Description

Class:

jwst.targ_centroid.TargCentroidStep

Alias:

targ_centroid

The targ_centroid step determines the location of a point source in the detector frame using the target acquisition (TA) verification image. The measured source position is used by downstream calibration steps, such as pathloss, to apply corrections that account for source positioning within the aperture.

This step is applicable to MIRI Low-Resolution Spectrometer (LRS) observations where a TA verification image is available. Pre-launch assumptions that telescope pointing information would be sufficient for accurate pathloss corrections have been superseded by in-flight experience, which demonstrates the need to use actual TA verification images when available.

Upon successful completion of this step, the status keyword S_TACNTR is set to “COMPLETE”, the position of the source in the TA verification is stored in the model’s ta_xpos and ta_ypos attributes (FITS keywords TA_XPOS and TA_YPOS), and the expected source position in the science exposure, applying any necessary filter offset, is stored in the model’s source_xpos and source_ypos attributes (FITS keywords SRCXPOS and SRCYPOS).

When run as part of the calwebb_spec2 pipeline, the step is executed after the srctype step.

Step Inputs

The targ_centroid step can accept either:

  • A single science exposure (ImageModel) along with a separate TA verification image file specified via the ta_file parameter, or

  • An association file that includes both the science exposure and the TA verification image as separate members.

If no TA verification image is found in either location, the step will be skipped. If the ta_file parameter is provided and an association with a TA verification image is also provided, the file specified by ta_file will take precedence.

Algorithm

The targ_centroid step performs the following operations:

  1. Load reference files: The step retrieves the following reference files from CRDS:

    • FILTEROFFSET: Provides column and row offsets specific to the observation filter

  2. Assign WCS to TA verification image: If the TA verification image does not already have a WCS assigned, the step invokes the assign_wcs step to compute and attach the WCS to the TA verification image data model.

  3. Determine reference position: The step computes the expected source location based on the TA verification image WCS and metadata.

  4. Extract cutout: A cutout of the TA verification image is extracted, centered on the expected source position.

  5. Find the centroid: The centroid of the source within the cutout is determined using the centroid_2dg() method, which fits the source with a 2D Gaussian. This yields the fitted x and y coordinates of the source in cutout pixel coordinates.

  6. Store TA position: The measured source position in the TA verification image is stored in the input data model’s ta_xpos and ta_ypos attributes for reference.

  7. Transform coordinates: The fitted position is transformed from TA verification image cutout coordinates to science image coordinates, accounting for any subarray offsets and/or dither offsets.

  8. Apply filter offsets: The FILTEROFFSET reference file corrections are applied to the final x and y position.

  9. Store source position: The corrected source position is stored in the output data model’s source_xpos and source_ypos attributes for use by subsequent calibration steps.

Step Outputs

The input science exposure is returned unmodified, except with new metadata attributes:

  • ta_xpos: The measured x-coordinate of the source in the TA verification image (0-indexed pixels)

  • ta_ypos: The measured y-coordinate of the source in the TA verification image (0-indexed pixels)

  • source_xpos: The x-coordinate of the source in the science detector coordinate system (0-indexed pixels).

  • source_ypos: The y-coordinate of the source in the science detector coordinate system (0-indexed pixels).

  • meta.cal_step.targ_centroid keyword set to “COMPLETE”