Description

Class:

roman.tweakreg.TweakRegStep

Alias:

tweakreg

Overview

This step uses the coordinates of point-like sources from an input catalog (i.e. the result from SourceCatalogStep saved in the meta.tweakreg_catalog attribute) and compares them with the coordinates from a Gaia catalog to compute corrections to the WCS of the input images such that sky catalogs obtained from the image catalogs using the corrected WCS will align on the sky.

Custom Source Catalogs

The default catalog used by tweakreg step can be disabled by providing a file name to a custom source catalog in the meta.tweakreg_catalog attribute of input data models. The catalog must be in a format automatically recognized by read(). The catalog must contain either 'x' and 'y' or 'x_psf' and 'y_psf' columns which indicate source image coordinates (in pixels). Pixel coordinates are 0-indexed.

For the tweakreg step to use user-provided input source catalogs, use_custom_catalogs parameter of the tweakreg step must be set to True.

In addition to setting the meta.tweakreg_catalog attribute of input data models to the custom catalog file name, the tweakreg_step also supports two other ways of supplying custom source catalogs to the step:

  1. Adding tweakreg_catalog attribute to the members of the input ASN table - see ModelLibrary for more details. Catalog file names are relative to ASN file path.

  2. Providing a simple two-column text file, specified via step’s parameter catfile, that contains input data models’ file names in the first column and the file names of the corresponding catalogs in the second column. Catalog file names are relative to catfile file path.

Specifying custom source catalogs via either the input ASN table or catfile, will update input data models’ meta.tweakreg_catalog attributes to the catalog file names provided in either in the ASN table or catfile.

Note

When custom source catalogs are provided via both catfile and ASN table members’ attributes, the catfile takes precedence and catalogs specified via ASN table are ignored altogether.

Note

  1. Providing a data model file name in the catfile and leaving the corresponding source catalog file name empty – same as setting 'tweakreg_catalog' in the ASN table to an empty string "" – would set corresponding input data model’s meta.tweakreg_catalog attribute to None. In this case, tweakreg_step will automatically generate a source catalog for that data model.

  2. If an input data model is not listed in the catfile or does not have 'tweakreg_catalog' attribute provided in the ASN table, then the catalog file name in that model’s meta.tweakreg_catalog attribute will be used. If model.meta.tweakreg_catalog is None, tweakreg_step will automatically generate a source catalog for that data model.

Alignment

The source catalog (either created by SourceCatalogStep or provided by the user) gets cross-matched and fit to an astrometric reference catalog (set by TweakRegStep.abs_refcat) and the results are stored in model.meta.wcs_fit_results. The pipeline initially supports fitting to any Gaia Data Release (defaults to GAIADR3).

An example of the content of model.meta.wcs_fit_results is as follows:

model.meta.wcs_fit_results = {
  "status": "SUCCESS",
  "fitgeom": "rshift",
  "matrix": array([[ 1.00000000e+00,  1.04301609e-13],
        [-1.04301609e-13,  1.00000000e+00]]),
  "shift": array([ 7.45523163e-11, -1.42718944e-10]),
  "center": array([-183.87997841, -119.38467775]),
  "proper_rot": 5.9760419875149846e-12,
  "proper": True,
  "rot": (5.9760419875149846e-12, 5.9760419875149846e-12),
  "<rot>": 5.9760419875149846e-12,
  "scale": (1.0, 1.0),
  "<scale>": 1.0,
  "skew": 0.0,
  "rmse": 2.854152848489525e-10,
  "mae": 2.3250544963289652e-10,
  "nmatches": 22
}

Details about most of the parameters available in model.meta.wcs_fit_results can be found on the TweakWCS’s webpage, under its linearfit module.

WCS Correction

The linear coordinate transformation computed in the previous step is used to define tangent-plane corrections that need to be applied to the GWCS pipeline in order to correct input image WCS. This correction is implemented by inserting a v2v3corr frame with tangent plane corrections into the GWCS pipeline of the image’s WCS.

Step Arguments

TweakRegStep has the following arguments:

Catalog parameters:

  • use_custom_catalogs: A boolean that indicates whether to ignore source catalog in the input data model’s meta.tweakreg_catalog attribute (Default=`False`).

    Note

    If True, the user must provide a valid custom catalog that will be assigned to meta.tweakreg_catalog and used throughout the step.

