In this Chapter we describe how to carry out observations with the ACS. We include a description of the operating modes, some suggestions on how to split exposures for cosmic ray rejection and a description of the use of subarrays and dithering patterns.

8.1 Operating Modes

8.1.1 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.

A total of nine supported apertures are accessible to WFC users. WFC1-FIX and WFC2-FIX select the geometric centers of the two WFC camera chips. The 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 approximately 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 8.5 for more details about ACS apertures, including the subarray apertures.

Usually each CCD is read from two amplifiers to minimize Charge Transfer Efficiency (CTE) problems and minimize 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 overscan which can be used to estimate the bias level and the read-out noise on each single image.

The ACS internal buffer can only store a single full frame WFC image. When this image is compressed, and depending on the compression factor, the buffer can store a number of 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 also 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 9, Overheads and Orbit-Time Determination, for more details about ACS overheads.

WFC CCD Subarrays

It is possible to read-out only a portion of a detector thus obtaining a subarray which has a smaller size than the full frame. Subarrays are mostly useful to reduce the 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 utilize WFC subarrays either by using 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 (true for all the supported subarrays). Users who define general subarrays that cross amplifier boundaries (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, but is available-but-unsupported. 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 8.1.

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 is 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, e.g., continuum subtraction of emission line data. 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 chip are shown in Table 8.1 and Figure 8.3. 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 8.1 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 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 4.1 and Table 4.2 can be used to determine the remaining two ramp filters which can be of interest for serendipitous observations. While all fifteen ramp filters can be used with the WFC, over specified regions of the WFC1 and WFC2 chips, only the five middle ramp filters are available with the HRC and they cover the region over the HRC chip defined by the HRC aperture (Table 8.2).

8.1.2 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. As a consequence, 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 has been chosen as 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 8.3 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 512x512 pixels in the C Amp readout corner, and a 512 pixel square aperture centered on the 1.8" 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 (Phase II terminology) POS TARGS, the observer can decide which mode to adopt.

8.1.3 SBC ACCUM Mode

The SBC ACCUM mode accumulates photons into a 1024 by 1024 array, 16 bits per pixel. At the end of the exposure the data are sent to the onboard recorder via the internal ACS memory buffer. The high-res mode used in the STIS MAMAs is not available for the SBC. Note that ACCUM is the only mode available for SBC observations since the Time Tag mode of STIS is also not available on ACS. The minimum SBC exposure time is 0.1 seconds and the maximum 1.0 hours. 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 9.2.

Up to 17 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.

8.1.4 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 which is however 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 5.2.2.

8.2 Patterns and Dithering

A number of different patterns are available for ACS to support dithered observations, i.e., observations where the pointing is shifted between frames. The size of the offsets can be very different depending on the purpose of offsetting the pointing between exposures. In particular, it is useful to distinguish between mosaicing and dithering. Mosaicing is done with the aim of increasing the area covered by a particular set of exposures, while providing a seamless joining of contiguous frames. Dithering is done for a variety of goals, namely

Patterns have been defined to allow ACS users to easily carry out both mosaicing and dithering. Using patterns allows exposures to be automatically associated in CALACS pipeline processing with the following restrictions: only pattern exposures obtained within a single visit and those patterns where the cumulative offset is under the ~100 arcsec guide star limitation can be associated. For the latter, these patterns include the dither patterns for all three cameras, the HRC and SBC mosaic patterns and the 2-point ACS-WFC-MOSAIC-LINE pattern. All patterns designed with POS TARGs will not be associated. These are described in detail on the ACS Dither web page: http://www.stsci.edu/hst/acs/proposing/dither.

The plate scale for the WFC varies by about �5%, and so a one pixel dither near the center will be 0.95 or 1.05 pixels near the corners. For this reason, dither patterns should strike a balance between being large enough to reject detector artifacts, and being as compact as possible to maintain the integrity of the pattern over the entire field-of-view. Large displacements will have varying sub-pixel properties across the image.

In addition to the plate scale variation associated with the significant ACS geometric distortion, there can also be a temporal variation of overall image alignment. Some CR-SPLIT images taken during SMOV testing, in which the two components were separated by the scheduling system across orbital occultations (about one hour gap), showed registration differences of about 0.5 pixels corner-to-corner. Thus for programs that wish to combine multiple images to create oversampled images at the resolution ACS is capable of providing, the user may need to allow for the general problem of combining distorted, misregistered images. A variety of tools are being made available within STSDAS and pyraf to assist with these tasks including PyDrizzle and Multidrizzle. See the ACS Drizzle Web page and the ACS Data Handbook at:

8.2.1 How to Obtain Dithered Data

Whenever possible, observers should make use of the pre-defined mosaic and dither patterns. For WFC exposures requiring a contiguous field of view, offsets by 2.5 arcsec or more are required to cover the interchip gap. The STSDAS Multidrizzle package is the recommended software package for processing dithered observations. It includes tools for rejecting CR affected pixels from data sets with a single image at each pointing so that CR-SPLITting observations at each pointing is not necessary. Multidrizzle enhances and simplifies the functionality of the STSDAS dither package. The following are suggestions on the optimal number of exposures for a dithered data set:

Given the relatively low read-out noise and the high throughput of the WFC, broad-band optical images longer than about 500 seconds will be background limited.

