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Subsections


7.3 Calibration Files

This section gives a description of all the calibration files used during the automated pipeline processing of LWS data. The contents of these files are fixed for each version of the processing and can only be updated when a new OLP version is issued.

The SPD level files are mainly concerned with how LWS output is turned from engineering units to photocurrent at each grating position and much of this information has remained unchanged since before launch although parameters such as discard times have been updated in-orbit. The files LCD2, LCFP and LCD3 are no longer used.

The Auto-Analysis level files are concerned with the astronomical calibration of LWS and have evolved with various versions of the LWS pipeline processing as the understanding of instrument calibration has improved. In particular the photometric and spectral response calibration of LWS has changed with each version. The LCTP and LCDK files have only been in existence since OLP Version 8.


7.3.1 SPD calibration files

The contents and usage of the SPD calibration files have changed as the pipeline has developed. Some of the calibration files still read by SPL are no longer used during the processing. Other files contain information which is no longer used. In the following sections only the information which is actually used is identified.

Each SPD calibration file contains a version number and date for identification purposes. The version numbers are contained in the keywords named XXXXVER, where XXXX is the four letter name of the calibration file. The date is contained in the keyword LDATE. The date and version numbers of the calibration files used by Derive-SPD are written into the header of the SPD and AAR files as a series of keywords named LSVERSn (n=1,2,...).


7.3.1.1 LCDT: Discard times file

Type of File:
FITS Header
Contents:


Table 7.34: LCDT calibration file keywords.
name type unit description
LCDTNSAM I none Threshold for switching from standard set of
      detector reset discard times to alternate set.
LCDTTRTn R ms Standard discard time for detector n (n=0...9)
      following detector reset. Only used if number of
      samples in ramp is less than or equal to LCDTNSAM.
LCDTTRAn R ms Alternate discard time for detector n (n=0...9)
      following detector reset. Only used if number of
      samples in ramp is greater than to LCDTNSAM.

Use:
To determine how much of every integration ramp has to be discarded. Used in Derive-SPD processing step `Construct ramps and discard unusable readouts', see Section 4.3.2.


7.3.1.2 LCAL: Readout limits for the analogue electronics

Type of File:
FITS header
Contents:

Table 7.35: LCAL calibration file keywords.
name type unit description
LCALDMI0 I - Detector SW1 minimum readout
LCALDMI1 I - Detector SW2 minimum readout
LCALDMI2 I - Detector SW3 minimum readout
LCALDMI3 I - Detector SW4 minimum readout
LCALDMI4 I - Detector SW5 minimum readout
LCALDMI5 I - Detector LW1 minimum readout
LCALDMI6 I - Detector LW2 minimum readout
LCALDMI7 I - Detector LW3 minimum readout
LCALDMI8 I - Detector LW4 minimum readout
LCALDMI9 I - Detector LW5 minimum readout
LCALDMX0 I - Detector SW1 maximum readout
LCALDMX1 I - Detector SW2 maximum readout
LCALDMX2 I - Detector SW3 maximum readout
LCALDMX3 I - Detector SW4 maximum readout
LCALDMX4 I - Detector SW5 maximum readout
LCALDMX5 I - Detector LW1 maximum readout
LCALDMX6 I - Detector LW2 maximum readout
LCALDMX7 I - Detector LW3 maximum readout
LCALDMX8 I - Detector LW4 maximum readout
LCALDMX9 I - Detector LW5 maximum readout

Use:
To determine which readouts are outside the limits of the electronics. Note that this is not the same as the `saturation' limits. These limits are specified in the LCDB file.


7.3.1.3 LCVC: Detector voltage conversion file

Type of File:
FITS header
Contents:
Two keywords are used in the conversion of raw detector readouts into voltages. The conversion is done using the formula: 

           V = (raw detector value - LCVCVOFF) * LCVCVFAC
where the keywords are: 

        LCVCVFAC= Conversion factor, Volts per raw value
        LCVCVOFF= Offset to be subtracted from raw value
The conversion factor and the offset value are REAL values.
Use:
To convert the readouts into voltages.







7.3.1.4 LCGA: Analogue amplification gains

Type of File:
FITS header

Contents:
Eighty keywords (all REAL values) specify the analogue amplification gain for each gain setting of each detector. Each keyword has the format: LCGADG<detector><gain>, where <detector> specifies the detector as a number between 0 and 9, and <gain> specifies the gain setting as a number between 0 and 7.
Use:
In Derive-SPD for the conversion of readouts to voltages.


