12.2 WFC3 UVIS Observing Mode
Observations with HST's WFC3 instrument can be done with the UVIS detector, and the Astronomer’s Proposal Tool (APT) has parameters for specifying these observations.
On This Page
Boldface type indicates the name of an APT parameter or a value for a parameter.
Black text indicates an important note.
Magenta text indicates available but unsupported parameters (requires prior approval from STScI).
Red text indicates restricted parameters (for STScI use only).
Brown text indicates text file parameters.
Items in brackets - <value> - are required values.
Items in square brackets - [<value>] - are optional.
There is one detector on WFC3 that can be used to obtain ultraviolet and optical data, and that is the UVIS.
Mode = ACCUM Config = WFC3/UVIS
ACCUM is the only observing mode for the UVIS channel. Photons are detected in the WFC3 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. Detector dimensions are specified as the number of columns by the number of rows (X by Y dimensions in the pipeline images). Parallel readout occurs along columns, then serial readout occurs along a row. (see Figure 6.16 in the WFC3 Instrument Handbook.) A full detector readout is 4206 columns by 4140 rows, which includes 110 columns of serial overscan (of which 50 are physical and 60 are virtual) and 38 rows of parallel overscan (all virtual). The light sensitive (imaging) area of the detector is 4096 × 4102 pixels (columns × rows). Subarrays are comprised only of the physical pixels, which are contained within a 4146 × 4102 pixel region. Subarrays may contain light sensitive pixels and physical overscan pixels, but they do not contain virtual pixels.
Aperture or FOV
Observers determine the placement of the FOV of a WFC3 image on a target by specifying the target coordinates, the WFC3 fiducial point, and the WFC3 detector readout, and the telescope orientation (via the ORIENT special requirement, or by default). HST will be pointed so that the target is imaged at the fiducial point, plus any displacement from that point, which is specified by the observer with the POSition TARGet <X-value>,<Y-value> special requirement. Each WFC3 Aperture has a predefined fiducial point (see Table 12.3: Apertures for WFC3/UVIS), or if a quadrant filter or grism spectral element is used, the fiducial point is determined from the combination of aperture and spectral element in use. The full detector is read out unless the aperture name ends in "SUB" (or if a subarray is specified via optional parameters).
With regard to fiducial points, two types of apertures are defined in Table 12.3: Apertures for WFC3/UVIS. The first type is designed for placing targets at the "optimum location" of a region on the detector: either the entire 4096 × 4102 pixel array (UVIS, UVIS-CENTER), one of the two physical 4096 × 2051 CCD chips (UVIS1, UVIS2), one of the 2048 × 2051 quadrants of the detector (UVIS-QUAD), a region near a read-out amplifier (an aperture with the suffix -CTE), or within a predefined subarray (an aperture with the suffix -SUB). The default location within these apertures will be routinely adjusted by STScI to reflect any changes in CCD performance (e.g., new charge transfer traps, bad columns, etc.). These apertures are appropriate for targets that are small compared to the scale size of defects in the chips.
The second set of apertures defines the “geometric center” of the region and will remain fixed in aperture coordinates. These will not be adjusted for changes in CCD characteristics. These apertures are designated with the suffix -FIX, and should be used to specify the location of the target relative to the CCDs. These “geometric center” apertures are appropriate for pointings designed to position an extended scene within the WFC3 FOV. For UVIS-FIX, the “geometric center” is on CCD chip 1, ~10 arcseconds above the gap between the two chips. The fiducial point of the UVIS-IR-FIX aperture is the same as that of the IR-FIX aperture. Switching between the UVIS and IR channels using those two apertures will not cause HST to repoint. The fiducial points of the UVIS1-FIX and UVIS2-FIX apertures are at the centers of those chips.
The UVIS aperture is required for exposures using the G280 spectral element. In this case the STScI ground system will substitute a special aperture that has approximately the same pointing, but is optimized for use with the grism. An undispersed (i.e., bandpass filter) image exposure should be taken in conjunction with the grism exposure using the G280-REF aperture to enable measurement of the grism exposure wavelength zero-point.
