Moving Target Calibration and Processing

The JWST calibration pipeline has the ability to co-add moving target exposures. Many steps in the pipeline are identical for moving and fixed targets; steps specific to moving targets are outlined on this page.

On this page

See also: JWST Science Calibration Pipeline Overview

JWST's science instrument calibration pipeline will support co-addition of moving target exposures. The calibration pipeline outputs products at 3 levels, with the unique steps for moving targets outlined below for each level.

Calibration pipeline level 1

Level 1 processing includes data formatting, science frame re-orientation, and calculation of WCS keyword values. For moving targets, the WCS is expanded to include 4 moving target keywords per integration and 2 additional keywords per observation. The first unique moving target keyword is created by triggering the moving target flag in APT:  MTWCS (moving target world coordinate system).

The right ascension and declination values of the reference pixel at the mid-time of the observation are recorded with the following keywords:

  • RA_REF0 
  • DEC_REF0 

The right ascension and declination values of the reference pixel at the mid-time of each integration are recorded with the following keywords: 

  • RA_REF 
  • DEC_REF 

Then the following keywords are calculated using the standard WCS keywords CRVAL1  and CRVAL2:

  • CRVALMT1  = CRVAL1  + (RA_REF0  - RA_REF)

Dither and mosaic offsets are preserved.

Calibration pipeline level 2

Level 2 processing, or single exposure calibration processing, outputs calibrated single exposure count rate images. These steps are very similar to those for fixed targets, since the moving target is essentially "fixed" to the detector frame through telescope tracking. One unique consideration for moving targets is that the stars trail through the scene. The trails can cause transient increases in detected signal in the pixels they cross. In Cycle 1, these transient signals will be treated as cosmic rays in the ramps-to-slopes.

Calibration pipeline level 3

Level 3 processing combines the Level 2 products through co-adding exposures and mosaicking. For moving targets, co-addition of exposures occurs in the target's co-moving frame. The pipeline will accomplish this by implementing existing fixed target algorithms, and instead of the normal WCS, using the unique moving target's WCS data. Hershel and Spitzer applied this approach and proved it workable; however, it was never implemented for the HST pipeline. Both imaging and spectroscopy will be supported.

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