7.3.1.5 LCJF: JF4 amplifier parameters

Type of File:
FITS Header
Contents:
Ten keywords specifying the fixed gain of the JF4 amplifier for each detector. Followed by ten keywords specifying the capacitances associated with each of the JF4 amplifiers. See Table 7.36 for the keyword names.

Table 7.36: LCJF calibration file keywords.
name type unit description
LCJFJG0 R - Detector SW1 JF4 gain
LCJFJG1 R - Detector SW2 JF4 gain
LCJFJG2 R - Detector SW3 JF4 gain
LCJFJG3 R - Detector SW4 JF4 gain
LCJFJG4 R - Detector SW5 JF4 gain
LCJFJG5 R - Detector LW1 JF4 gain
LCJFJG6 R - Detector LW2 JF4 gain
LCJFJG7 R - Detector LW3 JF4 gain
LCJFJG8 R - Detector LW4 JF4 gain
LCJFJG9 R - Detector LW5 JF4 gain
LCJFJC0 R farad Detector SW1 JF4 capacitance
LCJFJC1 R farad Detector SW2 JF4 capacitance
LCJFJC2 R farad Detector SW3 JF4 capacitance
LCJFJC3 R farad Detector SW4 JF4 capacitance
LCJFJC4 R farad Detector SW5 JF4 capacitance
LCJFJC5 R farad Detector LW1 JF4 capacitance
LCJFJC6 R farad Detector LW2 JF4 capacitance
LCJFJC7 R farad Detector LW3 JF4 capacitance
LCJFJC8 R farad Detector LW4 JF4 capacitance
LCJFJC9 R farad Detector LW5 JF4 capacitance

Use:
In Derive-SPD for the conversion of readouts to voltages.


7.3.1.6 LCDB: Saturation voltage thresholds for slope fitting

Type of File:
FITS Header
Contents:

This file contains the maximum allowable voltage before a point is regarded as being saturated. See Section 4.3.4 for a description of how saturated points are handled.

This file also contains values for the `de-biasing' parameters and the thresholds for switching between first and second order slope fitting. These values have not been used since the introduction of the $\Delta{V}/\Delta{t}$ method of slope fitting in OLP Version 7.


Table 7.37: LCDB calibration file keywords.
name type unit description
LCDBVM0 R Volts Detector SW1 maximum readout
LCDBVM1 R Volts Detector SW2 maximum readout
LCDBVM2 R Volts Detector SW3 maximum readout
LCDBVM3 R Volts Detector SW4 maximum readout
LCDBVM4 R Volts Detector SW5 maximum readout
LCDBVM5 R Volts Detector LW1 maximum readout
LCDBVM6 R Volts Detector LW2 maximum readout
LCDBVM7 R Volts Detector LW3 maximum readout
LCDBVM8 R Volts Detector LW4 maximum readout
LCDBVM9 R Volts Detector LW5 maximum readout

Use:
In Derive-SPD for identifying saturated points.


7.3.1.7 LCD1: First level deglitching parameters

Type of File:
FITS Header
Contents:
See Table 7.38 for the contents of this file.

Table 7.38: LCD1 calibration file keywords.
name type unit description
LCD1GFRA R none Glitches whose height is below this fraction of
the ramp height will be rejected
LCD1SPRA R none `Spikes' whose height is below this fraction of
the ramp height will be rejected
LCD1SDRJ R none Number of standard deviations from mean for point
to be marked as an outlier.
LCD1PGRJ I none Number of ramps to discard after a positive glitch.
LCD1NGRJ I none Number of ramps to discard after a negative glitch.
LCD1GRRJ L none Indicates if the whole of the glitched ramp should
be discarded.
LCD1PGRI I none For illuminator flashes, the number of ramps to
discard after a positive glitch.
LCD1NGRI I none For illuminator flashes, the number of ramps to
discard after a negative glitch.
LCD1GRRJ L none For illuminator flashes, indicates if the whole of
the glitched ramp should be discarded.

Use:
Contains various parameters which control the detection and removal of glitched points by first level deglitching.


7.3.1.8 LCGH: Glitch History file parameters

Type of File:
FITS Header
Contents:
The first keyword (LCGHGHMR) gives the maximum number of records that can be written to the LWS Glitch History file. This was introduced because of early fears that this file might grow uncontrollably. In fact this has not proved to be the case and this value is set to a high enough value to cope with all observations.


Table 7.39: LCGH calibration file keywords.
name type unit description
LCGHGHMR I - Maximum number of records

Use:
For writing data to the LWS Glitch History file.




