UIST array is not uniformly sensitive throughout. We see variations by ~0.5 magnitudes in the raw in the raw instrumental magnitudes between the most sensitive and the least sensitive regions. This is due to the variation of the quantum efficiency across the array. The figure below shows the instrumental magnitudes evaluated on a standard star at different regions of the array. The values in red show the instrumental magnitudes before applying the flat field correction and those in blue are after flat fielding. The centroid pixel of the star on the array and the offsets in arcsec from the centre are given in brackets in the top row for each point.
The figure below shows the variations of sensitivity across the array as seen on a J-band flat frame (normalized).