
|---|---|---|---|
OPTCRLY |
CH*8 |
Optical relay |
|
CAMMODE |
CH*8 |
Coronographic apodizer mask |
|
WAVELENGTH |
I |
Å |
Pivot wavelength |
HEADER_FILE |
CH*18 |
Name of generic header file |
|
DATA_FILE |
CH*18 |
Name of generic data file |