The HRC target acquisition mode is used to place a target under the occulting finger or the coronagraphic mask. Observations through two (non-polarizer) filters are allowed in ACQ
images to cut down the flux to acceptable levels for very bright targets. Due to the optical design of HRC the simultaneous use of two filters leads to a degraded imaging quality but which is still acceptable for a successful target acquisition. The ACS IDT has identified a number of filter combinations that effectively act as neutral density filters, and allow the observer to acquire a very bright target that would otherwise saturate the CCD. These filter pairs are F220W+F606W, F220W+F550M, and F220W+F502N in order of decreasing transmission. A more complete description of the target acquisition procedure is given in Section 6.2.2.
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