8.2.2 Supported Patterns

As with the other instruments, a suite of carefully designed ACS dither and mosaic "convenience patterns" is available for Phase II proposers. The goals of these patterns are to accomplish the removal of detector features (including the WFC interchip gap), and provide sub-pixel PSF sampling (optimized for the number of dither points). Both Line and Box patterns are available for each detector, with designation DITHER or MOSAIC depending on the intended purpose of the pattern. Default parameters are available for these convenience patterns, although observers may override these and specify their own patterns if desired. Detailed description of the use of these patterns and the syntax to employ in developing a Phase II proposal may be found in the Phase II Proposal Instructions, and on the ACS Dither web page: http://www.stsci.edu/hst/acs/proposing/dither.

8.2.3 How to Combine Dithered Observations

Because of the nonlinear geometric distortion of ACS, a shift by an integer number of pixels at the chip center will not, in general, correlate to an integer shift at the edge. Therefore, simple shift-and-add schemes are inadequate for the proper combination of ACS dither exposures for all but the smallest shifts (e.g., the 2x2 "hot pixel" dither pattern). In the case of WFC the effect can be very significant since a shift by 50 pixels, as required to bridge the interchip gap, will be different by 2.5 pixels at the edge of the CCD, causing stars in different exposures not to be aligned across the FOV when a simple shift is applied to the images.

The STSDAS dither package contains tools which can be used to effectively combine dithered ACS images. These include the low-level IRAF tasks such as drizzle itself but also high-level (Python) scripts which automate the process to a large degree. These require that the user be running the Pyraf environment rather than the IRAF "cl". For typical dithered ACS data the user will be able to use MultiDrizzle to combine datasets and simultaneously flag and ignore cosmic rays and other defects during the combination process. MultiDrizzle and PyDrizzle, which it uses as an interface layer to drizzle and other tools, are described in detail in the Data Handbook.

The quality of final images which can be obtained from well dithered datasets is limited by the optical PSF, the pixel size and charge diffusion effects and any broadening introduced by the combination process. Appropriate dither patterns and careful combination allow the last of these factors to be kept to a minimum. Rules of thumb which may be used to estimate the final PSF width are given in Fruchter & Hook 2002, PASP, 114, 144.

8.2.4 How to Determine the Offsets

Within a single visit the commanded relative positions and the positions that are actually achieved are in very good agreement, often to better than 0".01. Thus within one visit the commanded offsets are usually a very good starting point for image combination. On occasion the guide star acquisition leads to a false lock. In this case, the commanded position can be incorrect even by 0".5 or more. The jitter files allow the observer to track such false locks since they also contain information on the rms of the pointing, on the guide star separation and on the guide star separation rms. During false locks one or more of these indicators are normally anomalous. Across different visits the mismatch between commanded and achieved offsets can instead be significant. In these cases the offsets derived from the jitter files are better than the commanded ones, although they are only good to about 0".02 rms. For accurate combination of images the recommended strategy is to derive the offsets from cross-correlation of the images themselves, or by using matched object catalogues derived from the images. The dither package includes software to carry out such cross-correlations.

8.3 A Road Map for Optimizing Observations

Dithering and CR-SPLITting more than the minimum recommended values tends to yield higher quality images with fewer residual detector defects, hot pixels or CR signatures in the final combined image. In cases where hot pixels are of particular concern, dithering may be especially useful for simultaneous removal of hot pixels and cosmic rays. Unfortunately, splitting a given exposure time into several exposures reduces its signal-to-noise when the image is read-out noise limited. WFC images taken through the broad band filters and longer than about 500 seconds are background limited, while shorter exposures and narrow band images are read-out noise limited for all practical exposure times. Thus, the optimal number of CR-splits and dithering positions is a result of a trade-off between completeness of the hot pixel elimination, CR-rejection, final image quality, and optimal S/N. A schematic flow chart of this trade-off is given in Figure 8.1. The main steps in this, possibly iterative, process are the following:

8.4 CCD Gain Selection

As quantified in Table 7.3 both the WFC and HRC CCDs have selectable gain values near 1, 2, 4, and 8 electrons per digital number. Various factors should influence the gain selected in Phase II for your science program: level of support and calibrations provided, influence of associated readout noise on data quality, dynamic range on the bright end, and for the WFC in limited applications data compressibility.

