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.
JWST's science instrument calibration pipeline will support co-addition of moving target exposures. The calibration pipeline outputs products at 3 stages, with the unique steps for moving targets outlined below for each stage.
Calibration pipeline stage 1
Moving target header keywords
As for all JWST science data, stage 1 processing includes data formatting, science frame re-orientation, calculation of WCS keyword values, and "ramps-to-slopes" fitting. For moving target data, additional WCS entries are created to enable co-addition of data in the co-moving frame of the target. These keywords are created at the exposure level in stage 1 (these are carried over to the stage 2 products), and at the observation level in stage 3. See the stage 3 discussion below for further details about the header keywords for those products.
The right ascension and declination values of the target pixel at the mid-time of each exposure (stage 1 and 2 products) are recorded with the following keywords present in the SCI header:
Note that there are no comparable keywords giving the target position at the mid-time of each integration within an exposure at a given dither position. Such information can be reconstructed by retrieving the "uncal" products and accessing the "MOVING_TARGET_POSITION" table extension, and the
MT_APPARENT_DEC values. That extension also contains the heliocentric and observer-centric distances as a function of time during the exposure, along with the X, Y, and Z positions of the target and JWST.
Also, if multiple exposures are specified with the same spectral elements, the stage 1 and stage 2 products will have different values of
MT_DEC for the separate exposures.
It is worth noting that dither and mosaic offsets are preserved by a correction applied to the
CRVAL keywords in the headers of moving target products. In this case the standard keyword values are replaced by values that are corrected for the target motion during the exposure (or observation, at stage 3). In this case the RA and DEC of the reference pixel at the exposure (or observation) mid-time,
DEC_REF0, are calculated, and new values for
CRVAL2 calculated as:
Rejection of signal from fixed sources in moving target observations
Another unique consideration for moving targets is that the stars and other fixed objects trail through the scene. Due to the small pixels of the JWST instruments and PSF, fixed objects can cross one or many pixels during a single integration ramp, causing transient count rate increases or decreases as they enter and leave specific pixels. These transient signals are treated as cosmic rays in the "jump-detection" step (see Figure 1).
Calibration pipeline stage 2
As with fixed target observations, stage 2 processing, or single-exposure calibration processing, outputs calibrated single exposure images (e.g., cal, i2d). These steps are identical to those for fixed targets, since the moving target is "fixed" to the detector frame through telescope tracking.
For some applications users may be interested in stage 2 products for each integration within an exposure. This may be the case for observations containing multiple targets or features of interest that are moving at different rates (e.g., cloud features on Jupiter and/or Jupiter's satellites). At this time users have to generate these products off-line by installing the JWST pipeline package and processing the "rateints" files to produce "calints" files.
Calibration pipeline stage 3
There are similar updates to the
CRVAL values in the stage 3 headers, such that the final image is centered on the target location at the mid-time of the observation.
Stage 3 processing also includes an outlier detection step with a rejection threshold that may be too aggressive for some moving target observations. This is especially the case for observations with few dithers and/or complex scenes; observations of point-like sources, or observations with a large number of dithers, are less likely to be affected. For observations that are subject to this effect, the stage 3 products will have otherwise acceptable pixels rejected, resulting in those pixel values being replaced with 0 or NaN (see Figure 2). For this reason, it is highly recommended to make use of the stage 2 products or to run the pipeline on your local machine with the rejection threshold set to a higher sigma threshold.
The stage 3 outlier rejection step will also further erode signal from trailed stars in moving target observations (and of moving targets in fixed target observations).