Introduction

There is one detector on ACS that can be used to obtain optical and infrared data, and that is the WFC.

Mode = ACCUM Config = ACS/WFC

Photons are counted on the Wide Field Channel CCD as accumulated charge, which is read out at the end of the exposure and converted to DN at a selectable gain. The DN are stored as 16-bit words in a data memory array. A full detector readout is 4144 × 4136 pixels, which includes 24 leading pixels and 24 trailing pixels of overscan per line and 40 virtual overscan lines.

Aperture or FOV

Allowed apertures for this mode are: SBC or SBC-FIX or SBC-LODARK.

Note: that the prisms are not permitted when the Aperture is SBC-LODARK.

An appropriate ramp aperture (names contain "RAMP") must be specified when a ramp filter (names begin with "FR") is used, and may be specified for other spectral elements. Two readout options are allowed with ACS ramp filters: (1) full WFC readout (i.e., 4K × 4K pixels designated by "RAMP" apertures), or (2) one WFC quadrant readout (i.e., 2K × 2K pixels designated by "RAMPQ" apertures). Because the field of view of any ramp filter segment lies completely within one WFC quadrant, the RAMPQ apertures are usually more practical and preferred, as described below. No other apertures (subarray or otherwise) are supported for ramp filter observations. Moreover, STScI discontinued support of user-defined subarrays for all filters and grisms after SM4.

Because all four WFC quadrants are clocked simultaneously during readout, the amount of time (2.5 minutes) to read out the full 4K × 4K frame is the same as the time to read out one 2K × 2K quadrant. Consequently, there is no advantage to using the RAMPQ apertures from the perspective of readout time alone. However, users can record up to four images with the RAMPQ apertures (instead of just one image with the RAMP apertures) before triggering the data buffer dump to the solid state recorder. By dividing the buffer dump into smaller segments, the users can obtain shorter exposure without incurring large overheads during the buffer dump. This allows more efficient packing exposures in each HST orbit.

Only apertures WFC, WFC1 or WFC2 may be used with the following spectral elements: POL0UV, POL60UV, POL120UV, POL0V, POL60V, POL120V, and F892N (unless a ramp aperture and filter are also specified). The actual position of the target within the FOV for these spectral element/aperture combinations will be determined by the STScI based on detector performance.

The aperture WFC1-CTE is available to mitigate CTE loss. The WFC1-CTE aperture has the same area as the WFC1 aperture except that the reference position is 200 pixels from the upper-right corner of Chip 1, in both the AXIS1 and AXIS2 directions. Therefore WFC1-CTE is not appropriate for highly extended targets. Observations of targets placed here will be less affected by CTE loss than those placed at other commonly used apertures that are closer to the center of the detector.

For apertures WFC1-512, WFC1-1K, WFC1-2K, WFC2-2K and the quadrant ramp apertures (names end with "RAMPQ" except WFC2-MRAMPQ), the proposal processing software will assign a subarray encompassing the field of view of the aperture. Applicable overscan and bias calibrations are automatically available. See the ACS Instrument Handbook for details. These subarrays cannot be modified by the use of subarray Optional Parameters.

Target location on the detector is the same for a full-frame ramp aperture and the corresponding subarray readout quadrant ramp aperture.

For Apertures WFC<chip><amp>-<size>, where <chip> = 1 or 2, <amp> = A, B, C, or D, and <size> = 512, 1K, or 2K, the proposal processing software will assign a subarray whose dimensions are 2048 × 512, 2048 × 1024, or 2048 × 2048 pixels in the imaging area, to be read out through the <amp> readout amplifier. All of these subarrays will also have 24 columns of physical pre-overscan, making the full X size 2072. The 2K subarrays will also have 20 rows of virtual overscan, making the full Y size 2068. Note that the appropriate one of these 2K subarrays will be used by default when a polarizer/F892N or quadrant ramp Aperture is selected.

