APT Target Acquisition

Some JWST observations require a target acquisition to be specified, or include target acquisition as an option. 

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The need for separate target acquisition (TA) exposures is driven by the requirements of a particular science program as well as the particular observation template being used. Observations needing the highest positional accuracy for a target relative to an aperture or reference point require TA as part of the setup for the science observation. Other modes such as imaging do not generally require high positional accuracy to observe the relevant field of view. Between these extremes are special cases that may or may not require high positional accuracy, depending on the science goals. The table below shows the TA options for the available APT observation templates.

TA exposure information generally needs to be obtained using the JWST Exposure Time Calculator, and those APT observation templates needing TA will have a GUI section in the template for entering the TA exposure specifications. The APT GUI contains a box to enter a tracking number ([ETC workbook ID].[calculation number]) to provide tracking information, and shows a yellow caution if no entry is present.

APT template target acquisition requirements

The following Table shows the target acquisition status of the various APT observation templates as of APT 25.4.2:

Table 1. JWST target acq versus instrument observation template


TA required


No TA allowed



Image Full


TA as an option is under consideration for Cycle 2.



Default=yes, with NONE as an option.



Default=yes, with NONE as an option. 








Image SUB64P


Users should consider using the SUBARRAY_DITHER pattern to ensure there target is covered in both the short-wavelength and long-wavelength channels.



TA using the neutral-density squares for bright targets, and through clear portions of the coronagraph mount for faint targets.

Time SeriesX

Grism Time SeriesX




(See note 1 below.)



Default=yes (MSATA), but WATA, NONE and VERIFY_ONLY are other options. (See note 2 below.)



Default=yes (WATA), but MSATA, NONE and VERIFY_ONLY are other options. (See note 2 below.)



Default=yes (WATA), but MSATA and NONE are other options.

Default=yes (WATA), but NONE is an option.


Image Full




Default=yes, but NONE is an option when AMI used in full-frame mode. TA is strongly recommended though.



Default=yes, but NONE is an option when SOSS used in full-frame mode. TA is strongly recommended though.



Table Notes:

1) Any NIRSpec modes that use MSATA target acquisition will not need to have TA fully defined at the time of submission, since MSATA depends on the assigned orient, which is only available after the observation is accepted and assigned a place in the Long Range Plan. See NIRSpec MOS Observing Process for details.

 2) Some NIRSpec templates offer the option to skip target acquisition. If the selected option is VERIFY_ONLY, a science pointing verification image can be specified; if the selected option is NONE, no verification image is taken. 

Calculating target acquisition exposure specifications

See also: JWST ETC Target Acquisition

For TAs to be successful, the exposures of the TA target must provide at least the SNR recommended for the given instrument and mode (refer to instrument articles) but generally not saturate the detector of interest. Because of the importance of the TA for the success of the following science observation, great care must be taken in specifying the TA exposure parameters. Thus, the use of the JWST Exposure Time Calculator for calculating TA exposure specifications is highly recommended to determine the exposure parameters to enter into APT.  As of the APT 25.4.1 release in early December 2017, the TA sections of the relevant APT templates contain a box for you to enter the ETC workbook and specific calculation ID within the workbook that was used to provide the TA exposure specifications.

Each ETC workbook created has not only a name, but a numerical designation, and this is the identifier needed. Also, each calculation within the workbook has a calculation number. So as specific example, if your ETC workbook is number 1234, and your TA calculation of interest is calculation number 9, the entry in the ETC box in APT would be 1234.9. (Note: no leading zeroes.) Lack of a relevant entry in this box results is an APT warning. You should take care, however, not to update the indicated calculation in your ETC workbook after you enter the info in APT or it will no longer be representative of what you have entered in APT.

Adding ETC info to APT

 NOTE: ETC workbook and calculation ID info in this same format can optionally also be added to science exposure specifications. You may find this a convenient way to track for future reference which ETC calculations you used to determine the exposure specifications you enter into APT.



Latest updates

  • Updated for APT 25.4.2

    Updated for APT 25.4.1