# carrousel stsdas.hst_calib.hrs.z_calib

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## NAME

carrousel -- Calibrate GHRS carrousel position.

## DESCRIPTION

Using tables of dispersion coefficients (for the same grating mode) the carrousel calibration coefficient, C, is computed by the following equation:

```	carpos = C - B * arcsin(m * w / A)

where:
carpos - is the carrousel position,
m - spectral order ("1" for first order gratings),
w - wavelength at the center of the
photocathode, (i.e., sample position 280),
B = 10430.378  (carrousel steps per radian),
A - constant for each grating, and
C - coefficient updated by this task.
```

The input dispersion coefficients are used to compute the central wavelength, w, at sample position 280.0 at each carrousel position observed by solving the following equation using Newton's iterative method;

```     280.0 = a0 + (a1 * m * w) + (a2 * m * m * w * w) +
(a3 * m) + (a4 * w) + (a5 * m * m * w) +
(a6 * m * w * w)
```

In this equation, the spectral order number, m, is set to "1" for the first order gratings. The central orders, m=42 for Echelle A and m=25 for Echelle B, are used in the two Echelle modes.

An effective carrousel position is computed for each wavelength that is computed from dispersion coefficients, using the previous calibration coefficients in the table specified by cctabin. The average of the difference between the computed and actual carrousels is used as an offset to the input value of the coefficient C.

## PARAMETERS

dctab [file name template]
Names of the input dispersion coefficient tables to be processed. All table rows must be for the same grating mode. These tables are produced by the routine wavecal and can be concatenated for use by the carrousel task using the tmerge task in the ttools package.
cctabin [file name]
Name of the input carrousel calibration table; this table has the columns GRATING, CAP_A, LIT_A, CAP_B, LIT_B, CAP_C, LIT_C and LIT_D. If the table has multiple rows, then the first row in which the grating mode corresponds to that of the input table dctab is used. The values of LIT_A, LIT_B, LIT_C, and LIT_D are not used by this task but are copied to the output table.
Name of the output table that is to contain the updated carrousel calibration coefficients. The values of LIT_A, LIT_B, LIT_C, LIT_D, CAP_A and CAP_B are copied directly from cctabin, however, the value of CAP_C is computed by this task.

## EXAMPLES

1. Perform the carrousel calibration using dispersion coefficients in contained in the tables dcg2_1 through dcg2_8. Previous carrousel calibration coefficients will be taken from oldcctab and the updated calibration will be placed in newcctab.

```hr> tmerge dcg2*.tab dcg2lib.tab append
hr> carrousel dcg2lib.tab oldcctab newcctab
```