Table 10.6: Allowed Aperture, Spectral Element & Readout Combinations summarizes rules for Aperture and Spectral Element combinations, and whether a full-frame, optional subarray, or fixed subarray readout is done.

Spectral Element

For the available ACS/WFC spectral elements, see Table 10.3: Spectral Elements for use with ACS/WFC Configuration.

When F892N or a polarizer is specified, STScI will automatically assign a subarray containing the entire FOV provided by those spectral elements. The subarray is one-quarter the size of the full WFC array. Those subarray parameters may not be overridden.

For restricted parameter Click here to expand/collapse Therefore, F892N and the polarizers may not be specified when Optional Parameter CTE is specified as JCTWFS or JCTWE.

Wavelength

If a ramp filter (any spectral element beginning with the letters “FR”) is specified, enter the desired central wavelength in Ångstroms. Table 10.5: Wavelength Ranges for the WFC Ramp Filters gives the allowed minimum and maximum wavelength for each ramp filter.

A wavelength should not be specified if a ramp filter is not being used.

Optional Parameters

CR-SPLIT

= 2 - 8; NO (default)

Specifies the number of sub-exposures into which the exposure is to be split for the purpose of cosmic ray elimination in post-observation data processing (see the ACS Instrument Handbook). The specified exposure time will be divided equally among the number of CR-SPLIT exposures requested. If CR-SPLIT=NO, the exposure is taken without splitting. If a pattern is also specified (see Chapter 7: Pointings and Patterns), the specified number of sub-exposures will be taken at each pattern point.

In May 2020, the number of hot pixels were measured to be ~1.5% of the total number of available pixels (see Figure 3 and Figure 5 in McDonald et. al.  ACS ISR 2020-05 “Anneal Efficacy in the Advanced Camera for Surveys Wide Field Channel”). This is similar to the number of pixels affected by cosmic rays in a 1000 sec exposure (between 1.5% and 3%). Hot pixels are growing at a rate of ~0.1% per year (see Figure 5 in ACS ISR 2020-05). The standard CR-SPLIT approach allows for cosmic-ray subtraction, but without additional dithering, does not allow for correction of hot pixels. Hence, we recommend that users who would otherwise have used a single CR-SPLIT, now use some form of dithering instead. For example, a simple ACS-WFC-DITHER-LINE pattern has been developed based on integer pixel offsets (see 7.2 ACS Patterns). This will allow the simultaneous removal of both hot pixels and cosmic ray hits in post-observation processing.

GAIN

= 0.5, 1, 1.4, 2 (default) (e/DN)

Specifies the gain of the CCD electronics in e/DN.

AUTOIMAGE

= YES (default), NO

Controls the automatic scheduling of image exposures for the purpose of spectra zero point determination of grism observations. By default, a single short image through a standard filter will be taken in conjunction with each Exposure Specification using the grism for external science observations. A value AUTOIMAGE=NO will disable the automatic scheduling of the image exposure for the Exposure Specification on which it is specified. The parameter is allowed only on external science observations using the grism.

For available but unsupported parameters (SIZEAXIS2, SIZEAXIS1, CENTERAXIS2, CENTERAXIS1, COMPRESSION, AMP)

Click here to expand/collapse

SIZEAXIS2

= FULL (default); 16 - 2046 (pixels)

Specifies the size of a WFC subarray in detector coordinates along the AXIS2 direction. Care must be taken in specifying a WFC detector subarray to ensure that the subarray is fully contained in one of the two WFC chips that compose the WFC detector array.

SIZEAXIS2 may not be used when the Spectral Element specification includes F892N or a polarizer, or when a fixed subarray Aperture is specified (see Table 10.6: Allowed Aperture, Spectral Element & Readout Combinations).

The default value FULL will result in a full detector readout of 4144 × 4136 pixels, unless Optional Parameter SIZEAXIS1 has a numerical value, in which case the readout will be SIZEAXIS1 × 2046 pixels. Subarrays will be centered at the reference position of the Aperture used for the exposure. CENTERAXIS2 may be used to override this default behavior.