7.3.1.9 LCD2: Second level deglitching parameters

Type of File:
FITS Header
Contents:
This file relates to a deglitching stage used during early versions of the pipeline. This file is still currently read by SPL, but its contents are not used.
Use:
Not used




7.3.1.10 LCFP: Parameters for electronic filters

Type of File:
FITS Header
Contents:
Ten keywords containing the time constants for the high pass filter for each detector. The times are specified in seconds.

These values have not been used since the introduction of the $\Delta{V}/\Delta{t}$ method of slope fitting in OLP Version 7.

Use:
Not used




7.3.1.11 LCD3: Third level deglitching parameters

Type of File:
FITS Header
Contents:
This file relates to a deglitching stage used during early versions of the pipeline. This file is still currently read by SPL, but its contents are not used.
Use:
Not used


7.3.2 Auto-Analysis calibration files

7.3.2.1 General information

All Auto-Analysis calibration files contain a set of standard keywords in their header to identify the version and validity of the file. These keywords can be used to check that the calibration files are valid for the data that is being processed. The keywords are checked during the automatic processing of the data in the OLP pipeline.

The keywords can be found in Table 7.40


Table 7.40: Auto-Analysis calibration files general keywords.
name type unit description
LDATE C - date of creation or update
LVER I - version number
LMODEL C - identifies instrument model (always FM)
LVLSTART I - UTK of start of validity
LVLEND I - UTK of end of validity
LVLBIAn I - indicates the bias level for which
      the file is valid for each detector
      (0-9). If LVLBIA0 is set to $-$1
      file is independent of bias, other
      bias levels are then not present

The version number and date of all the calibration files used by Auto-Analysis are written into the header of the LSAN file as a series of keywords named LVERSn (n=1,2,...).


7.3.2.2 LCIR: Illuminator reference file

Type of File:
FITS binary table
Contents:
The LCIR file is a calibration file which contains a reference photocurrent value for each ramp in an illuminator flash for each detector. The LCIR is a FITS binary table extension file of shape 1, i.e. there is only one LCIR file which covers the whole mission.

The file contains an illuminator `type' number as part of each record, allowing it to contain reference data for more than one `type' of illuminator flash. The comments in the LCIR file header should describe each flash type stored in the file.

For each flash type the LCIR file contains only the data starting from when the first illuminator was switched on and ending when the last illuminator was switched off. The background measurements at the beginning and end of the flash are not contained in the LCIR file. The the reference photocurrent values in the LCIR file have had the background photocurrent already subtracted.

Each LCIR record contains a status flag which allows selected points to be ignored when ratioing against the flash data. This can be used to mask out data from illuminator levels which do not provide useful data. Any photocurrent value in the LCIR file which is set to zero will also be ignored in the same way. Values may be zero because of glitches in the reference data.

The header of the LCIR file contains keywords which specify NSD, the number of standard deviations for median clipping the data. These keywords are: LCIRNSDB (Number of Standard deviations to use for median clipping of background) and LCIRNSDF (Number of Standard deviations to user for median clipping of flash data).

The record structures is given in Table 7.41


Table 7.41: LCIR calibration file record structure.
name offset Number type unit description
LCIRTYPE 0 1 I*4 - Number identifying type of
          illuminator flash
LCIRPHC 4 10 R*4 A Reference photocurrent,
          with background subtracted
LCIRPHCU 44 10 R*4 A Uncertainty in reference
          photocurrent
LCIRSTAT 84 1 I*4 - Status word. 1=use this value;
          0=ignore this value
LCIRICS 88 1 I*4 - Illuminator commanded status

Use:
For the flux calibration of the data.


7.3.2.3 LCGW: Grating position to wavelength conversion parameters

Type of File:
FITS binary table.
Contents:
This file contains the parameters required for converting the grating LVDT readout values into wavelength. Since the wavelength calibration changes over time this file contains different sets of coefficients for different time periods. Each record of the LCGW file contains one set of coefficients, which are valid for a particular time period. Rather than use a time key directly, the LCGW file uses the revolution number to identify which record is valid for which time period. Each record contains the start and end revolution numbers for which it is valid.

The record structure of the LCGW file is shown in Table 7.42.

The header of the LCGW file also contains keywords which contain fixed values for use in the conversion. These keywords are shown in Table 7.43.


