Pixel sizes in VIRCAM
Below are two maps of the pixel sizes across the focal plane as defined by pawprint v20090126_00001_st.fit which is a typical example from several pawprints analysed the same way. The maps show the variation in pixel size over the whole focal plane. The overall variation is:
0.3363 <= dx <= 0.3413
0.3351 <= dy <= 0.3413
derived directly from the fitted WCS obviating the need to go through the exercise of differencing the positions of stars in various parts of the field.
Given that the pixels are 20u in on a side, this means that the projected plate scale is:
58.60 <= scale_x <= 59.47 micron/arcsec
58.60 <= scale_y <= 59.68 micron/arcsec
a similar vein the measured diameter of the available field from the
fitted WCS is d = 1.6406 degrees.