SIZEAXIS1

= FULL (default); 16 - 4140 (pixels)

Specifies the size of a WFC subarray in detector coordinates along the AXIS1 direction. Care must be taken in specifying a WFC detector subarray to ensure that the subarray is fully contained in one of the two WFC chips that compose the WFC detector array.

SIZEAXIS1 may not be used when the Spectral Element specification includes F892N or a polarizer, or when a fixed subarray Aperture is specified (see Table 10.6: Allowed Aperture, Spectral Element & Readout Combinations).

When SIZEAXIS1 is given a numerical value, it must be even. The default value FULL will result in a full detector readout of 4144 × 4136 pixels, unless Optional Parameter SIZEAXIS2 has a numerical value, in which case the readout will be 4140 x SIZEAXIS2 pixels. Subarrays will be centered at the reference position of the Aperture used for the exposure. CENTERAXIS1 may be used to override this default behavior.

CENTERAXIS2

= 9 - 2039, 2057 - 4087, TARGET

Specifies the center pixel in the AXIS2 direction. CENTERAXIS2 may be specified only if SIZEAXIS2 is also specified as a number. The default is to center a subarray at the aperture reference position.

The value TARGET is used to center a subarray on the target’s detector coordinates, which may differ from the aperture reference if a POSition TARGet offset is specified.

CENTERAXIS2 values between 9 - 2039 are on WFC Chip 2 and the values between 2057 - 4087 are on WFC Chip 1.

CENTERAXIS1

= 11 - 4135, TARGET

Specifies the center pixel in the AXIS1 direction. CENTERAXIS1 may be specified only if SIZEAXIS1 is also specified as a number. The default is to center a subarray at the aperture reference position.

The value TARGET is used to center a subarray on the target’s detector coordinates, which may differ from the aperture reference if a POSition TARGet offset is specified.

COMPRESSION

= DEF (default), NONE, 1.3 - 3.5

Specifies the compression factor to be used in determining a fixed size for the WFC image after onboard compression. If the actual compression achieved onboard is greater than the specified value, fill data will be added to the compressed science data. If the actual compression factor is less than the specified value, science data will be lost. A value NONE will disable onboard compression. The default value will depend on the target type and the GAIN parameter. The table of default compression factors can be found in the ACS Instrument Handbook. Compression of images with low gain values should be avoided because an actual compression factor of less than 1.3 is likely and thus science data will be lost.

AMP

= A, B, C, D (values allowed for subarrays)

= ABCD, AD, BC (values allowed for full-frame)

= AD, BC (only values allowed when CTE = JCTWFS or JCTWE)

Indicates  the  readout  amplifier(s)  to  use  for  this  exposure.  For  full-frame exposures, the default is ABCD. For subarrays, the default readout amplifier will be the one closest to the subarray. If the exposure will be a subarray care must be taken to ensure that the readout amplifier specified is on the same WFC chip as the subarray. Refer to Table 10.6: Allowed Aperture, Spectral Element & Readout Combinations and the ACS Instrument Handbook for details.

AMP may not be specified if one of the amplifier-specific subarray apertures is specified, if one of the quadrant ramp apertures (names end with "RAMPQ") is specified, or if a polarizer or F892N is specified.

The value AD is not recommended for use with the aperture WFC1-CTE. If this aperture is selected, AMP should include B since this aperture’s reference position is near readout amplifier B.

AMP must be specified when CTE = JCTWFS or JCTWE. Only amplifier values AD and BC are permitted for CTE.

For restricted parameters (CTE, READTYPE, SPEED)

Click here to expand/collapse

CTE

= NONE (default), JCTWFS, JCTWE

Indicates that the exposure specified will be for the purpose of making a charge transfer efficiency (CTE) measurement. The parameter value specifies which type of CTE measurement to obtain. JCTWE is for an EPER image, while JCTWFS is for an FPR image. CTE images must be specified with TUNGSTEN as the target, except as noted below.