8.4.1 WFC Gain

GAINs 1 and 2 are fully supported for the WFC, since GAIN=1 provides the smallest readout noise, while GAIN=2 (or above) is needed to sample the available full well depth. It is the goal, now closely achieved, to provide equal calibration support for data taken in these two supported gains, although more calibration data will be taken in the default GAIN=1 setting. Calibration support will not be provided for the "available-but-unsupported" GAIN=4 and 8 settings; users proposing their use should provide special justification and discussion of calibrations to be used. Note that WFC auto-parallel data is taken with GAIN=2.

While the readout noise is lower at GAIN=1, the advantage over GAIN=2 (< 0.3 e- extra rms) is modest. GAIN=2 has the offsetting advantage of completely sampling the full well depth of nearly 85,000 e- thus providing a > 0.3 magnitude dynamic range extension before saturation is reached. The latter could be advantageous even for programs in which the prime targets are very faint, if serendipitous objects in the field of view can be used to support image-to-image registration solutions as needed for optimal dithered image combinations. Furthermore, charge is conserved even beyond filling the full well depth, for point sources at GAIN=2 it is possible to obtain valid aperture photometry several magnitudes beyond saturation by summing over all pixels bled into. Both GAINs 1 and 2 provide better than critical sampling of the readout noise supporting robust background sky-level determination even at low values. Recent evidence also suggests that a minor electronic cross-talk feature is relatively less pronounced with GAIN=2, then the default GAIN=1.

The large pixel count for WFC can create data rate problems if images are acquired as quickly as possible over multiple orbits. The available-but-unsupported mode COMPRESSION is more effective when the noise is undersampled which could result in special circumstances for which the GAIN values of 4 or 8 are preferred.

8.4.2 HRC Gain

GAINS 2 and 4 are fully supported for the HRC, and analogous to the supported WFC values provide a low readout noise case and a GAIN that provides sampling of the physical full well depth.

GAIN = 4 on the HRC, which is needed if high dynamic range on the bright end is desired, does not provide critical sampling of the readout noise. Not only is the readout noise penalty in going from GAIN = 2 to 4 non-trivial, but background estimation will be less robust without critical noise sampling. As with WFC, when the full well depth is sampled with GAIN = 4 the detector response remains accurately linear up to and even well beyond saturation. Compression is not an issue for the small HRC images, therefore rationales for use of the unsupported GAIN = 8 are not anticipated. GAIN = 1 is available-but-unsupported, but the very modest improvement of readout noise in comparison to GAIN =2 (< 0.2 e- higher rms) seems unlikely to present compelling need for its use.

8.5 ACS Apertures

As discussed in Section 3.2, the ACS consists of three cameras: the WFC, the HRC and the SBC. The WFC is constructed of two CCDs each nominally 2048 by 4096 pixels, with their long sides adjacent to form a roughly square array, 4096 pixels on a side. The HRC CCD and the SBC MAMA detectors are each 1024 pixels square.

8.5.1 WFC Apertures

The active image area of each WFC detector is 4096 by 2048. The mean scale is 0.049 arcsec/pixel and the combined detectors cover an approximately square area of 202 arcseconds on a side. In establishing reference pixel positions we have to consider the overscan pixel areas which extend 24 pixels beyond the edges in the long direction. So each CCD must be regarded as a 4144 by 2048 pixel area. The gap between the two CCDs is equivalent to 50 pixels. In Figure 8.2 the letters A, B, C and D show the corner locations of the four readout amps.

We define apertures named WFC1 and WFC2 which represent the two CCDs, with their reference points at the geometric center of each chip, at pixel positions (2072,1024). If the appearance of new hot pixels makes these apertures undesirable, we will define new reference positions nearby. However, we keep two other apertures named WFC1-FIX and WFC2-FIX at the original central locations. For extended sources, choosing new positions may not be of any advantage and it may be more effective to use these fixed positions.

The aperture WFC encompasses both detectors and has its reference point near the overall center but about 10 arcsec away from the interchip gap. This has been chosen to be position (2072,200) on the WFC1 CCD. Again, this is the initial selection for the aperture named WFC which might be shifted later, but the reference point for WFC-FIX will remain at this value. Selection of WFC1, WFC2 or WFC only changes the pixel where the target will be positioned. In all three cases data is normally delivered in a file containing two imsets, one for each detector. See Section 11.1 for details of the ACS data format. Reading out a subarray, which consists of part of only one of the chips, is done only if requested.

WFCENTER is similar to WFC, but is placed at the center of the combined WFC full field. The center is defined as the average of the four corners in the distortion corrected space. Because of the scale variation this does not appear at the center in pixel space, but rather is on WFC2 about 20 pixels from the edge. Selection of WFCENTER can be of use in obtaining observations with maximum overlap at unique orientations and for mosaics.

