Calibration Access and Data Handbook

Procedure

The routine returns the linear position (deltaX, deltaY) from the distorted grid position for a specified detector location. The input may consist of a list of positions. The offset can be calculate in two different ways. Either the offset is obtained by interpolation of the coarsely mapped distortion or by direct evaluation of an up to 7th order polynomial. The way of calculation is selected by setting the input parameter calc_mode.

• If calc_mode is set to 0, then the distortion vector is calculated by interpolation the distortion map.
• If calc_mode is set to 1, then the distortion vector is calculated by evaluation of the polynomial.

calc_mode is set to 0:
the linear position (deltaX, deltaY) at the detector location (rawX, rawY) is derived from the distortion map stored in the extension FILTER-FilterId (e.g. FILTER-U, FILTER-V, FILTER-GRISM1 etc.).
As the distortion map is only coarsly sampled an interpolation is required to derive the distortion at the requested position in the same way as for Cal_omDistortion.

calc_mode is set to 1:
the linear position (deltaX,deltaY) at the location (rawX, rawY) is computed using an up to 7th order polynomial. The coefficients of the polynomials are stored in the columns XPOLYCOEF, YPOLYCOEF of the extension POLYNOM_MAP2. The processing is identical to that of Cal_omDistortion.

Note that the offsets (deltaX, deltaY) are subtracted from the detector coordinates (rawX, rawY), because the offsets are defined as the difference between the measured and the expected (i.e. linear grid) position.