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.
  

7.3.1 WFC ACCUM Mode

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 77,400 electrons and consequently, all GAIN values larger than 1 will allow the observer to count up to the full well capacity (ACS ISR 2020-02). For GAIN=1, only 75% of full well capacity is reached when the DN value saturates at 65,535. The readout noise of the WFC CCD is about 4 electrons RMS and thus is critically sampled 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 7.7) 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 is 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) losses and readout time. As a result, the two 2K by 2K halves of a single chip will have slightly different readout noise. The WFC chips have both a physical prescan and a virtual overscan that can be used to estimate the bias level and the readout noise on each single image. The present flight software does not allow reading an ACS frame directly into the HST onboard recorder. Images must first be stored in the internal buffer. When more than one WFC full-frame 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.

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, polarizers, or HRC filters (which produce a vignetted field of view on WFC). The supported WFC subarray modes were made available to users in October 2016, at the beginning of Cycle 24. As noted in Section 2.1, the post-SM4 WFC electronics have the property that differences in CCD readout timing can result in a significant difference in bias structure. This was observed between WFC full-frame and subarray images prior to the update in October 2016. It is also the case that the profile of CTE trailing varies markedly with different CCD readout timings, particularly in the dwell time between parallel shifts. Lastly, subarray modes in place before October 2016 were reading out all 4096 columns of the CCD and retaining only a small portion of them. Because of this, some subarray readout overheads were actually larger than the full-frame readout overhead. The subarray update in October 2016 revised the HST flight software so that all WFC subarray modes have exactly the same readout timing as the full-frame readout.

There are three geometry choices at each amplifier corner, resulting in twelve supported subarray modes, listed in Table 7.7. The readout areas are rectangles with 2048 columns (plus 24 columns of physical prescan) and either 512, 1024, or 2048 rows. The full 2048 columns are retained for all subarrays, so that the scene-dependent bias shift (ACS ISR 2012-02) can also be corrected exactly as with full-frame readout. Situating the subarrays at the amplifier corner mitigates the impact of the degraded CTE on source photometry, astrometry, and morphology. At the time of writing, the WFC amplifier with the lowest readout noise is amplifier B. Therefore, subarrays that use amplifier B are recommended over other amplifiers whenever possible. The reference pixel and extent of the subarrays are listed in Table 7.7. Calibration frames will be provided for supported subarrays only. Users who propose non-supported subarrays must request their own subarray bias images, which will typically be scheduled during the Earth-occultation portion of their HST visits. More information about pre-defined subarray apertures can be found in Section 7.7.

The bias-striping effect (ACS ISR 2011-05) is present in all post-SM4 subarrays. If de-striping is desired, it must be performed by the user with acs_destripe_plus (see Section 5.2.6). If pixel-based CTE correction is desired, de-striping beforehand (if appropriate) is highly recommended. Table 7.6 contains guidance for de-striping and CTE-correcting subarrays based on their observation dates.

Table 7.6: WFC subarray modes and processing considerations.

^a Perform processing with CALACS with CTE correction enabled.
^b Perform processing with acs_destripe_plus with CTE correction enabled.

Ramp Filters

 In practice, the observer specifies a ramp filter and a central wavelength and the 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 Tables 7.7 and 7.8 and Figure 7.4. An image of a ramp filter overlaid on a schematic of WFC ramp filter apertures is given in Figure 7.5.

Unlike WFPC2, ACS ramp filter observations at different wavelengths are obtained at the same location on the CCD, thus simplifying data processing. To select the desired wavelength, the ramp filter is automatically rotated to move the central wavelength specified by the observer over the reference point of the relevant aperture. 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 7.7, which lists the reference points for the apertures corresponding to the inner IRAMP, middle MRAMP, and outer ORAMP filter segments for each ramp filter. 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. See Figures 7.4 and 7.5 for visualizations of the ramp filters and WFC CCDs.

For any ramp filter observation, three ramp filters will end up in the FOV even though the target is properly positioned for only the requested one. Table 5.1 and Table 5.2 can be used to determine if the remaining two ramp filter segments are useful for serendipitous observations. The user can request either full-frame readout or the 2K subarray readout containing only the primary ramp filter's illumination region.

7.3.2 SBC ACCUM Mode

The SBC ACCUM mode accumulates photons into a 1024 by 1024 pixel 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 second and the maximum is 1.0 hour. The minimum time between SBC exposures is 40 seconds.

The maximum number the memory buffer can accommodate is 65,535 counts per pixel in a given observation. When accumulated counts per pixel exceed this number, the values will wrap, i.e., the memory resets to 0. The accumulated counts per pixel can be kept below this value by breaking individual exposures into multiple identical exposures, each of which is short enough that fewer than 65,535 counts are accumulated per pixel. Note that the SBC, like the STIS MAMAs, has no readout 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.