12.3 WFC3 IR Observing Mode

Observations with HST's WFC3 instrument can be done with the IR detector, and the Astronomer’s Proposal Tool (APT) has parameters for specifying these observations.

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Format definitions

Boldface type indicates the name of an APT parameter or a value for a parameter.

(red star) 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 infrared data, and that is the IR.

Mode = MULTIACCUM Config = WFC3/IR

MULTIACCUM is the only observing mode for the IR channel. An exposure in MULTIACCUM mode begins with an array reset followed by an initial readout. Next, one or more nondestructive readouts are obtained at user-selectable times. All of the readouts, including the initial readout, are recorded onboard and returned to the ground for analysis. The difference between each successive pair of reads is the image data accumulated between reads.

There are two major advantages of this approach. First, the multiple readouts provide a way to record what is happening in a pixel before it saturates, increasing dynamic range. Second, the multiple readouts can be compared to remove cosmic ray effects. See the WFC3 Instrument Handbook for more information.

Aperture or FOV

Placement of the target on the detector is controlled by the specified Aperture, the POSition TARGet <X-value>,<Y-value> special requirement (if used), the telescope orientation (via the "ORIENTation <angle1> TO <angle2>" special requirement or  by  default), and  in  some instances the Spectral Element. The apertures for the IR channel and their valid combinations with spectral elements are defined in Table 12.4: Apertures for WFC3/IR. The current values of the aperture coordinates of the Aperture+Spectral Element combinations in Table 12.3: Apertures for WFC3/UVIS may be found on the HST Apertures Web Page.

The IR aperture is designed for placing targets at the “optimum center” of the detector. The default location within this aperture may be adjusted by STScI to reflect any changes in detector performance. This aperture is appropriate for targets that are small compared to the scale size of defects in the chips.

The IR-FIX aperture defines the geometric center of the detector and will remain FIXED in aperture coordinates. This location will not be adjusted for changes in detector characteristics, and should be used to specify the location of the target relative to the detector. This geometric center aperture is appropriate for pointings designed to position an extended scene within the WFC3 FOV.

Three apertures are provided that use the same pointing of the telescope as used for three associated UVIS apertures. Using an associated pair of apertures for UVIS and IR exposures will avoid a small angle maneuver between the exposures. The IR apertures are IR-UVIS, IR-UVIS-CENTER, and IR-UVIS-FIX, which are associated with, respectively, the apertures UVIS, UVIS-CENTER, and UVIS-FIX.

IR subarrays are specified by selecting the appropriate aperture. The IRSUBnn or IRSUBnn-FIX apertures will result in the use of the subarray readout mode of the IR detector with the size of the subarray being that indicated by the aperture name (nn = 512, 256, 128, or 64). The use of the subarray readout mode will result in different sample times than for full detector readouts listed in SAMP-SEQ=RAPID table. Not all combinations of subarray size and sample sequence are supported. See the discussion under the SAMP-SEQ Optional Parameter for more details. The subarray readouts will have a border of five reference pixels added around the edge of the subarray used for imaging, making the total data sizes 1024 × 1024 (full-frame), 74 × 74, 138 × 138, 266 × 266, and 522 × 522 pixels.

The IRSUBnn apertures will place the target at the "optimum center" of the corresponding subarray; note that these positions may be different for the different subarrays. The default position of each of these apertures will be updated by STScI to reflect changes in instrument performance. These apertures are appropriate for targets that are small compared to the scale size of defects on the detector.

The IRSUBnn-FIX apertures define the geometric center of the subarray and will remain fixed in aperture coordinates. These locations will not be adjusted for changes in detector performance.

Five apertures are specialized for use with the two IR grisms (G102 and G141) and to obtain band pass filter images for use as wavelength zero-point references. According to the FOV, the apertures are named GRISMmm, where mm = 1024 (full frame), 512, 256, 128, 64. The subarrays are the same as the IRSUBnn apertures. The fiducial pixel for each Aperture+Spectral Element combination is optimized to best position the first-order spectrum in the FOV. For GRISM1024GRISM512, and GRISM256, the same fiducial pixel is used for G102 and G141 and for reference band pass filter exposures. For GRISM128 and GRISM64 different fiducial pixels are used for G102 and G141 that best center each first-order spectrum in the FOV. The fiducial pixel for a bandpass filter exposure with those two apertures is midway between the two grism fiducial pixels.

For available but unsupported parameter (IRSUBnn)

The IR aperture is required for exposures using the G102 or G141 spectral elements. In this case the STScI ground system will substitute a special aperture which has approximately the same pointing but is optimized for use with the selected grism. If an undispersed image exposure is taken in conjunction with the grism exposure, it is recommended that the appropriate grism reference aperture (G102-REF or G141-REF) be used on the undispersed image exposure to provide an optimal pointing offset between the two exposures.

For consistency with Cycle 18 proposals, the subarray aperture + grism spectral element combinations shown in Table 12.4: Apertures for WFC3/IR also will be Available But Unsupported. However, to position the first-order spectrum within the sub-array, the POSition TARGet special requirement must be used.