For sets of observations which take place over a substantial part of a year, the telescope roll limitations will require measurements to be taken over most of the angular range. On sky, the WFC aperture is roughly square and it is natural to design observations in steps of 90 degrees to consistently cover the same area. There will be some region at the edges not covered at all four orientations. However, a square area of side 194.8 arcseconds centered on WFCENTER, and with edges parallel to the V2 and V3 axes, is overlapped at all four positions. In designing a mosaic which combines observations at 90 degree steps, a translation of about 190 arcseconds between pointings would provide continuous coverage.

8.5.2 Ramp Filter Apertures

Each ramp filter consists of three segments that can be rotated across the WFC field of view as indicated in Figure 8.3. The IRAMP filter can only be placed on WFC1 in a location which will define the aperture WFC1-IRAMP and the ORAMP only on WFC2 creating the aperture WFC2-ORAMP. The MRAMP filter can lie on WFC1 or WFC2 with corresponding apertures WFC1-MRAMP and WFC2-MRAMP. The approximate aperture locations are indicated in Figure 8.3, while actual data obtained during ground calibrations are overlayed on an image of a ramp filter in Figure 8.4. Operationally, a fixed reference point will be defined for each detector and filter combination. Then the ramp filter will be rotated to place the required wavelength at the reference position.

The reference positions for all defined apertures are given in Table 8.1 in pixels and in the telescope V2,V3 reference frame, where values are measured in arcseconds. The values given here are based on in-flight calibration results. The x and y axis angles are measured in degrees from the V3 axis towards the V2 axis. This is in the same sense as measuring from North to East on the sky. The "extent" of the ramp filter apertures given in Table 8.1 are the FWHM of the monochromatic patches (visible in Figure 8.4) measured from a small sample of ground calibration data. To use a ramp filter in a Phase II program, specify the filter name and the wavelength, and choose aperture "WFC". The scheduling software will then automatically rotate the filter to the appropriate wavelength, and point at the reference point of the aperture that is associated with the chosen filter (e.g., WFC2-ORAMP, if an ORAMP filter has been chosen). The specific aperture names WFC1-IRAMP, WFC2-ORAMP, WFC1-MRAMP and WFC2-MRAMP should not generally be listed explicitly in a Phase II program, but they are accessible as "available-but-unsupported" choices (see Section 2.5).

8.5.3 The Small Filter Apertures

When a filter designed for the HRC is used on the WFC, it only covers a small area on either WFC1 or WFC2. The projected filter position may be placed on either chip by selection of the filter wheel setting. Figure 8.3 shows how the filter projection may be placed so as to avoid the borders of the chips. The apertures WFC1_SMFL and WFC2_SMFL are defined for this purpose and are automatically assigned when a WFC observation is proposed using an HRC filter. Reference positions at or near the center of these apertures are defined so that a target may be placed in the region covered by the chosen filter.

The axis angles given in Table 8.1 do not refer to the edges of the apertures as drawn, but rather to the orientation of the x and y axes at the WFC reference pixel. These angles vary slightly with position due to geometric distortion.

For the polarizers and F892n used with WFC, the default will be to read out a subarray. The subarray will be a rectangular area with sides parallel to the detector edges which encompasses the indicated filtered areas. For ramp filters the default will be to readout the entire WFC detector, unless a polarizer is used with the ramp filter, in which case a subarray is readout. Users cannot override the small filter subarrays.

***Table 8.1: WFC Aperture Parameters***
Aperture Name readout area	Extent (arcsec)	Reference pixel	Reference V2,V3 (arcsec)	x-axis angle	y-axis angle
Aperture Name readout area	Extent (arcsec)	Reference pixel	Reference V2,V3 (arcsec)	(degrees from V3 through V2)
`WFC`4096 × 4096	202 × 202	(2072,200) on WFC1	(259,239)	92.2	177.5
`WFCENTER`4096 × 4096	202 × 202	(2114,2029) on WFC2	(261,252)	92.1	177.5
`WFC1`4096 × 4096	202 × 202	(2072, 1024)	(261,198)	92.5	177.4
`WFC1-FIX`4096 × 4096	202 × 202	(2072, 1024)	(261,198)	92.5	177.4
`WFC2`4096 × 4096	202 × 202	(2072, 1024)	(257,302)	91.7	177.8
`WFC2-FIX`4096 × 4096	202 × 202	(2072, 1024)	(257,302)	91.7	177.8
`WFC1-IRAMP`4096 × 4096	25 × 65	(680,1325)	(194,187)	93.1	176.6
`WFC1-MRAMP`4096 × 4096	35 × 80	(3096,1024)	(312,196)	92.2	177.8
`WFC2-MRAMP`4096 × 4096	40 × 80	(1048,1024)	(206,304)	91.9	177.5
`WFC2-ORAMP`4096 × 4096	35 × 65	(3494,708)	(328,316)	91.4	178.2
`WFC1-SMFL`2048 × 2048	70 × 80	(3096,1024)	(312,196)	92.2	177.8
`WFC2-SMFL`2048 × 2048	70 × 80	(1048,1024)	(206,304)	91.9	177.5
`WFC1-POL0UV` 2048 × 2048	90 × 80	(3096,1024)	(312,196)	92.1	177.8
WFC1-POL0V 2048 × 2048	90 × 80	(3096,1024)	(312,196)	92.2	177.8
WFC2-POL0UV 2048 × 2048	90 × 80	(1048,1024)	(206,304)	91.9	177.5
WFC2-POL0V 2048 × 2048	90 × 80	(1048,1024)	(206,304)	91.9	177.5
WFC1-512 512 × 512	25 × 25	(3864,1792)	(352,158)	92.1	178.0
WFC1-1K 1024 × 1024	50 × 50	(3608,1536)	(338,170)	92.1	177.9
WFC1-2K 2048 × 2048	101 × 101	(3096,1024)	(312,196)	92.2	177.8
WFC2-2K 2048 × 2048	101 × 101	(1048,1024)	(206,304)	91.9	177.5

