Selecting the "Imaging" observing mode moves the imaging mirror into the beam in place of the first prism. Generally one of the filters in the middle or lower wheels is put in the beam, with the other two wheels in the "Clear_I" ("I" for imaging) or "Clear_S" positions. The Clear_I positions are offset from the Clear_S positions to account for an extra small rotation of the wheels. This allows the undersized facility filters to illuminate part of the CCD which is not affected by bad columns and is also more or less parallel to the rows and columns of the CCD. The unvignetted field is 2' x 8' with good images over the full field. Full-sized user filters should use the Clear_S positions in the other wheels. As of June 2001 the Ellis R filter is the only full-field filter available. Figure 8 shows the instrument configuration for imaging.

Figure 8: The optical layout for the imaging configuration.

The 80%-energy-enclosed circles the in ESI imaging mode are smaller than 20 microns (~0.2"). This is the best optical imager among the Keck instruments in terms of image quality and flatness of the focal plane.

The ESI filters are inserted in the middle and lower wheels. To cover the full field requires 13.5 x 3.6 inch filters. At delivery, the filters are 6.4 x 3.6 inches and the imaging field is 2 x 3.5 arcminutes. The delivered filters are Johnson B and V, a special R (see Figure 9) and Gunn-i. Richard Ellis has kindly donated a full-field R filter, the "Ellis-R", for general use.

Figure 9: Throughput plots for the delivered ESI filters.