Introduction

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. (For more information see Section 6.4 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.

Table 12.3: Apertures for WFC3/UVIS 

Spectral Element

See Table 12.22: Spectral Elements for use with WFC3/UVIS

Wavelength

This parameter does not apply to WFC3 observations and should be left blank.

Optional Parameters

CR-SPLIT
= 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.

If the exposure is a Spatial Scan (see Special Observation Requirements), numerical values of CR-SPLIT are not permitted.

BIN
=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.

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.

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.

Special Requirements

,,,,"">"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.

Related Links

12.3 WFC3 IR Observing Mode

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