HELP · SEE_ALSO

## NAME

carrousel -- Calibrate GHRS carrousel position.

## USAGE

`carrousel dctab cctabin cctabout`

## 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.

- cctabout = "carrousel" [file name]
- 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

## 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, yfit (for computation of `LIT_A`, `LIT_B`, `LIT_C` and `LIT_D`) and
wavecal (for computation of the dispersion coefficient tables).