NAME

fitoffsety -- Correct dispersion coefficients for wavelength offsets.

USAGE

fitoffsety offtab dcin dcout

DESCRIPTION

This routine corrects dispersion coefficients for spectral offsets found by the waveoffset task. If grating mode data is used, then a least squares polynomial of the wavelength offset as a function of the sample (or diode) position is computed. The coefficients of the fit are subtracted from the corresponding dispersion coefficients (i.e., coefficients for same power of sample or diode position). For the prism mode, only an average diode or sample shift is computed and is then added to the XZERO header keyword value in the dispersion coefficient file (produced by the dispfity task).

PARAMETERS

offtab [file name]

Name of the offset table previously generated by the waveoffset task. This table has the columns WAVELENGTH, DIODE, DELTAW', DELTAD', and COUNTS.

dcin [file name]

Name of the input dispersion coefficient table. This file contains the dispersion coefficients for the reference spectrum that was used by the waveoffset task to generate the offtab file. This table is left unchanged. Updated dispersion coefficients are written to the file specified by the dcout parameter.

dcout = "offsetdc" [file name]

File name for the output table containing dispersion coefficients.

(order = 1) [integer, min=0, max=3]

Order of the polynomial to fit for the grating modes. The order parameter is ignored for the prism modes. If order is set to "0", then a constant average shift is computed.

(mincount = 0.0) [real]

Minimum count value for spectral offsets. Rows in the input table offtab that have a value in the COUNTS column lower than mincount are not used in the fit.

EXAMPLES

1. Correct the dispersion coefficients in the file dc1.tab using offsets in off.tab. Use a quadratic fit to the offsets and place the updated dispersion coefficients in a file called dc2.tab. Reject rows in off.tab that have total counts less than 1000.0.

fo> fitoffsety dc1 off dc2 order=2 mincount=1000.0

BUGS

REFERENCES

Howard Bushouse, STSDAS

HELP

For assistance using this or any other tasks, please contact help@stsci.edu or call the help desk at 410-338-1082.

SEE ALSO

waveoffset, dispfity