Table 7.42: LCGW calibration file record structure.
name Offset number type unit description
LCGWSREV 0 1 I*4 - Revolution number of start
          of validity for this record
LCGWEREV 4 1 I*4 - Revolution number of end of
          validity for this record
LCGWCOEF 8 5 R*4 - Conversion coefficients
LCGWADET 28 10 R*4 deg Angle for each detector


Table 7.43: LCGW keywords.
name type unit description
LCGWLINE R - Number of lines per $\mu m$ on grating
LCGWOdet I - Order number for each detector
      (det=`SW1'...`LW5')

Use:
Wavelength calibration of the grating.


7.3.2.4 LCGR: Grating relative response file

Type of File:
FITS primary array
Contents:
This FITS file contains the grating relative wavelength responsivities for each detector at selected wavelengths. The file applies for a particular bias level. The data are normalised to the wavelength at which the absolute responsivity is measured. The relative responsivity includes a correction for the aperture size of the instrument, assuming the source is a point source in the centre of the beam. The unit of the relative responsivity therefore is cm$^{2}$. The file is written as a FITS primary array with three axis (NAXIS=3). The axis are defined as given in Table 7.44.


Table 7.44: LCGR calibration file structure.
name Number of type unit description
  elements      
NAXIS1 4 R*4 $\mu m$ Wavelength
    R*4 $\mu m$ Uncertainty in wavelength
    R*4 cm$^{2}$ Relative Responsivity
    R*4 cm$^{2}$ Uncertainty in responsivity
NAXIS2 10 R*4 - Detector number
NAXIS3 4096 R*4 - Grating LVDT value

The header of the file contains keywords which specify the range of grating positions which are used within the file and the `nominal' wavelength range for each detector. The nominal range is where the calibration is of acceptable quality. Everything outside the nominal range has poor calibration and is flagged with a warning flag in the LSAN status word. These keywords are shown in Table 7.45.


Table 7.45: LCGR keywords.
name type unit description
LSTARPOS I - First valid grating position in file
LENDPOS I - Last valid grating position in file
LSTRNOMn R $\mu m$ Start of nominal wavelength range
      for detector n (n=0...9)
LENDNOMn R $\mu m$ End of nominal wavelength range
      for detector n (n=0...9)

Use:
Relative responsivity correction for grating mode.


7.3.2.5 LCFW: Fabry-Pérot wavelength calibration parameters

Type of File:
FITS Header
Contents:
This FITS file contains the parameters required for converting FP position into wavelength. The parameters are the coefficients for the third order polynomial that is used to describe the dependence of the wavelength on the position of the FP etalons. Because of the precision of the FP wavelength calibration, these values should be read into real*8 variables. Table 7.46 gives the keywords for this file.


Table 7.46: LCFW calibration file keywords.
name type unit description
LCFWFLC0 D - FPL zeroth order coefficient
LCFWFLC1 D - FPL first order coefficient
LCFWFLC2 D - FPL second order coefficient
LCFWFLC3 D - FPL third order coefficient
LCFWFSC0 D - FPS zeroth order coefficient
LCFWFSC1 D - FPS first order coefficient
LCFWFSC2 D - FPS second order coefficient
LCFWFSC3 D - FPS third order coefficient

Use:
Wavelength calibration of Fabry-Pérot data.







7.3.2.6 LCGB: Grating spectral bandwidth correction factors

Type of file:
FITS binary table
Contents:
The correction factor for the grating spectral bandwidth for each of the ten LWS detectors. For each detector the factor and its uncertainty is given. The record structure can be found in Table 7.47.


Table 7.47: LCGB calibration file record structure.
name offset number type unit description
LCGBDET 0 1 C*3 - Detector
LCGBSPAR 3 1 I*1 - Filler
LCGBSB 4 1 R*4 $1/\mu$m Bandwidth
LCGBSBU 8 1 R*4 $1/\mu$m Uncertainty in bandwidth

Use:
In Auto-Analysis to correct fluxes for the spectral bandwidth for grating spectra.


7.3.2.7 LCDK: Fixed dark current calibration file

Type of file:
binary extension FITS file
Contents:
The file contains the dark current values for each detector.








7.3.2.8 LCTP: Fabry-Pérot throughput correction calibration file

Type of file:
FITS header
Contents:
The file contains the coefficients of the polynomial fits of $\eta\,\times\,T (\lambda)$ (product of the transmission efficiency $T(\lambda)$ and the effective spectral element width $\eta(\lambda)$) for FPS and the two sections of FPL (SW4-LW1 and LW2-LW5)
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Next: 8. Getting Started with Up: 7. Guide to Instrument Previous: 7.2 Product Files -
ISO Handbook Volume III (LWS), Version 2.1, SAI/1999-057/Dc