QC PLOTS
	CURRENT	HISTORY
Xccd - Xsky for Q1
Xccd - Xsky for Q2
Xccd - Xsky for Q3
Xccd - Xsky for Q4
Yccd - Ysky for Q1
Yccd - Ysky for Q2
Yccd - Ysky for Q3
Yccd - Ysky for Q4
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VIMOS astrometric fields are typically observed once or twice each month. They are needed to provide the sky-to-CCD transformation matrix. The fields that are normally used are named 'Region E', 'Region M', and Region 'N'. Monitoring of the transformation coefficients is a prerequisite for enabling preimage-less MOS observations (PILMOS).

The WCS can be used to transform celestial coordinates onto a virtual plane that is free of instrument distortions. The sky-to-CCD transformation converts these virtual coordinates into real (pixel) coordinates on the detectors. The transformation matrix has the following form:

Xccd = sum [ a_ij * X^j * Y^i ],
Yccd = sum [ b_ij * X^j * Y^i ], with i, j = 1, ..., m.

The pipeline recipe vmskyccd is executed with m=3. Only the constant and linear coefficients are used for the QC parameters, i.e.:

Xccd = a_00 + a_01 * X + a_10 * Y
Yccd = b_00 + b_01 * X + b_10 * Y .

QC1 parameters

For monitoring the astrometric quality, nine virtual data points (X, Y) = (512, 610), (1024, 610), (1536, 610), (512, 1220), (1024, 1220), (1536, 1220), (512, 1830), (1024, 1830), (1536, 1830) are converted into CCD coordinates (Xccd, Yccd). The QC1 parameters ref_xsky_xccd_1024_1220 and ref_ysky_yccd_1024_1220 contain the difference between both, i.e. ref_xsky_xccd_1024_1220 = Xccd - X for (X, Y) = (1024, 1220). For the other parameters, this central difference is subtracted, e.g. delta_xsky_xccd_512_610 = Xccd - X - ref_xsky_xccd_1024_1220 for (X, Y) = (512, 610). This allows to distinguish between the overall shift of the transformation (measured by ref_xsky_xccd_1024_1220, etc.) and changes in the distortion (measured by delta_xsky_xccd_512_610, etc.).

History