The UVIS-QUAD, UVIS-QUAD-FIX, and UVIS-QUAD-SUB apertures are allowed only with one of the quadrant filters (see Table 12.22: Spectral Elements for use with WFC3/UVIS), and one of these apertures must be specified if a quadrant filter is used. The choice of aperture only affects the telescope pointing; it does not restrict the area of the detector that is read out, except for UVIS-QUAD-SUB which will read out only the quadrant of the detector corresponding to the filter specified. UVIS-QUAD and UVIS-QUAD-FIX refer to the full array unless Optional Parameters describing user-defined subarrays are specified.
Apertures with sub-array readouts are provided in order to reduce operational overhead time when imaging targets that do not require the full FOV of the UVIS channel. 2K × 2K apertures for each of the four quadrants are provided (UVIS1-2K2A-SUB, UVIS1-2K2B-SUB, UVIS2-2K2C-SUB, and UVIS2-2K2D-SUB). For the Amplifier C quadrant, 1K × 1K and 512 × 512 subarray apertures are provided both near the center of the FOV (UVIS2-M1K1C-SUB and UVIS2-M512C-SUB), and near the amplifier (UVIS2-C1K1C-SUB and UVIS2-C512C-SUB).
Full-frame apertures (UVIS2-C1K1C-CTE and UVIS2-C512C-CTE) are provided for placement of targets nearer to the C readout amplifier (same reference positions as the UVIS2-C1K1C-SUB and UVIS2-C512C-SUB apertures, respectively) to allow full-frame readout with better CTE performance than for targets placed at the reference positions of other full-frame apertures.
For available but unsupported
Additional sub-array apertures for CCD chip 1 are available. They are the full-chip aperture UVIS1-2K4-SUB, the 512 × 512 sub-arrays in the corners (UVIS1-C512A-SUB and UVIS1-C512B-SUB), and the 512 × 512 sub-array near the middle of the UVIS FOV, UVIS1-M512-SUB. These apertures are intended to support the WFC3 calibration program.
Additional apertures on CCD chip 2 are provided to support the WFC3 calibration program. They are the full-chip aperture UVIS2-2K4-SUB, the 512 × 512 sub-array in a corner UVIS2-C512D-SUB, and a 512 × 512 sub-array near the middle of the UVIS FOV, UVIS2-M512-SUB.
The UVIS aperture is required for ACCUM exposures that specify optional parameter CTE = EPER.
Table 12.3: Apertures for WFC3/UVIS
Two-CCD mosaic with the reference point at a targetable
location near the geometric center (Optimum Center).
Initial version of UVIS that remains fixed even if UVIS is modified later. Geometric center of full two-CCD FOV.
Two-CCD mosaic with the reference point at the "center" of a distortion corrected view. This point is equidistant from each pair of opposite corners and falls on UVIS2 close to the gap. Useful for designing mosaics, especially if combining images with different orientations.