Table 12.4: Apertures for WFC3/IR


Spectral Element



Fnnn1, G102, G141

Full frame, Optimum center of detector



Full frame, Geometric center of detector



Full frame, Fiducial points match paired UVIS
apertures inTable 12.3: Apertures for WFC3/UVIS


Fnnn, G102, G141

Optimum center of the 64 × 64 subarray



Geometric center of the 64 × 64 subarray


Fnnn, G102, G141

Optimum center of the 128 × 128 subarray



Geometric center of the 128 × 128 subarray


Fnnn, G102, G141

Optimum center of the 256 × 256 subarray



Geometric center of the 256 × 256 subarray


Fnnn, G102, G141

Optimum center of the 512 × 512 subarray



Geometric center of the 512 × 512 subarray


G102, G141, Fnnn

Full frame G102 or G141 spectra


G102, G141, Fnnn

512 × 512 subarray for grism spectra,
or grism reference


G102, G141, Fnnn

256 × 256 subarray for grism spectra,
or grism reference


G102, G141, Fnnn

128 × 128 subarray for grism spectra,
or grism reference


G102, G141, Fnnn

64 × 64 subarray for grism spectra,
or grism reference



G102 reference aperture for undispersed exposures



G141 reference aperture for undispersed exposures

1 Fnnn’ denotes any of the band pass filters, but neither of the grisms 

Spectral Element

See 12.5.2 WFC3 IR Spectral Elements.


This parameter should be left blank.

Optional Parameters


=RAPID, SPARS5, SPARS10, SPARS25, SPARS50, SPARS100, SPARS200, STEP25, STEP50, STEP100, STEP200, STEP400, MIF600, MIF900, MIF1200, MIF1500

A required parameter specifying the name of a predefined sequence of times from the start of the exposure at which the nondestructive readouts (samples) are performed. The structure and purpose of each class of sequence (RAPID, STEP, SPARS, MIF) is described in the paragraphs below. SPARS5 was introduced in Cycle 23. The number of readouts (up to 15, plus one for the initial readout) taken for each exposure is controlled by the NSAMP parameter (see below).

SAMP-SEQ=RAPID gives the sample times (defined as the time from the start of the initial readout to the start of a given readout) for each sequence and image size. Different types of sequences are provided. The RAPID sequence provides linear sampling as fast as possible (limited by the readout time for the selected image size) and is intended for bright targets that could saturate in the other sample sequences. All sample sequences with the full detector apertures are supported. But note that only a limited number of combinations of subarray size and sample sequence are supported.

Sequences STEP25, STEP50, STEP100, STEP200, and STEP400 begin with four rapid samples (five readouts), switch to logarithmic spacing up to the given number of seconds (25-400), and then continue with linear spacing for the remainder of the sequence with adjacent steps separated by 25-400 seconds depending on the selected sequence. These sequences are intended to compensate for any nonlinearities near the start of the exposure and to provide increased dynamic range for images that contain both faint and bright targets.

Sequences SPARS5, SPARS10, SPARS25, SPARS50, SPARS100, and SPARS200 begin with one rapid sample (two readouts) then provide linear spacing to allow observers to "read up the ramp" at evenly spaced intervals. The variety of sampling intervals allows this basic strategy to be applied over a wide range in target flux. 

For available but unsupported sequences

Sequences MIF600, MIF900, MIF1200, and MIF1500 provide pseudo-Fowler sampling (described in the WFC3 Instrument Handbook) of the signal, using total exposure times of 600 seconds through 1500 seconds in 300 second increments. They start with a series of rapid readouts (7 reads, but the initial read is to be ignored in processing due to potential reset anomalies), followed by three widely spaced readouts, and finish with six rapid readouts at the end of the exposure. The six initial useful readouts and the six final readouts are to be averaged then differenced to form the final difference image. The middle reads are provided for cosmic ray and saturation detection. These sequences should always be specified with NSAMP = 15.

The different sequences are designed to efficiently fill the orbital visibility period with one, two, or several exposures. See the WFC3 Instrument Handbook for recommendations on which sequences to use in different situations.


A required parameter specifying the number of samples in a predefined sequence that should actually be taken, not counting the initial readout. SAMP-SEQ=RAPID defines 15 sample times for each sequence. If an NSAMP value smaller than 15 is used, samples will be taken at only the first NSAMP times from this table.If an NSAMP value smaller than 5 is used, the flux determined by "up the ramp" fitting will be less reliable.

The total number of readouts will be NSAMP plus one (for the initial readout), giving a maximum of 16 readouts for a single execution of a MULTIACCUM exposure. Each readout will be recorded and will appear in the final data set.

For restricted parameters (Gain)

= 2.0, 2.5(default), 3.0, 4.0 (e/DN)

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

Number of Iterations

Enter the number of times this exposure should be iterated. This option should be used in observational situations when two or more identical exposures should be taken of the same field. If the Number_Of_Iterations is n, the exposure will be iterated n times.

If the exposure is a Spatial Scan (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. This will sacrifice orbital visibility time. Consider alternating the Scan_Direction instead.

Time Per Exposure

Time_Per_Exposure must be DEF in this Mode. The exposure time is unnecessary, because it is specified by SAMPSEQ and NSAMP.

Table 12.5: Predefined Sample Sequences for MULTIACCUM Mode. provides a link to the individual MULTIACCUM mode tables. The tables show the sequence of 15 sample times corresponding to the different SAMP-SEQ values, in seconds from the start of the initial readout to the start of the readout for the given sample. These values are given to the nearest millisecond. 

Table 12.5: Predefined Sample Sequences for MULTIACCUM Mode.

For available but unsupported MULTIACCUM mode tables

Predefined Sample Sequences for MULTIACCUM Mode

Special Requirements

See Special Observations Requirements SPATIAL SCAN  for information on executing an exposure as a Spatial Scan.

Special requirement SAME POSition AS <exposure> 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 requirements SAME ALIGNMENT and PARallel WITH are 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 mode parameters (SAA CONTOUR)


This is Available-but-Unsupported for WFC3/IR MULTIACCUM.

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