8.5.4 Polarizer Apertures

Apertures have been provided for use with the polarizer sets similar to the SMFL apertures. These apertures are selected automatically when a polarizing spectral element is used, and a single WFC chip quadrant readout is obtained. The aperture parameters given in Table 8.1 are valid for all three polarizing filters in each polarizer set, UV or visible, to the stated significant figures.

8.5.5 HRC Apertures

The HRC has an area of 1062 by 1024 including 19 physical overscan pixels at each end in the x direction. The active area is 1024 by 1024 pixels. The mean scales along the x and y directions are 0.028 and 0.025 arcseconds/pixel, thus providing a field of view of about 29 by 26 arcseconds in extent. The anisotropy and variation of scales is discussed in a later section of this handbook. The reference point for the aperture labelled HRC-FIX, and initially for HRC, is at the geometric center, (531,512). As with the WFC apertures, there may be reason to move the HRC reference point later.

The HRC is equipped with two coronagraphic spots, nominally 1.8 and 3.0 arcseconds in diameter and a coronagraphic finger, 0.8 arcseconds in width. Apertures HRC-CORON1.8, HRC-CORON3.0 and HRC-OCCULT0.8 are defined to correspond to these features. In addition we define a target acquisition aperture, HRC-ACQ designed for acquiring targets which are subsequently automatically placed behind a coronagraphic spot or the occultation finger. The positions of the coronagraphic spots have been found to fluctuate. Observations will need to incorporate a USE OFFSET special requirement to allow current values to be inserted at the time of the observation (see the Phase II Proposal Instructions).

A substantial region masked out by the occulting finger can be present in the HRC data (Figure 8.5). The occulting finger is not retractable -- it will be in every HRC exposure. However as with any other detector feature or artifact, the "lost" data can be recovered by combining exposures which were suitably shifted with respect to each other. A dither pattern, ACS-HRC-DITHER-LINE has been defined for this purpose and spans the area flagged for the HRC occulting finger (~1.6 arcsec or ~56 pixels wide), with an extra ~0.3 arcsec or ~10 pixels of overlap. More details can be found in the Phase II Instructions Handbook.

8.5.6 SBC Apertures

The SBC aperture is 1024 pixel square. There are no overscan pixels to consider. The x and y scales are 0.034 and 0.030 arcseconds/pixel leading to a coverage on the sky of 35 by 31 arcseconds. The reference point will initially be at (512,512). As with the CCDs we will maintain an SBC-FIX aperture which will always have this same position even if SBC has to be altered. MAMA detectors slowly lose efficiency with each exposure, therefore the SBC reference point may be shifted if the initial position shows this effect to a measurable degree.

The (512,512) reference point falls at the same position in (V2,V3) as the HRC, namely (207, 471) and the x and y axis angles are -83.9 and 0.4 degrees.

***Table 8.2: HRC Aperture Parameters***
Aperture Name active area	Extent (arcsec)	Reference pixel	Reference V2,V3 (arcsec)	x-axis angle	y-axis angle
HRC 1024 × 1024	29 × 26	(531, 512)	(206,472)	-84.1	0.1
HRC-FIX 1024 × 1024	29 × 26	(531, 512)	(206,472)	-84.1	0.1
HRC-CORON1.8 200 × 200	6 × 5	(564,466)¹	(205,471)¹	-84.1	0.1
HRC-CORON3.0 200 × 200	6 × 5	(467,794)¹	(208,479)¹	-84.2	0.1
HRC-OCCULT0.8 200 × 200	6 × 5	(443,791)	(209,477)	-84.2	0.2
HRC-ACQ 200 × 200	6 × 5	(735,575)	(200,474)	-84.1	0.0
HRC-512 512 × 512	15 × 13	(276,256)	(214,465)	-84.1	0.2
HRC-SUB1.8 512 × 512	15 × 13	(570,468)	(205,471)	-84.1	0.1

¹These values fluctuate and will be updated at the time of the observation.

