QC PLOTS
	CURRENT	HISTORY
X0 and Y0
translational matrix coefficients
rotational matrix coefficients
image quality
ellipticity
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The Mask to CCD Transformation Matrix translates mask coordinates into CCD pixels. It is determined with special MOS masks having pinholes on a regular grid. Measurements using these mask are typically executed once a day for health check purposes. Since August 2010, these frames are also used to check the VIMOS focus.

Example MOS pinhole mask image

The transformation matrix has the following form:

X = a_xx * x + a_xy * y + x_0
Y = a_yx * x + a_yy * y + y_0

with (x,y) being mask coordinates in mm and (X,Y) CCD coordinates in pixels. X direction is along the slit orientation, y direction is perpendicular to the slits (i.e. in dispersion direction).

The matrix coefficients a_xx, etc. and the zeropoints x_0 and y_0 are determined from the pinhole image for each quadrant. The diagonal coefficients a_xx and a_yy give the scale factor between mask and CCD (around 8.4 pixels/mm). The coefficients a_xy and a_yx give the rotation and are nearly zero.

QC1 parameters

From the pinhole images, also QC parameters for checking the focus are calculated. These are image quality (as FWHM in pixels of the major axis fo the pinhole ellipses), ellipticity, and orientation (of the pinhole ellipses). Each quadrant is divided into 3x3 sub cells (squares). All parameters and their standard deviations are calculated in each cell and globally per quadrant.

QC1 parameters

History