# UPDATEHDR: Functions for Updating WCS with New Solutions

The functions in this module support updating the WCS information in distorted images with the alignment solution determined by `tweakreg` or saved in a shiftfile.

Authors: | Warren Hack, Mihai Cara
License: | http://www.stsci.edu/resources/software_hardware/pyraf/LICENSE

`drizzlepac.updatehdr.``create_unique_wcsname`(fimg, extnum, wcsname)

  This function evaluates whether the specified wcsname value has already been used in this image. If so, it automatically modifies the name with a simple version ID using wcsname_NNN format.

  Parameters: | fimg : obj PyFITS object of image with WCS information to be updated extnum : int Index of extension with WCS information to be updated wcsname : str Value of WCSNAME specified by user for labelling the new WCS
  Returns: | uniqname : str Unique WCSNAME value

`drizzlepac.updatehdr.``update_from_shiftfile`(shiftfile, wcsname=None, force=False)

  Update headers of all images specified in shiftfile with shifts from shiftfile.

  Parameters: | shiftfile : str Filename of shiftfile. wcsname : str Label to give to new WCS solution being created by this fit. If a value of None is given, it will automatically use ‘TWEAK’ as the label. [Default =None] force : bool Update header even though WCS already exists with this solution or wcsname? [Default=False]

`drizzlepac.updatehdr.``update_wcs`(image, extnum, new_wcs, wcsname='', reusename=False, verbose=False)

  Updates the WCS of the specified extension number with the new WCS after archiving the original WCS.

  The value of ‘new_wcs’ needs to be the full HSTWCS object.

  Parameters: | image : str Filename of image with WCS that needs to be updated extnum : int Extension number for extension with WCS to be updated/replaced new_wcs : object Full HSTWCS object which will replace/update the existing WCS wcsname : str Label to give newly updated WCS reusename : bool User can choose whether to over-write WCS with same name or not. [Default: False] verbose : bool, int Print extra messages during processing? [Default: False]

`drizzlepac.updatehdr.``updatewcs_with_shift`(image, reference, wcsname=None, reusename=False, fitgeom='rscale', rot=0.0, scale=1.0, xsh=0.0, ysh=0.0, fit=None, xrms=None, yrms=None, verbose=False, force=False, sciext='SCI')

  Update the SCI headers in ‘image’ based on the fit provided as determined in the WCS specified by ‘reference’. The fit should be a 2-D matrix as generated for use with ‘make_vector_plot()’.

  Parameters: | image : str or PyFITS.HDUList object Filename, or PyFITS object, of image with WCS to be updated. All extensions with EXTNAME matches the value of the ‘sciext’ parameter value (by default, all ‘SCI’ extensions) will be updated. reference : str Filename of image/headerlet (FITS file) which contains the WCS used to define the tangent plane in which all the fit parameters (shift, rot, scale) were measured. wcsname : str Label to give to new WCS solution being created by this fit. If a value of None is given, it will automatically use ‘TWEAK’ as the label. If a WCS has a name with this specific value, the code will automatically append a version ID using the format ‘_n’, such as ‘TWEAK_1’, ‘TWEAK_2’,or ‘TWEAK_update_1’. [Default =None] reusename : bool User can specify whether or not to over-write WCS with same name. [Default: False] rot : float Amount of rotation measured in fit to be applied. [Default=0.0] scale : float Amount of scale change measured in fit to be applied. [Default=1.0] xsh : float Offset in X pixels from defined tangent plane to be applied to image. [Default=0.0] ysh : float Offset in Y pixels from defined tangent plane to be applied to image. [Default=0.0] fit : arr Linear coefficients for fit [Default = None] xrms : float RMS of fit in RA (in decimal degrees) that will be recorded as CRDER1 in WCS and header [Default = None] yrms : float RMS of fit in Dec (in decimal degrees) that will be recorded as CRDER2 in WCS and header [Default = None] verbose : bool Print extra messages during processing? [Default=False] force : bool Update header even though WCS already exists with this solution or wcsname? [Default=False] sciext : string Value of FITS EXTNAME keyword for extensions with WCS headers to be updated with the fit values. [Default=’SCI’]

  Notes

  The algorithm used to apply the provided fit solution to the image involves applying the following steps to the WCS of each of the input image’s chips:

  1. compute RA/Dec with full distortion correction for
       reference point as (Rc_i,Dc_i)
  2. find the Xc,Yc for each Rc_i,Dc_i and get the difference from the
       CRPIX position for the reference WCS as (dXc_i,dYc_i)
  3. apply fit (rot&scale) to (dXc_i,dYc_i) then apply shift, then add
       CRPIX back to get new (Xcs_i,Ycs_i) position
  4. compute (Rcs_i,Dcs_i) as the sky coordinates for (Xcs_i,Ycs_i)
  5. compute delta of (Rcs_i-Rc_i, Dcs_i-Dcs_i) as (dRcs_i,dDcs_i)
  6. apply the fit to the chip’s undistorted CD matrix, the apply linear
       distortion terms back in to create a new CD matrix
  7. add (dRcs_i,dDcs_i) to CRVAL of the reference chip’s WCS
  8. update header with new WCS values