GetWCSAdjustStep

This step calculates shifts to get good relative alignment between tiles in a mosaic. Since longer wavelengths may not have sources (and hence astrometry may fail catastrophically), we can instead defined a reference band to calculate a shift per-visit (since JWST cycles between filters), and apply that shift to all tiles in that visit. Our testing shows that taking a long NIRCam band that is ideally stellar dominated (i.e. F300M) works best for NIRCam, and a shorter wavelength, non-PAH-dominated MIRI band (i.e. F1000W) is ideal for MIRI.

By default, this step uses tweakreg, but can also be swapped out for cross-correlation via optical flow. This may be more optimal if you don’t have any point source dominated wavebands, such as if you only have F770W, for instance.

API

class pjpipe.GetWCSAdjustStep(directory, progress_dict, target, alignment_dir, procs=1, bands=None, method='tweakreg', alignment_catalogs=None, group_dithers=None, degroup_nircam_modules=False, tweakreg_parameters=None, reproject_func='interp', overwrite=False, custom_catalog_function=None, custom_catalog_function_kwargs=None)[source]

Gets a table of WCS corrections to apply to visit groups

Experience has shown that the relative JWST guide star uncertainty is very small, but there are significant absolute corrections between guide stars. Thus, we can use the same visit as a correction for all visits, for example using F1000W at F2100W where tweakreg doesn’t work so well.

Here, we take some template bands and loop over with tweakreg, writing out a table of shifts/matrices to apply to other bands. For multiple dithers etc., will take an average correction.

Alternatively, we can take a cross-correlation approach. Here, we instead make a stacked image of the dithers for each mosaic tile, then loop over and cross-correlate to get a shift. This should work better than tweakreg for bands where there aren’t many stars present, e.g. F770W.

Parameters:

  • directory – Directory of target

  • progress_dict – The progress dictionary the pipeline builds up. This is used to figure out what subdirectories we should be looking in

  • target – Target to consider

  • alignment_dir – Directory for alignment catalogs

  • bands – List of target bands to pull corrections out for

  • method – Method to align images together. Can be “tweakreg” (default) or cross_corr. N.B. for “cross_corr”, all dithers will be stacked by default to calculate the cross-correlation. This can also be a dictionary to distinguish between e.g. NIRCam and MIRI, like {‘nircam’: ‘tweakreg’, ‘miri’: ‘cross_corr’}

  • alignment_catalogs – Dictionary mapping targets to alignment catalogs

  • procs – Number of processes to run in parallel. Defaults to 1

  • group_dithers – Which band type (e.g. nircam) to group up dithers for and find a single correction. Defaults to None, which won’t group up anything

  • degroup_nircam_modules – Whether to degroup NIRCam A and B modules. Currently, the WCS is inconsistent between the two, so should probably be set to True if you see “ghosting” in the final mosaic. Defaults to False

  • tweakreg_parameters – Dictionary of parameters to pass to tweakreg. Defaults to None, which will use observatory defaults

  • reproject_func – Which reproject function to use. Defaults to ‘interp’, but can also be ‘exact’ or ‘adaptive’

  • overwrite – Whether to overwrite or not. Defaults to False

do_step()[source]

Run the WCS adjust step

get_visit_transforms(in_dir)[source]

Get visit transforms out of the shifted files

Parameters:

in_dir – Directory containing files with shifts

make_stacked_images(dithers, in_dir, out_dir, procs=1)[source]

Function to parallellise up making stacked dither images

Parameters:

  • dithers – List of dithers to go

  • in_dir – Where to find files

  • out_dir – Where to save stacked images to

  • procs – Number of simultaneous processes to run. Defaults to 1

parallel_make_stacked_image(dither, in_dir, out_dir)[source]

Light wrapper around parallelising the stacked image

Parameters:

  • dither – Dither group to consider

  • in_dir – Where to find files

  • out_dir – Where to save stacked images to

run_cross_corr(band_full, out_dir)[source]

Get transforms per-visit, using a cross-correlation between stacked dithers

Parameters:

  • band_full – Band to consider

  • out_dir – Output directory to save files with shifts to

run_tweakreg(band_full, out_dir)[source]

Run tweakreg to get shifts

Parameters:

  • band_full – Band to consider

  • out_dir – Output directory to save files with shifts to