8.6 Fixing Orientation on the Sky

Determining the orientation of an image requires knowledge of the telescope roll and the angle of the aperture relative to the telescope's coordinate frame. A target may need to be specially oriented on a detector, particularly when spectroscopy is to be performed.

All HST aperture positions and orientations are defined within an orthogonal coordinate system labelled V1,V2,V3, in which V1 is nominally along the telescope roll axis. Apertures are therefore in the V2,V3 plane. The V3 position angle p is defined as the angle of the projection of the V3 axis on the sky, measured from North towards East with the aperture denoting the origin. This is almost identical to the telescope roll angle. (There is a small difference between roll angles measured at the V1 axis and those measured at the aperture. This can amount to several tenths of a degree depending on the target declination.) When the position angle is zero, V3 points North and V2 points East. In the V2V3 coordinate system, aperture orientations are defined by

_x and

_y, the angles their x and y axes make with the V3 axis, measured in an anti-clockwise direction. (The value of

_x as illustrated would be considered negative.) Hence, the angles these axes make with North are found by adding the axis angles to the position angle.

The science image header supplies the value of ORIENTAT, the angle the detector y axis makes with North, which is equal to p +

_y. Another angle keyword supplied is PA_APER which is the angle the aperture y axis makes with North. Both angles are defined at the aperture so using them does not involve the displacement difference. Normally the aperture and detector y axes are parallel and so PA_APER = ORIENTAT. Several STIS slit apertures were not aligned parallel to the detector axes and so this distinction was meaningful, but ACS has no slit apertures so this difference will probably not arise. In any case, we recommend to use PA_APER.

Beyond establishing the direction of the aperture axes, it will often be required to know the orientation of a feature, such as the plane of a galaxy, within an image. Conversely, we need to know what direction within an image corresponds to North. To this end we define a feature angle

within the aperture as measured on the science image, anti-clockwise from the y-axis so that it is in the same sense as the previously defined angles. For an orthogonal set of aperture axes the direction of this feature would be PA_APER +

and the image direction of North would be the value of

which makes this angle zero, namely -PA_APER, still measured in an anti-clockwise direction from the y axis.

The x and y axes projected on the sky are not necessarily orthogonal. For all instruments prior to the ACS the departure from orthogonality has been negligible, but for the ACS the angle between the axes is about 85 degrees. Figure 8.7 realistically represents the alignment of the ACS apertures and shows that the apertures are not square. The x and y axes indicated are those that will be used for the science images. The V2,V3 coordinates can be calculated from the x, y coordinates according to

where s_x and s_y are scales in arcsec per pixel along the image x and y axes. V2₀ and V3₀ are the coordinates of the aperture origin, but they do not enter into the angle calculations. Figure 8.7 shows that a rotation from x to y is in the opposite sense to a rotation from V2 to V3. This will be the arrangement for ACS apertures. This is significant in defining the sense of the rotation angles. For a direction specified by displacements

x and

y in the image, the angle

is arctan(-

y).

Because of the oblique coordinates, the angle

_s on the sky will not be equal to

. To calculate the sky angle, it is convenient to define another set of orthogonal axes x_s, y_s, similar to the V2V3 but rotated so that y_s lies along y, and x_s is approximately in the x direction. Let

_y-

_x be the angle between the projected detector axes and for simplicity let their origins be coincident.Then the transformation is

The equation as written will place the angle in the proper quadrant if the ATAN2 Fortran function or the IDL ATAN function is used. To get the true angle East of North, for a feature seen at angle

in the image, calculate

_s and add to PA_APER.

To find the value of

corresponding to North we need the value of

_ssuch that PA_APER+

_s= 0. So substitute -PA_APER for

_s in the equation to get the angle

in the image which corresponds to North. The values of the scales and axis angles for all instruments are maintained on an Observatory Science Group web page:

For the ACS apertures, the values in Table 8.3 have been derived from results of operating the ACS in the Refractive Aberrated Simulator. These should not be considered as true calibrations but they indicate some aperture features, such as the non-orthogonality of the aperture axes and the x and y scale differences for HRC and SBC.

***Table 8.3: Plate scales and axis angles for the 3 ACS channels***
	s_x	s_y	_x	_y	_y-_x
	arcsec/pixel	degrees
WFC	.0494	.0494	92.2	177.5	85.3
HRC	.0284	.0248	-84.1	0.1	84.2
SBC	.0338	.0301	-85.4	-1.0	84.4

8.6.1 Determining Orientation for Phase II

A particular orientation is specified in an HST Phase II proposal using yet another coordinate system: U2,U3. These axes are opposite to V2 and V3, so, for example, U3 = -V3. The angle ORIENT, used in a Phase II proposal to specify a particular spacecraft orientation, is the position angle of U3 measured from North towards East. The direction of the V3 axis with respect to North is PA_APER -

_y and so

The IRAF task rotate in the package images.geom takes an image and rotates it counter-clockwise by a specified angle. To orient an image so that its y axis becomes North, the angle to specify is PA_APER. The x axis will then point approximately 5 degrees North of East. Orients can be checked by using the Visual Target Tuner (VTT) in the APT.