Optimum center of CCD 1
Geometric center of CCD 1
Optimum center of CCD 2
Geometric center of CCD 2
Same fiducial point as IR-FIX in Table 12.4: Apertures for WFC3/IR
Grism reference aperture for undispersed exposures
Optimum center of quadrant corresponding to selected quadrant filter (offset from the center of the quadrant toward the nearest corner of the detector by about 8 to 10 arcsec in X and in Y)
Geometric center of quadrant corresponding to selected quadrant filter
Full-frame with target at same reference position as UVIS2-C1K1C-SUB (see below); for better CTE performance
Full-frame with target at same reference position as UVIS2-C512C-SUB (see below); for better CTE performance
Optimum center of the quadrant corresponding to the selected quadrant filter, with a 2072 × 2050 subarray (2072 includes 23 pixels of the physical overscan and 2049 image pixels) to read out approximately on that quadrant
Optimum center of CCD 1 using a 4142 × 2050 subarray (4142 includes 46 pixels of physical overscan)
Optimum center of CCD 2 using a 4142 × 2050 subarray (4142 includes 46 pixels of physical overscan)
512 × 512 subarray on CCD 1 near the center of UVIS FOV, for best image quality
512 × 512 subarray on CCD 2 near the center of UVIS FOV, for best image quality
2048 × 2050 full quadrant subarrays (optimum center - same center as UVIS-QUAD for matching N,M,W filter exposures to quadrant filter exposures)
1024 × 1024 subarray, quadrant C near detector center (optimum center)
1024 × 1024 subarray, near amp C (optimum center)
512 × 512 subarray on CCD 2 near the center of UVIS FOV; for best image quality
536 × 512 subarray (536 includes 23 pixels of physical overscan and 513 image pixels) located in the corner of CCD 1 near either amp A or B (optimum center)
536 × 512 subarray (536 includes 23 pixels of physical overscan) located in the corner of CCD 2 near amp C (optimum center)
536 × 512 subarray (536 includes 23 pixels of physical overscan and 513 image pixels) located in the corner of CCD 2 near amp D (optimum center)
See Table 12.22: Spectral Elements for use with WFC3/UVIS
This parameter does not apply to WFC3 observations and should be left blank.
= 2 - 8, NO (default)
Specifies the number of sub-exposures into which the original exposure is to be split for the purpose of cosmic ray elimination in post-observation data processing (see the WFC3 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. The use of CR-SPLIT is not recommended in general. Consider dithering rather than repeating identical pointings.
For available but unsupported (BIAS)
If the target is BIAS, DARK, DARK-NM, TUNGSTEN, or DEUTERIUM, then the only legal value for CR-SPLIT is NO. If CR-SPLIT is not specified, an implicit CR-SPLIT=NO will be applied to the exposure.
If the exposure is a Spatial Scan (see Special Observation Requirements), numerical values of CR-SPLIT are not permitted.
=NONE (default), 2, 3 (pixels)
Specifies the number of CCD pixels in each dimension that are binned to a single signal value as the detector is read out. If the value NONE is specified, or the optional parameter is not provided, the exposure will be read out unbinned. A value of 2 produces 2 × 2 binning; a value of 3 produces 3 × 3 binning. See the discussion of binning in the WFC3 Instrument Handbook.
BIN=2 or 3 are not permitted in conjunction with the CCD subarray parameters SIZEAXIS2 or SIZEAXIS1 or with any subarray aperture (-SUB). BIN=2 or 3 are not permitted in conjunction with CTE=EPER.
For available but unsupported
Subarrays are specified by a user in the AXIS frame, which is an image-based coordinate system defined in the WFC3 Instrument Handbook, Chapter 6 and shown in Figure 12.1: UVIS Aperture Diagram. The AXIS1 coordinate varies along rows of the detector and AXIS2 varies along detector columns. Restrictions on which of the physical pixels (i.e., rows and columns) may be included in a user-specified subarray are also presented in Chapter 6 of the WFC3 Instrument Handbook. With the Optional Parameters listed below, a user specifies the lengths of the rectangular sides of the subarray and the position of center of the subarray, both of which have units of pixels.
SIZEAXIS2 = FULL (default), 16 - 2050 (pixels)
Specifies the number of pixels of a UVIS subarray along the AXIS2 direction (i.e., along a column). Care must be taken in specifying a UVIS detector subarray to ensure that the subarray is fully contained on one of the two CCD chips that comprise the UVIS detector. SIZEAXIS2 may not be specified if the Aperture is a subarray aperture (-SUB).
The default value of FULL will result in a full-detector readout of 4206 × 4140 pixels (which includes the various overscan pixels), unless Optional Parameter SIZEAXIS1 is not FULL, in which case the readout will be SIZEAXIS1 × 2050 pixels. Subarrays will be centered at the reference position of the Aperture used for the exposure, unless Optional Parameter CENTERAXIS2 is used to specify the center of the subarray in the AXIS2 direction. Note that for all subarrays, the target will be placed on the detector at the Aperture reference position, plus any POSition TARGet offset specified on the exposure.
