NAME

waveoffset -- Compute the spectral offset.

USAGE

waveoffset input1 wave1 input2

DESCRIPTION

This routine computes the offset of a spectrum (input2) from a reference spectrum (input1) and its wavelength (wave1), as a function of wavelength and diode position. The input spectra are divided into a number of equally sized bins (specified by the nbins parameter). The second spectrum for each bin is cross-correlated with the corresponding bin in the first spectrum. The position of the maximum in the correlation matrix is found. An offset is found by locating the maximum of the quadratic through that position and its two neighbors. The offsets for all bins are placed in an output table as both diode and wavelength offsets. PARAMETERS

input[file name]

Name of the reference spectral data file.

wave[file name]

Name of the wavelength file corresponding to input1.

input[file name]

Name of the spectral data file whose offset is to be computed.

table = "waveoffset" [file name]

Name of the output offset table file. The output table from this task is used as input to the fitoffsety task, which corrects dispersion coefficients for the offsets. This table has the following columns:

   'WAVELENGTH' - Wavelength at the center of each bin (double).
   'DIODE'      - Diode, or sample position, at bin centers (double).
   'DELTAW'     - Offsets in wavelength units (double).
   'DELTAD'     - Offsets in diode units (double).
   'COUNTS'     - Total counts in bin for 'input2' (double).

cross = "cross_corr.hhh" [file name]

Name of the output image containing the correlation values for each bin. This image is two-dimensional: the first dimension is the search space (2 * maxdist + 1); the second dimension is the number of bins.

(nbins = 5) [integer, min=1, max=100]

Number of bins into which the data is to be divided.

(maxdist = 10) [integer, min=1, max=100]

Maximum distance---in data points---in which to search for the best correlation. If the correlation is found at the edge of the search area (i.e., at distance maxdist), it is assumed that the correlation was not found and the values of the offsets are set to INDEF.

EXAMPLES

1. Compute the offset between upper and lower apertures of the spectra in file h40blue. The upper aperture is in the first group of the file and the lower aperture in the second group. The wavelengths for the first group are stored in wh40blue (group 1). Results are written to a table called offsets.tab.

fo> waveoffset h40blue[1] wh40blue[1] h40blue[2] tab=offsets

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

fitoffsety