When CTE is specified as JCTWE or JCTWFS, Optional Parameter AMP must also be specified, and FLASHCUR, FLASHEXP, and GAIN are the only other Optional Parameters that may be specified.

CTE may not be specified if a subarray aperture is used (WFC1-512, WFC1-1K, WFC1-2K, or WFC2-2K).

To obtain a bias subtraction image with a CTE timing pattern, specify: The desired CTE value, The target as DARK, not BIAS, and Time_per_Exposure as 0.5 S.

READTYPE

= DSINT (default), CLAMP

Indicates the type of readout for WFC exposures. The default value DSINT is for the dual slope integrator readout method. The value CLAMP is for the clamp and sample readout method.

SPEED

= FULL (default), HALF

Indicates the speed of readout for WFC exposures. The default value FULL is for the pre-SM4 readout timing. The value HALF is for a half speed readout which may be desirable for reducing read noise.

FLASH =

0 (default - means no post-flash), 1 - 5733

Specifies the number of electrons per pixel to add to the image by illuminating the detector with the post-flash LED. The upper limit corresponds to the longest flash permitted.

Estimate the number of background electrons expected using the ACS ETC. Also refer to ACS ISR 2012-04 “Assessing ACS/WFC Sky Backgrounds” (Sokol et al.) to see expected background levels for the given exposure time. If the background in the area of interest is less than 20 electrons, pixel based CTE corrections may be off by a significant amount (or may not work very well). Add post flash to elevate the background to 20 electrons or more in the area of your target or region of interest.

Note that the level of the ACS post-flash is highly spatially variable and the commanded value of flash is the value seen in the central region, and other regions have lower values. See ACS ISR 2014-01 “Post-Flash Capabilities of the Advanced Camera for Surveys Wide Field Channel (ACS/WFC)” (Ogaz et al.) for more details. You should evaluate and assess the need for it based on the anticipated sky background levels and the scientific and technical purposes behind the design of your intended observations. If you have an assigned ACS Contact Scientist, that person may be able to help, or if not, you may contact the ACS Help Desk at https://stsci.service-now.com/hst.

To summarize: use of ACS post-flash may be recommended by the ACS ETC in some circumstances where there will be low background in an exposure, especially due to use of very short exposure times or narrow-band filters, etc. (See also ACS ISR 2012-04 by Sokol et al.) However, use of this capability should be considered very carefully and may also greatly complicate your data analysis in many situations, due to the highly nonuniform nature and significant variation in the flash level over the ACS/WFC field of view which is documented more fully in ACS ISR 2014-01 (Ogaz et al., 2014). You can also read more about the ACS post-flash in the example and discussion in Section 9.6.6 of the ACS Instrument Handbook. You may also request help from your assigned Contact Scientist, if you have one, or you may request help from the ACS group through the help desk at https://stsci.service-now.com/hst.

Number of Iterations

Enter the number of times this Exposure Specification should be iterated, each with the specified Time_per_Exposure.

CR-SPLIT and multiple iterations are mutually exclusive capabilities. If Number_of_Iterations > 1 on an external exposure, CR-SPLIT=NO must be specified.

Time Per Exposure

Enter the exposure time, in seconds, for the Exposure Specification. If Number_of_Iterations = 1, the Time_per_Exposure is divided equally among the CR-SPLIT copies, if any. If Number_of_Iterations > 1, each iteration comprises a single exposure whose duration is Time_per_Exposure.

Note that exposure time for an individual WFC exposure, after any CR-SPLIT is applied, must be an integer multiple of 0.1 second and in the range of 0.5 to 3600 sec. The value 0.6 sec. is not allowed.

Related Links

10.3 ACS Solar Blind Channel, SBC

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