SIZEAXIS1= FULL (default), 16 - 4142 (pixels)
Specifies the number of pixels of a UVIS subarray along the AXIS1 direction (i.e., along a row). The numerical value, if specified, must be an even number. Care must be taken in specifying a UVIS detector subarray to ensure that the subarray is fully contained on one of the two CCD chips that comprise the UVIS detector. SIZEAXIS1 may not be specified if the Aperture is a subarray aperture (-SUB).
The default value of FULL will result in a full-detector readout of 4206 × 4140 pixels (which includes the various overscan pixels), unless Optional Parameter SIZEAXIS2 is not FULL, in which case the readout will be 4142 × SIZEAXIS2 pixels. Subarrays will be centered at the reference position of the Aperture used for the exposure, unless Optional Parameter CENTERAXIS1 is used to specify the center of the subarray in the AXIS1 direction. Note that for all subarrays, the target will be placed on the detector at the Aperture reference position, plus any POSition TARGet offset specified on the exposure.
CENTERAXIS2 = 9 - 2043, 2059 - 4093, TARGET
Specifies the center pixel in the AXIS2 direction (i.e., along a column). CENTERAXIS2 may be specified only if Optional Parameter SIZEAXIS2 is also specified. The default is to center the 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 position if a POSition TARGet Special Requirement is present on the exposure (see "POSition TARGet <X-value>,<Y-value>"). Note that for all subarrays, the target will be placed on the detector at the Aperture reference position, plus any POSition TARGet offset specified on the exposure. Note also that if the target is BIAS, DARK, DARK-NM, DEUTERIUM, or TUNGSTEN, CENTERAXIS2=TARGET is not allowed.
CENTERAXIS1 = 11 - 4137, TARGET
Specifies the center pixel in the AXIS1 direction (i.e., along a row). CENTERAXIS1 may be specified only if Optional Parameter SIZEAXIS1 is also specified. The default is to center the 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 position if a POSition TARGet Special Requirement is present on the exposure (see "POSition TARGet <X-value>,<Y-value>"). Note that for all subarrays, the target will be placed on the detector at the Aperture reference position, plus any POSition TARGet offset specified on the exposure. Note also that if the target is BIAS, DARK, DARK-NM, DEUTERIUM, or TUNGSTEN, CENTERAXIS1=TARGET is not allowed.
AMP = A, B, C, D, AC, AD, BC, BD, ABCD
Specifies the readout amplifier or combination to use for the exposure. If the entire detector (both CCD chips) is being read out, then the single amplifier selections are not allowed. If a subarray is being used, the value of AMP must correspond to the CCD chip containing the subarray, and only a single amplifier may be used (i.e., value must be either A, B, C, or D).
GAIN = 1.0, 1.5 (default), 2.0, 4.0 (e/DN)
Specifies the gain of the CCD electronics in e/DN.
CURRENT = LOW, MEDIUM, HIGH
Specifies the current to be used for the Deuterium lamp exposures. This optional parameter may only be used if the target is DEUTERIUM. At LOW and MEDIUM currents, the lamp may be powered on for a maximum of 255 minutes. For thermal reasons, at HIGH current, the lamp may not be on for more than 60 minutes; after any set of exposures using HIGH current, the lamp will automatically be powered off for at least 60 minutes to allow for cool down of the calibration system.
For available but unsupported and restricted
INJECT = NONE (default), YES
= CONT, LINE10, LINE17, LINE25
Specifies whether to do charge injection or not. Charge injection is used for correcting the effects of charge transfer inefficiency. (See the description of the optional parameter FLASH (below), the supported mode for reducing the effects of CTI.)
The specific mode of charge injection may be specified. The values LINE10, LINE17, and LINE25 specify the spacing of lines (rows) receiving injected charge ("discrete" charge injection); the lines receiving charge are fixed to the CCDs and do not change with the specification of subarrays. CONT specifies that every line (row) of the detector will receive injected charge ("continuous" charge injection).
