Operating Modes

ACCUM for each of the cameras. This is the standard data taking mode used by observers.

ACQ (acquisition). This is the mode used to acquire a target for coronagraphic observations. ACQ is 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 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 5 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).

Fifteen 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 will be useful for facilitating mosaics and obtaining observations at multiple orientations. WFC, WFC1, and WFC2 are located near the field of view center and the centers of chips 1 and 2, respectively. 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 ACS internal buffer can store only a single full frame WFC image. When this image is compressed, and depending on the compression factor, the buffer can store additional HRC and SBC images. As a consequence of the implementation of the compression strategy, under no circumstance can more than one full frame WFC image be stored in the buffer. Note that the adopted policy is not to compress primary WFC observations. 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. 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:

They can be specified only on a single WFC chip;

They may have physical but no virtual overscan;

They cannot include the CCD edge (i.e. the maximum subarray size is 4140 by 2046); and

They are read through a single amplifier and may take longer to readout than a full-frame image, depending on size and location.

Users can use WFC subarrays either by specifying a supported pre-defined subarray (which is recommended) or by defining their own general subarrays. For supported subarrays, the dark, flat, and bias frames used for calibration will simply be extracted from available full-frame images. Tests have shown that this does not degrade the quality of the dark, flat-field or bias corrections, as compared to full-frame data. However, this is true only for subarrays that fall entirely within a single amplifier quadrant and include a corner section with the overscan (true for all the supported subarrays). 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. In some special cases where a general subarray is cleverly defined so as to include a physical overscan region, no separate bias frames are needed.

Pre-defined subarrays are the appropriate choice for observing a small target when lessening the data volume is desired. These supported subarrays for WFC are invoked by using the named apertures WFC1-1K, WFC1-2K, and WFC1-512. On WFC1, at the amplifier B corner there are square apertures WFC1-512, WFC1-1K, and WFC1-2K with light collecting areas being squares with sides of length 512, 1024, and 2048 pixels. A 2048 pixel aperture is available at the amplifier D corner of WFC2 called WFC2-2K. These all incorporate 22 columns of the physical overscan pixels. These have been chosen bearing in mind that as charge transfer efficiency degrades with radiation damage to the detectors, there is an advantage in being close to the readout amplifier. The reference pixel and extent of the subarrays are listed in Table 7.7. 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 http://www.stsci.edu/hst/acs/faqs/subarrays.html. 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 chips are shown in Table 7.7 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 7.7 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 chip, while the outer segment corresponds to the WFC2 chip. The middle segment can be used with either of the WFC chips but is used by default with WFC1. 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. Or, if desired, the user can define a general subarray to readout only a portion of the chip. Table 5.1 and Table 5.2 can be used to determine the remaining two ramp filter segments which can be of interest for serendipitous observations. While all fifteen ramp segments can be used with the WFC, only the five middle ramp segments are available with the HRC. Ramps used with the HRC cover the region over the HRC chip defined by the HRC aperture (Table 7.9). Please refer to Section 7.7.2 and Section 5.3.1 for further information.

7.3.2 HRC ACCUM Mode

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 7.9 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.

HRC CCD Subarrays

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.

7.3.3 SBC ACCUM Mode

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 HRC images and/or a single, compressed WFC image.

7.3.4 HRC ACQ Mode

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.