In this mode the HRC CCD accumulates signal during the exposure in response to photons. The charge is read out at the end of the exposure, and translated by the A-to-D converter into a 16 bit data number (DN) ranging from 0 to 65,535. The number of electrons per DN can be specified by the user as the GAIN
value. The full well of the HRC CCD is about 155,000 electrons. In order not to overflow the 16 bit pixel word size, one needs to use GAIN=4
. In many applications GAIN=2
is adequate since it still allows critical sampling of the read-out noise of HRC (about 4.7 electrons rms) and, for this reason, it is the default GAIN
ratio. For typical HRC observations the observer should specify the HRC
aperture which is approximately located at the center of the field of view in a location free of detector blemishes and hot pixels. The HRC-
FIX
aperture is located at the geometric center of the field-of-view. Additional apertures are used for coronagraphic observations - see Table 7.8 for more details of HRC apertures.
Up to 16 HRC images can be stored in the ACS buffer. Alternatively, HRC images can share the buffer with some SBC images and/or a single compressed WFC image. The number of HRC images will depend in the latter case on the WFC compression factor.
Similarly to the WFC, a subarray is obtained when only a portion of the detector is read-out and transmitted to the ground. Generally the smaller size of the HRC CCD reduces the usefulness of subarrays. However, subarrays are used during on-board coronagraphic target acquisition, which is similar to the STIS target acquisition and cannot be changed. A square subarray of 512 x 512 pixels in the C Amp readout corner and a 512 pixel square aperture centered on the 1.8 arcseconds coronagraphic spot are available. In addition, on an available-but-unsupported basis, nearly arbitrary sizes and locations for subarrays can be specified. When coupling use of subarrays with PATTERNs or POS TARGs, the issue arises of whether to keep the subarray fixed in pixel space, or have it track and stay centered on the target. With PATTERNs, the subarray stays fixed in pixel space. When using POS TARGs, the observer can decide which mode to adopt.
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