Optimum Aperture for Mosaicing: What aperture should I use for large mosaics? Reply A new ACS aperture, WFCENTER, has recently been defined. Its main feature is that it encompasses the full Wide Field Camera field of view, and has its reference point at the center of the field of view. This aperture is designed for observations which are expected to be repeated with relative rotations of 90 or 180 degrees on the same field. Since the reference point lies very close to the inter-chip gap, WFCENTER is *not* the preferred aperture when the need is simply to center a target in the full field of view of the Wide Field Camera. For this, use aperture WFC. The aperture WFCENTER may be used in configuration ACS/WFC in ACCUM mode in the same manner as WFC with no special restrictions. WFCENTER will not yet work properly if you include it in your RPS2 file because software changes need to be made. If you feel that WFCENTER is needed for your science program, please consult your Program Coordinator.