BLADE = A (Available)
= B (Restricted)
Allowed only when the "Time Per Exposure" is less than 60 seconds. For short exposures, the two sides of the UVIS shutter produce notably different PSFs. Programs that can benefit from using the superior side may specify Optional Parameter BLADE = A. Approval from your Contact Scientist is required.
Engineering and calibration proposals may also specify BLADE = B.
CTE =NONE (default), EPER
Indicates that the exposure specified will be for the purpose of making a charge transfer efficiency (CTE) measurement. The value EPER is for obtaining an Extended Pixels Edge Response image. CTE measurement images must be specified with TUNGSTEN as the target, except as noted below. When CTE is specified as EPER, GAIN is the only other optional parameter that may be specified, and aperture UVIS must be specified.
Note: To obtain a bias subtraction image with a CTE timing pattern, specify
• CTE = EPER
• Target = DARK or DARK-NM (not BIAS)
• Time_Per_Exposure = 0.5 S.
FLASH =0 (default) - 40
Specifies the number of electrons per pixel to add to the image by illuminating the detector with the post-flash LED. When associated exposures are created by specifying multiple iterations, or by applying a PATTERN, or by using the optional parameter CR-SPLIT, this value applies to each member of the association. Updated advice and documentation on charge transfer efficiency (CTE) losses can be found on the CTE landing page and references therein. This guidance will also be located under current status on WFC3's main page.
If FLASH is specified, neither FLASHCUR nor FLASHEXP may be specified.
FLASHEXP = 0.0 - 409.5 (seconds; no default) (Restricted)
Specifies the "exposure time" of the LED post-flash. If FLASHEXP is specified, FLASHCUR must also be specified and FLASH may not be specified. FLASHCUR = LOW, MEDIUM, HIGH (no default) (Restricted) Specifies the LED current to use for the post-flash. If FLASHCUR is specified, FLASHEXP must also be specified and FLASH may not be specified.
Number of Iterations
Enter the number of times this exposure should be iterated, and the duration in seconds of each iteration. There are many observational situations when two or more identical exposures should be taken of the same field (e.g., to keep a bright object from blooming by keeping the exposure time short). If the Number_Of_Iterations is n, the entire exposure will be iterated n times.
The value entered for the Time_Per_Exposure is the exposure time for each iteration of the specified exposure. For instance, specifying Number_Of_Iterations = 10 and a Time_Per_Exposure of 10 seconds will produce a total exposure time of 100 seconds. This differs from the situation with a CR-SPLIT, when the total exposure time will be apportioned among shorter exposures: specifying an exposure time of 10 seconds and CR-SPLIT=2 results in two exposures of 5 seconds each.
Note: 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
Time_Per_Exposure must be an integer multiple of 0.1 second and in the range of 0.5 to 3600 sec. The value of 0.6 sec is not allowed.
If the exposure is a Spatial Scan (see Special Observations Requirements) and Number of Iterations > 1, a small slew will be inserted between the exposures so the scans will repeat the same path on the detector each time. Depending on detector setup and slew length, this may sacrifice orbital visibility time. Consider alternating the Scan_Direction instead.
For exposures that specify BIAS as the target, the exposure time must be 0.
, , , "">"SPATIAL SCAN <Scan_Rate>, <Scan_Orient>, <Scan_Direction>, <Scan_Line_Separation>, <Scan_Number_Lines>"
See Special Observations Requirements for information on executing an exposure as a Spatial Scan. Special Requirement SAME POSition AS is not permitted on and may not refer to a Spatial Scan exposure. Spatial Scan exposures are not permitted in Coordinated Parallel containers or in Pure Parallel visits.
Special requirement PARallel WITH is not permitted on and may not refer to a Spatial Scan exposure. Pure Parallel visits may not contain Spatial Scan exposures.
For Available but Unsupported
This is Available-but-Unsupported for WFC3/UVIS ACCUM.
Version Cycle 30 June 2022
Edit suggested by the WFC3 team
Version Cycle 30 April 2022
PROPINST-91384 (Document update to WFC3 op FLASH upper limit)