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31.1 Pipeline Calibration Overview

The basic steps of the calibration pipeline, calfos, are summarized in Figure 31.1 and Table 31.1.

Figure 31.1: Pipeline Processing by calfos

Table 31.1: Calibration Steps and Reference Files for FOS Pipeline


Processing Step

Reference File or Table


Compute propagated error at each point in spectrum. Error file calibrated with science file and propagated statistical errors written to .c2h.


Convert from raw counts to count rates by dividing each data point by exposure time and correcting for disabled diodes. Diode numbers are taken from ddthfile or from the unique data log.



Correct for image motion in the FOS X direction (dispersion) induced by magnetic field. Uses a model of the earth's magnetic field along with scale factors from table ccs7. This step should be applied for observations taken before April 5, 1993, after which the on-board GIM correction is used.



Correct raw count rates for saturation in detector electronics using paired-pulse correction table (coccgr2).



Correct for particle-induced background using default reference background (bachfile) if no background spectrum was obtained as part of the observation.



If BAC_CORR is set to PERFORM, and the default background file (bachfile) is used, this file can be scaled to the expected mean count rate for the spacecraft geomagnetic position using the ccs8 reference table and subtracted from the count rate spectra by setting GMF_CORR to PERFORM. Scaled background is written to the .c7h file.



Remove background scattered light. The scattered light is determined by calculating the mean value of diodes not illuminated by the selected grating. This mean is then subtracted from the observed spectrum. Un-illuminated diodes are found in the CCS9 reference table.



Correct for diode to diode sensitivity variations by multiplying by the flatfield response file (fl1hfile). For paired aperture or spectropolarimetry observations, a second flatfield file is used.



If a sky spectrum was observed, the background is subtracted and the sky smoothed using a median and mean filter. Uses filter widths table (ccs3), aperture size table (ccs0), emission line positions (ccs2), and sky shift table (ccs5).

ccs0, ccs2, ccs3, ccs5


Compute vacuum wavelength scale for each object or sky spectrum using coefficients (ccs6).



Correct for relative aperture throughputs. Object data are normalized to the reference aperture used to derive the average inverse sensitivity used in AIS_CORR. This step is required if AIS_CORR is used. Additionally, object data are divided to correct for changes in aperture throughput due to changes in OTA focus.

ccsa, ccsb, ccsc


POLARIMETRY ONLY: Convert from count rate to absolute flux units by multiplying by inverse sensitivity curve. Uses inverse sensitivity file (iv1hfile) and, for paired aperture or spectropolarimetry, file (iv2hfile).



Convert from count rate to absolute flux units by multiplying by inverse sensitivity curves. This step replaces FLX_CORR and is different in that an average inverse sensitivity, determined from calibration of all epochs, is used. APR_CORR must be performed for this step to have meaning.



Correct for changes in instrument sensitivity over time by dividing the object data by an appropriate correction factor.



Perform ground software mode dependent corrections for time-resolved, rapid readout, or spectropolarimetry observations. For RAPID, write total flux and sum of statistical errors to groups 1 and 2 of .c3h file. For PERIOD mode, write pixel-by-pixel averages of all slices to groups 1 and 2 of .c3h file. For spectropolarimetry, data from individual waveplate positions is used to make Stokes parameters I, Q, U, and V and linear and circular polarization position angle spectra.

ccs4, rethfile


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Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 01/14/98 14:47:13