Upper panel: format of images on the IFS detector. The top image is an overlay of an detector flat field (obtained using the IFS internal calibrating sphere) and of a flat field image obtained through the BIGRE. The whole detector is a square of 2048  ×  2048 pixels, shown with vertical and horizontal sides. The area within the red square represents the projection of the BIGRE on the detector; this is a rectangle, which sides make an angle of 10.5° with the detector; this is the angle between the lenslet array and the spectra, and could not be avoided. Some 9% of the BIGRE useful area is lost due to vignetting. The black region at the corner of the detector image is the area of the detector for which no internal flat field is possible due to vignetting by the cold filter holder. This is completely outside the area covered by BIGRE, so that there is no field loss due to the cold filter holder. Lower panel: portion of an image obtained with a white lamp with IFS in the Y − H mode.