8.7 Parallel Observations

8.7.1 Parallel Observing

Parallel observing allows HST to operate several instruments simultaneously, in addition to the instrument that executes the primary observations. While the primary instrument observes a fixed target at user-specified coordinates, the parallel instrument observes at coordinates 5 to 10 arcminutes away, depending on the parallel instrument. The HST field of view following SM3B (Figure 3.3) shows the general locations of the instrument apertures adjacent to one another on the sky. Accurate relative positions for all instruments can be found on STScI's Observatory web page under "Pointing"¹. The recommended method of determining the field of view for any instrument is the Visual Target Tuner (VTT). A Digital Sky Survey (or user supplied) image of the primary target area is displayed with an HST field of view overlay. Any desired coordinate and ORIENT combination for the primary target will then display the possible pointings of any instrument operated in parallel. If the primary exposure will execute at a known (absolute) orient, the VTT will display the exact field of view for any instrument executed in parallel. If the primary exposure will execute at a random (nominal) orient or range of orient values, the VTT allows the HST field-of-view to be rotated interactively about the primary pointing. The VTT can be an invaluable resource for parallel observing programs, especially those designed for or restricted to specific pointings for the parallel FOV.

Certain operating limits are in place to restrict use of configurations, modes, parameters, elements and requirements allowed for each instrument while used in parallel. Details on these limits are documented in the Cycle 14 Call For Proposals, Section 4.2: "Parallel Observations". General information on ACS specific parallel operations are documented in the following sections for each of the three types of ACS parallel observing: coordinated, auto and pure.

ACS Coordinated Parallels

Coordinated parallel observations are specified in the same Phase II observing program as the primary observations via the prime + parallel group containers in APT. A single ACS channel may be used for a coordinated parallel observation, with, and only with, another instrument. Unlike NICMOS, coordinated parallels cannot be used to operate any of the ACS channels simultaneously. ACS exposures may not be used as both the prime and parallel exposures within the same parallel container. In order to operate ACS channels simultaneously, the use of ACS auto-parallels are described in the following section. The filter choice for auto-parallels is restricted and thus implemented as auto-parallels instead of coordinated parallels.

In order to protect the ACS SBC detector from inadvertent over illumination, the ACS/SBC configuration may be used as a coordinated parallel only if an exact spacecraft orientation (ORIENTation) is specified, the coordinates of the parallel field are determined and the parallel target or field passes the same bright-object screening applied to SBC primary observations. The Visual Target Tuner will greatly assist in writing this type of ACS parallel program.

Currently ACS and WFPC2, used together in a coordinated parallel program, result in the ACS auto-parallel capability being disabled. This restriction is made to prevent potential conflicts between ACS buffer dumps and WFPC2 readouts. The user may attempt to restore autoparallels via PAREXP=MULTIPLE.

ACS Auto-Parallels

The ACS auto-parallel capability is intended to increase the scientific return of the instrument by adding exposures with the parallel detector while interfering as little as possible with the observer's primary program. When either the WFC or HRC is the primary channel, and the exposure in that channel meets the requirements stated below, an auto-parallel observation will be automatically scheduled in the parallel channel during Phase II processing. Parallel detector exposures will be added automatically with the longest possible exposure time that does not interfere with the primary program. In order for an auto-parallel to be scheduled, the primary observation must meet the specifications that depend primarily on the exposure time and the filter selection of the primary exposure.

The user has three control options: This is done by selecting the PAREXP optional parameter. A user may either choose to explicitly add the auto-parallels by choosing PAREXP=MULTIPLE; choose to have no auto-parallels added by selecting PAREXP=NONE; or leave the special requirement set to the default, DEF. When DEF is selected, auto-parallels will be added according to the same primary exposure requirements as those for the MULTIPLE option. When MULTIPLE is selected an auto-parallel will be added for each CR-SPLIT part of the primary exposure (see Figure 8.8). There is a simple algorithm that the Phase II software follows in order to determine if an auto-parallel is feasible:

HRC Primary	WFC Auto-parallel	WFC Primary	HRC Auto-parallel
F892N	F775W	F475W	F850LP
F606W	F625W	F658N	F550M
F502N	F550M	G800L	F625W
G800L	F850LP	F502N	F775W
F555W	F606W	F606W	F555W
F775W	F502N	F850LP	G800L
F625W	G800L	F550M	F502N
F550M	F658N	F625W	F606W
F850LP	F475W	F775W	F892N
F330W	F660N	F660N	F330W
F250W	F814W	F814W	F250W
F220W	F435W	F435W	F220W

primary exposure type	multiple auto-parallel scenario (n 1)
HRC	n × 495
WFC compressed	n × 406
WFC uncompressed	n × 465

	This data loss will be considered routine and it will be the policy of STScI not to repeat observations scheduled as auto-parallels which exhibit partial data loss due to the data compression.

