JWST ETC NIRCam Target Acquisition
The JWST Exposure Time Calculator (ETC) has a NIRCam Target Acquisition (TA) mode which allows the user to estimate the exposure time necessary to achieve the signal to noise for the required centroid accuracy.
The JWST Near Infrared Camera (NIRCam) instrument uses Target Acquisition (TA) for its Coronagraphy, Time Series and Grism Time Series observing templates. The JWST ETC includes a Target Acquisition calculation mode for NIRCam. This mode can be used to calculate the signal-to-noise ratio (SNR) for user specified exposure parameters, and covers all three of the relevant templates. The minimum recommended integrated (within the extraction aperture) SNR is ≥ 30 for NIRCam TA which is required to obtain centroid accuracy better than 0.1 pixels.
Creating a TA calculation
The steps involved in creating a TA calculation are: (1) define the TA scene with a source having appropriate spectral type and magnitude on the Scenes and Sources page, and (2) Specify the instrument and detector setup on the Calculations page.
Defining the TA Scene and Source
Creating a calculation
Target Acquisition is one of the modes available for all JWST instruments within the ETC. To initialize a TA calculation select Target Acquisition from the NIRCam instrument drop-down menu. Once selected, the calculation will utilize a default scene and source. This can be changed at the users discretion by accessing the Configuration pane where you can select the Scene tab to change the pre-defined TA source.
The ETC supports TA for the following NIRCam observing templates: Coronagraphy, Time Series and Grism Time Series. For each mode, there are only a few choices users can make regarding the Instrument Setup. The Detector Setup is much less restricted, allowing use of all exposure patterns, but values of NGROUPS are limited to a few values ranging from 3 to 65 (the maximum is determined by limitations in the on-board data acquisition system).
Time Series and Grism Time Series Target Acquisition
Target acquisition for both Time Series and Grism Time Series observations utilizes subarrays on the LW detectors that are 32 pixels (2.16") on a side. The only filter supported is F335M. The Instrument Setup for these target acquisitions is therefore very simple, involving only a single choice in the Acq Mode menu.
Coronagraphic target acquisition
For the Coronagraphic Imaging template, the instrument configuration includes choosing the desired occulting mask from the list of 5 available masks:
- For MASK210R and MASKSWB (in the SW channel), the TA subarray is 128 pixels (4.0") square.
- For MASK335R, MASK430R and MASKLWB (in the LW channel), the TA subarray is 64 pixels (4.0") square.
The next choice is whether the target that will be used for TA is in the "bright" or "faint" regime. For "bright" targets, the source will be placed behind a neutral density square for the TA exposure; for "faint' targets the source will be imaged through a transparent region. The division between "bright" and "faint" targets depends on the target spectrum, and on whether the SW or LW channel will be used. Roughly speaking the division occurs at KVega = 6.5 (SW) and KVega = 5.0 (LW) for early type main sequence stars.
- For MASK210R and MASKSWB the filter choices are F210M (Faint) or F210M + ND square (Bright).
- For MASK335R, MASK430R and MASKLWB the filter choices are F335M (Faint) or F335M + ND square (Bright).
Target acquisition for Coronagraphic Imaging, Time Series, and Grism Time Series uses specific subarrays as noted above under Instrument Setup. The subarray sizes can not be selected by users.
Users must choose the exposure pattern to use, and the value for NGROUPS. For all TA modes the allowed selections are:
- PATTERN =
] (i.e., all NIRCam patterns are available)
- NGROUPS = ].
The maximum value of NGROUPS is limited by on-board data acquisition limitiations.
|TA Acq Mode||Coronagraphic mask||Subarray||Filter|
|Time Series or Grism Time Series †||--||SUB32 Time Series TA||F335M|
|MASK210R||SUB128 (128 × 128)|
Use for sources fainter than KVega=6.4
F210M+ND square (Bright)
|MASK335R||SUB64 (64 × 64)|
Use for Kmag fainter than KVega=4.9
|F335M+ND square (Bright)|
|MASK430R||SUB64 (64 × 64)||F335M (Faint)|
|F335M+ND square (Bright)|
|MASKSWB||SUB128 (128 × 128)||F210M (Faint)|
|F210M+ND square (Bright)|
|MASKLWB||SUB64 (64 × 64)||F335M (Faint)|
|F335M+ND square (Bright)|
† Time Series and Grism Time Series have identical configurations in the ETC.
Target Acquisition is the only offered strategy for NIRCam TA. It consists of a square aperture 9 × 9 pixels on the side, and no sky subtraction. There are no user selectable parameters. If the scene has multiple sources, the user should select the intended TA source from the Aperture centered on source drop-down menu in the Strategy tab.
Minimum recommended SNR
The exposure specification for the TA should be chosen to obtain at least the minimum recommended integrated (within the extraction aperture) SNR (~30). At lower SNR a warning will be issued, and centroid accuracy may not be adequate to fully support acquisition of the target and/or calibration of the data.
Saturation limits for target acquisition
Saturation is also a concern for TA exposures, and should be avoided if possible. Assuming an early type main sequence spectral type, the brightest targets that can be observed in the various TA modes are:
- Short wavelength Coronagraphic Imaging: KVega = -0.9
- Long wavelength Coronagraphic Imaging: KVega = -1.8
- Time Series and Grism Time Series: KVega = See "Time Series Saturation Limits" section below
Time Series Saturation Limits
For Imaging Time Series and Grism Time Series templates, targets that cause saturation in some pixels can still be used for target acquisition, depending on the centroid accuracy required for a particular observation. The NIRCam Time-Series Imaging Target Acquisition and NIRCam Grism Time-Series Target Acquisition pages, in figures 3 - 5, present the observed centroid accuracy versus KVega and number of saturated pixels. Observers are encouraged to check these plots in order to determine the limiting magnitude or number of saturated pixels which can be tolerated for their required accuracy and science case. For sources which are too bright and saturate too many pixels for the required centroid accuracy, users may be required to perform TA on a nearby offset target with a more suitable brightness.
For the particular case of full saturation (in the 1st group) of one pixel, the ETC will report an SNR of 0, but TA will still work. Approximate magnitude ranges corresponding to these cases are:
No saturation: KVega = 7.3
Partial Saturation: 6.9 < KVega< 7.2
Full Saturation: 6.5 < KVega< 6.8 (ETC reports SNR=0, but TA will still work)
The ETC will issue a "TA May Fail" if the number of saturated pixels in the centroid box exceeds one pixel. TA will still work in this case, although observers are encouraged to check figures 3-5 on the Grism Time-Series Target Acquisition or Time-Series Imaging Target Acquisition pages to see that their analysis can accept the number of ETC-reported saturated pixels.