11.4 Mode = ACQ
Config = ACS/HRC (Unavailable for Cycle 16)

For HRC science observations using one of the coronagraphic apertures (HRC-OCCULT0.8, HRC-CORON1.8, HRC-CORON3.0, HRC-SUB1.8), you will need to perform a target acquisition (ACQ Mode exposure) prior to the science observations in order to accurately position the target behind the occulter. In ACQ mode, the ACS onboard flight software locates the brightest target in the acquisition subarray of the HRC FOV by determining the total flux within a sequence of overlapping "checkboxes" within that subarray. The target position is located within the brightest checkbox and HST is repositioned to place the target at the position of the small spot (HRC-CORON0.8, HRC-SUB1.8). A slew to a different aperture for an ACCUM exposure will be separate and requires a small amount of additional time.

11.4.1 Aperture or FOV

HRC-ACQ is the only valid aperture for this Mode.

11.4.2 Spectral Element

In addition to any of the allowed filters or polarizer/filter pairs specified in Table 11.2, the filter pairs F220W & F606W, F220W & F550M or F220W & F502N are allowed for ACQ mode exposures. These paired filters should be used for bright targets that would otherwise saturate the HRC detector. The neutral density equivalents of these paired filters can be found in the ACS Instrument Handbook.

11.4.3 Wavelength

If a ramp filter (any spectral element beginning with the letters "FR") is specified, enter the value of the desired central wavelength in Ångstroms. Table 11.5: Wavelength Ranges for the WFC/HRC Ramp Filters gives the allowed minimum and maximum wavelength for each ramp filter.

Note: A wavelength should not be specified if a ramp filter is not being used.

11.4.4 Optional Parameters

There are no optional parameters available to the GO in this mode.

11.4.5 Number of Iterations

The Number_of_Iterations must be 1 in this Mode.

11.4.6 Time Per Exposure

The procedures to determine the exposure time for ACQ exposures are given in the ACS Instrument Handbook. Observers should take care to obtain a signal-to-noise ratio of at least 40 from their sources, integrated over the checkbox. Note that the gain of the CCD electronics is 4 e/DN for ACQ exposures. The acquisition overhead is typically dominated by factors other than the exposure time itself, and observers may wish to consider adding an extra pad to the exposure time to ensure the required signal-to-noise, taking care not to saturate the CCD pixels or to exceed the maximum acquisition exposure time.

Note that Time per Exposure for individual HRC exposures must be an integer multiple of 0.1 second and must be in the range of 0.1 to 300 sec.