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Advanced Camera for Surveys Instrument Handbook for Cycle 20 > Chapter 7: Observing Techniques > 7.3 Operating Modes

ACS supports two types of operating modes:
ACCUM for each of the cameras. This is the standard data taking mode used by observers.
ACQ (acquisition). This was the mode used to acquire a target for coronagraphic observations. ACQ was only available with the HRC.
In this mode the WFC 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 WFC CCD is about 85,000 electrons and consequently, all GAIN values larger than 1 will allow the observer to count up to the full well capacity. For GAIN=1 only 75% of full well capacity is reached when the DN value saturates at 65,535. The read-out noise of the WFC CCD is about 4 electrons rms and thus it is critically sampled even at GAIN=2. WFC can make use of a user-transparent, lossless, on-board compression algorithm, the benefits of which will be discussed in the context of parallel observations. The algorithm is more effective with higher GAIN values (i.e., when the noise is undersampled). Note that only GAIN=2 is supported by STScI.
Several supported apertures (see Table 7.6) are accessible to WFC users. WFC1-FIX and WFC2-FIX select the geometric centers of the two WFC camera chips. WFCENTER corresponds to the geometric center of the combined WFC field, and will be useful for facilitating mosaics and obtaining observations at multiple orientations. Due to maximal CTE loss, WFCENTER is not recommended for a single compact target. WFC, WFC1, and WFC2 are located near the field of view center and the centers of chips 1 and 2, respectively (see Figure 7.3), Their locations were chosen to be free of detector blemishes and hot pixels, and they are the preferred apertures for typical observations. See Section 7.7 for more details about ACS apertures, including subarray apertures.
Usually each CCD is read from two amplifiers to minimize charge transfer efficiency (CTE) problems and read-out time. As a result, the two 2K by 2K portions in a single chip may have slightly different read-out noise. The WFC chips have both physical and virtual overscans that can be used to estimate the bias level and the read-out noise on each single image.
The present flight software does not allow reading an ACS frame directly into the HST on-board recorder. Images have to be first stored in the internal buffer. The ACS internal buffer can store only a single full frame WFC image. When this image is compressed, the buffer can store additional SBC images, depending on the compression factor. Regardless of the compression strategy, no more than one full frame WFC image can be stored in the buffer. It is STScI’s policy not to compress primary WFC observations. When more than one WFC image is obtained during an orbit, a buffer dump must occur during the visibility period so as to create space in the buffer for a new WFC image.
If each exposure is longer than approximately 339 seconds, buffer dumps can occur during the integration of the following image with no impact on observing efficiency.
Conversely, short, full frame, integrations with the WFC during the same orbit will cause buffer dumps to be interleaved with observations and will negatively affect the observing efficiency. See Chapter 8 for more details about ACS overheads.
WFC CCD Subarrays
It is possible to read-out only a portion of a detector with subarrays, which have a smaller size than the full frame. Subarrays are used to reduce data volume, to store more frames in the internal buffer (thus avoiding the efficiency loss due to buffer dumps), or to read only the relevant portion of the detector when imaging with ramp filters or with HRC filters (which produce a vignetted field of view on WFC). WFC subarrays have some limitations:
Users can use WFC subarrays either by specifying a supported pre-defined subarray (which is recommended) or by defining their own general subarrays. Calibration frames will be provided for supported subarrays only. Users who define general subarrays that cross amplifier boundaries or do not include a corner (not advised) must request their own subarray bias images, and these will typically be scheduled during the following occultation.
Pre-defined subarrays are the appropriate choice for observing a small target when lessening the data volume is desired. On WFC1, at the amplifier B corner there are supported square apertures WFC1-512, WFC1-1K, and WFC1-2K with light collecting areas being squares with sides of length 512, 1024, and 2048 pixels. These apertures incorporate 22 columns of the physical overscan pixels. Placing the subarrays at the amplifier corner mitigates the impact of degraded CTE on source photometry, astrometry, and morphology. The reference pixel and extent of the subarrays are listed in Table 7.6. More information about pre-defined subarrray apertures can be found in Section 7.7
To define a general subarray, the available-but-unsupported parameters SIZEAXIS1, SIZEAXIS2, CENTERAXIS1, and CENTERAXIS2 can be used. More practical information about defining subarrays can be found at:
When polarizers or the small HRC filter F892N are used with the WFC, the aperture WFC must be selected and a subarray is forced by the system. If the user chooses to use a polarizer with a ramp filter, then they may select an available-but-unsupported ramp aperture, but a subarray is still read out.
Ramp Filters
Unlike WFPC2, ACS ramp filter observations at different wavelengths are obtained at the same location on the CCD, thus simplifying data processing. In practice the observer specifies a ramp filter and a central wavelength; the filter wheel is automatically rotated to place the central wavelength at the reference point of the relevant aperture. The different ramp apertures and their reference points on the WFC CCDs are shown in Table 7.6 and Figure 7.4.
To select the desired wavelength, the ramp filter is rotated to move the appropriate part of the filter over the specified pointing. Observations with different ramp filters do not generally occur at the same pointing. The precise location where a given observation will be performed can be found from Table 7.6 where for each ramp filter we list the fiducial pointing for the inner IRAMP, middle MRAMP, and outer ORAMP filter segment. The inner segment corresponds to the WFC1 CCD, while the outer segment corresponds to the WFC2 CCD. The middle segment can be used with either of the WFC CCDs but only the WFC1 aperture is supported.
For any ramp filter observation three ramp filters will end up in the FOV even though the target is properly positioned only for the requested one. However, the user can define a general subarray to read out only the relevant portion of the CCD. Table 5.1 and Table 5.2 can be used to determine if the remaining two ramp filter segments are useful for serendipitous observations. While all fifteen ramp segments can be used with the WFC, only the five middle ramp segments were available with the HRC. Ramps used with the HRC cover the region defined by the HRC aperture (Table 7.8). Please refer to Section 7.7 and Section 5.3.1 for further information.
The SBC ACCUM mode accumulates photons into a 1024 by 1024 array, with 16 bits per pixel. The data are sent to the onboard recorder via the internal ACS memory buffer at the end of the exposure. ACCUM is the only mode available for SBC observations; the Time Tag mode of the STIS MAMAs is not available on ACS. The minimum SBC exposure time is 0.1 seconds and the maximum is 1.0 hour. The minimum time between SBC exposures is 40 seconds. Note that the SBC, like the STIS MAMAs, has no read-out noise. As a consequence there is no scientific driver for longer exposure times apart from the small overhead between successive images, described in Section 8.2.
Up to 16 SBC images can be stored in the internal buffer. SBC images can also share the buffer with a single, compressed WFC image.

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