ACS Pure Parallels

In ACS pure parallel observations, an observation is taken with ACS on an essentially random area of the sky while another instrument is making prime observations. The WFC will be a prominent instrument in pure parallel observing due to its sensitivity and field of view. The HRC is also available for pure parallel observing, however no SBC pure parallels will be allowed due to bright object concerns.

Unlike the previous two types of parallel programs, pure parallels contain only parallel visits. Use of the GO/PAR proposal category will make any visit in the program a pure parallel.

The ACS default (archival) pure parallel program continued to execute for the community until midway through Cycle 13 when all of the "Default" HST archival pure parallel programs were discontinued to prolong the lifetime of transmitters on HST. This non-proprietary data currently comes from programs 9575, 9584 and 9701. A list of all pure parallel datasets in the HST archive is updated regularly at:

Pure parallel observations are executed at every possible opportunity, although there are many constraints which can render pure parallels unschedulable in any given orbit. Pure parallels will always be given lower priority than primaries and are thus scheduled only on a non-interference basis.

ACS Auto-Parallels with ACS Coordinated and ACS Pure Parallels

ACS auto-parallels can be added to ACS pure and ACS coordinated parallels by default if scheduling constraints allow. However auto-parallels cannot be added to observations that make use of EXPAND or MAX DUR special requirements. Therefore ACS pure parallels can either be crafted to expose for the maximum duration allowed in each individual orbit by using EXPAND/MAX DUR or have auto-parallels added, but not both.

8.8 Two-Gyro Guiding

At some future date HST may be operated with only two gyroscopes. A primary impact of operating with two gyros will be a reduction in the available observing time during an orbit, and a decrease in the times of year when a given target can be observed. Other potential impacts may include additional spacecraft jitter and degradation of the effective PSF, as well as slightly poorer pointing stability between different exposures..

Advanced Camera for Surveys Instrument Handbook for Cycle 14

Chapter 8:
Observing Techniques

8.1 Operating Modes

8.1.1 WFC ACCUM Mode

WFC CCD Subarrays

Ramp Filters

8.1.2 HRC ACCUM Mode

HRC CCD Subarrays

8.1.3 SBC ACCUM Mode

8.1.4 HRC ACQ Mode

8.2 Patterns and Dithering

8.2.1 How to Obtain Dithered Data

8.2.2 Supported Patterns

8.2.3 How to Combine Dithered Observations

8.2.4 How to Determine the Offsets

8.3 A Road Map for Optimizing Observations

8.4 CCD Gain Selection

8.4.1 WFC Gain

8.4.2 HRC Gain

8.5 ACS Apertures

8.5.1 WFC Apertures

8.5.2 Ramp Filter Apertures

8.5.3 The Small Filter Apertures

8.5.4 Polarizer Apertures

8.5.5 HRC Apertures

8.5.6 SBC Apertures

8.6 Fixing Orientation on the Sky

8.6.1 Determining Orientation for Phase II

8.7 Parallel Observations

8.7.1 Parallel Observing

ACS Coordinated Parallels

ACS Auto-Parallels

ACS Pure Parallels

ACS Auto-Parallels with ACS Coordinated and ACS Pure Parallels

8.8 Two-Gyro Guiding

Advanced Camera for Surveys Instrument Handbook for Cycle 14

Chapter 8: Observing Techniques

8.1 Operating Modes

8.1.1 WFC ACCUM Mode

WFC CCD Subarrays

Ramp Filters

8.1.2 HRC ACCUM Mode

HRC CCD Subarrays

8.1.3 SBC ACCUM Mode

8.1.4 HRC ACQ Mode

8.2 Patterns and Dithering

8.2.1 How to Obtain Dithered Data

8.2.2 Supported Patterns

8.2.3 How to Combine Dithered Observations

8.2.4 How to Determine the Offsets

8.3 A Road Map for Optimizing Observations

8.4 CCD Gain Selection

8.4.1 WFC Gain

8.4.2 HRC Gain

8.5 ACS Apertures

8.5.1 WFC Apertures

8.5.2 Ramp Filter Apertures

8.5.3 The Small Filter Apertures

8.5.4 Polarizer Apertures

8.5.5 HRC Apertures

8.5.6 SBC Apertures

8.6 Fixing Orientation on the Sky

8.6.1 Determining Orientation for Phase II

8.7 Parallel Observations

8.7.1 Parallel Observing

ACS Coordinated Parallels

ACS Auto-Parallels

ACS Pure Parallels

ACS Auto-Parallels with ACS Coordinated and ACS Pure Parallels

8.8 Two-Gyro Guiding

Chapter 8:
Observing Techniques