NAME

yfit -- Compute Y-deflection calibration coefficients.

USAGE

yfit cctabin ypostab cctabout

DESCRIPTION

Using a table of Y-deflections as a function of carrousel position and spectral order, this task will determine the coefficients giving the Y-deflection by using a least squares fit, described as follows:

          ydef = a + (b * w) + (c * w * w) + (d * m * w)
    
     where:
          w - is the wavelength computed with the carrousel 
	      calibration stored in 'cctabin',
          m - is the spectral order,
          a, b, c and d - are fitted coefficients. (d is not 
	      used for first order gratings (i.e., it is set 
	      to 0.0) and c is not used for Echelle modes.

PARAMETERS

cctabin [file name]

Name of the input carrousel calibration table that is to be updated. The carrousel coefficients CAP_A, CAP_B, and CAP_C are copied directly from the input table to the output table. The Y-deflection coefficients LIT_A, LIT_B, LIT_C, and LIT_D are updated by this task. If cctabin has multiple rows, the first row in which the grating mode matches that of ypostab will be used.

ypostab [file name]

Name of the input Y-deflection table to be processed. For first order grating modes, table rows must be supplied for at least four carrousel positions. For the Echelle mode, at least two carrousel positions, each having two spectral orders, must be present in the table. This input table is the one generated by the yfind task.

cctabout = "cctabout" [file name]

Name for the output file that will contain the updated carrousel calibration table produced by this task. The cctabout table will have one row containing the updated calibration of the selected row of cctabin.

EXAMPLES

1. Compute the Y-deflection calibration for the input Y-position table ypostab.tab. The table oldcctab.tab contains the previous calibration and newcctab.tab will contain the updated calibration.

hr> yfit oldcctab ypostab newcctab

BUGS

REFERENCES

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

yfind