HELP · SEE_ALSO

## 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.