  • catalog_format: A str indicating one of the catalog output file format supported by astropy.table.Table (Default=’ascii.ecsv’).

    Note

    • This option must be provided whenever use_custom_catalogs = True.

    • The full list of supported formats can be found on the astropy’s Built-In Table Readers/Writers webpage.

  • catfile: Name of the file with a list of custom user-provided catalogs (Default=’’).

    Note

    • This option must be provided whenever use_custom_catalogs = True.

  • catalog_path: A str indicating the catalogs output file path (Default=’’).

    Note

    All catalogs will be saved to this path. The default value is the current working directory.

Reference Catalog parameters:

  • expand_refcat: A boolean indicating whether or not to expand reference catalog with new sources from other input images that have been already aligned to the reference image (Default=False).

Object matching parameters:

  • minobj: A positive int indicating minimum number of objects acceptable for matching (Default=15).

  • searchrad: A float indicating the search radius in arcsec for a match (Default=2.0).

  • use2dhist: A boolean indicating whether to use 2D histogram to find initial offset (Default=True).

  • separation: Minimum object separation in arcsec (Default=1.0).

  • tolerance: Matching tolerance for xyxymatch in arcsec (Default=0.7).

Catalog fitting parameters:

  • fitgeometry: A str value indicating the type of affine transformation to be considered when fitting catalogs. Allowed values:

    • 'shift': x/y shifts only

    • 'rshift': rotation and shifts

    • 'rscale': rotation, shifts, and scale

    • 'general': rotation, shifts, scale, and skew

    The default value is “rshift”.

    Note

    Mathematically, alignment of images observed in different tangent planes requires fitgeometry='general' in order to fit source catalogs in the different images even if misalignment is caused only by a shift or rotation in the tangent plane of one of the images.

    However, under certain circumstances, such as small alignment errors or minimal dithering during observations that keep tangent planes of the images to be aligned almost parallel, then it may be more robust to use a fitgeometry setting with fewer degrees of freedom such as 'rshift', especially for “ill-conditioned” source catalogs such as catalogs with very few sources, or large errors in source positions, or sources placed along a line or bunched in a corner of the image (not spread across/covering the entire image).

  • nclip: A non-negative integer number of clipping iterations to use in the fit (Default = 3).

  • sigma: A positive float indicating the clipping limit, in sigma units, used when performing fit (Default=3.0).

Absolute Astrometric fitting parameters:

Parameters used for absolute astrometry to a reference catalog.

  • abs_refcat: String indicating what astrometric catalog should be used. Currently supported options are (Default=’GAIADR3’): 'GAIADR1', 'GAIADR2', or 'GAIADR3'.

    Note

    If None or an empty string is passed in, TweakRegStep will use the default catalog as set by tweakreg_step.DEFAULT_ABS_REFCAT.

  • abs_minobj: A positive int indicating minimum number of objects acceptable for matching. (Default=15).

  • abs_searchrad: A float indicating the search radius in arcsec for a match. It is recommended that a value larger than searchrad be used for this parameter (e.g. 3 times larger) (Default=6.0).

  • abs_use2dhist: A boolean indicating whether to use 2D histogram to find initial offset. It is strongly recommended setting this parameter to True. Otherwise the initial guess for the offsets will be set to zero (Default=True).

  • abs_separation: Minimum object separation in arcsec. It is recommended that a value smaller than separation be used for this parameter (e.g. 10 times smaller) (Default=0.1).

  • abs_tolerance: Matching tolerance for xyxymatch in arcsec (Default=0.7).

  • abs_fitgeometry: A str value indicating the type of affine transformation to be considered when fitting catalogs. Allowed values:

    • 'shift': x/y shifts only

    • 'rshift': rotation and shifts

    • 'rscale': rotation and scale

    • 'general': shift, rotation, and scale

    The default value is “rshift”. Note that the same conditions/restrictions that apply to fitgeometry also apply to abs_fitgeometry.

  • abs_nclip: A non-negative integer number of clipping iterations to use in the fit (Default = 3).

  • abs_sigma: A positive float indicating the clipping limit, in sigma units, used when performing fit (Default=3.0).

  • save_abs_catalog: A boolean specifying whether or not to write out the astrometric catalog used for the fit as a separate product (Default=False).

Further Documentation

The underlying algorithms as well as formats of source catalogs are described in more detail on the TweakWCS webpage.