pixtosky: Coordinate transformation to sky coordinates

This task allows a user to perform coordinate transformations with the full WCS and distortion model on source positions from an input image to sky coordinates. This task serves as a replacement for the IRAF.STSDAS task xy2rd, albeit with the added capability of understanding the full distortion model provided in the headers of images updated for use with astrodrizzle and tweakreg.

pixtosky transforms pixel positions given as X,Y values into positions on the sky based on the WCS information and any recognized distortion keywords from the input image header.

 Warren Hack


input : str
full filename with path of input image, an extension name [‘sci’,1] should be provided if input is a multi-extension FITS file.

Optional Parameters

x : float, optional
X position from input image
y : float, optional
Y position from input image
coords : str, deprecated
[DEPRECATED] full filename with path of file with x,y coordinates Filename given here will be ignored if a file has been specified in coordfile parameter.
coordfile : str, optional
full filename with path of file with x,y coordinates
colnames : str, optional
comma separated list of column names from ‘coords’ files containing x,y coordinates, respectively. Will default to first two columns if None are specified. Column names for ASCII files will use ‘c1’,’c2’,... convention.
separator : str, optional
non-blank separator used as the column delimiter in the coords file
hms : bool, optional (Default: False)
Produce output in HH:MM:SS.S format instead of decimal degrees?
precision : int, optional
Number of floating-point digits in output values
output : str, optional
Name of output file with results, if desired
verbose : bool (Default: False)
Print out full list of transformation results


ra : float
Right Ascension of pixel. If more than 1 input value, then it will be a numpy array.
dec : float
Declination of pixel. If more than 1 input value, then it will be a numpy array.


This task performs a full distortion-correction coordinate transformation based on all WCS keywords and any recognized distortion keywords from the input image header. The transformation recognizes the conventions for describing distortion implemented as part of the SIP and Paper IV conventions used with AstroDrizzle. Input images can be updated to use these conventions through the use of the updatewcs module the STWCS package.

See Also



This task can be run from either the TEAL GUI or from the Python command-line. These examples illustrate the syntax that can be used to run the task in a couple of common modes.

  1. Run the task using the TEAL GUI under PyRAF:

    >>> import drizzlepac
    >>> epar pixtosky
  2. Convert a single X,Y position (100,100) from an calibrated ACS image (j8bt06nyq_flt.fits) into an undistorted sky position (RA,Dec) without using the TEAL GUI:

    >>> from drizzlepac import pixtosky
    >>> r,d = pixtosky.xy2rd("j8bt06nyq_flt.fits[sci,1]", 100, 100)
  3. Convert a list of X,Y positions from the file 'xyfile.dat' for a calibrated ACS image (j8bt06nyq_flt.fits) into undistorted sky positions and write out the result to the file ‘radec.dat’ without using the TEAL GUI:

    >>> r,d = pixtosky.xy2rd("j8bt06nyq_flt.fits[sci,1]", coordfile="xyfile.dat",
    ...       output="radec.dat")
  4. The set of X,Y positions from 'input_flt.fits[sci,1]' stored as the 3rd and 4th columns from the ASCII file 'xy_sci1.dat' will be transformed and written out to 'radec_sci1.dat' using:

    >>> from drizzlepac import pixtosky
    >>> r,d = pixtosky.xy2rd("input_flt.fits[sci,1]", coordfile='xy_sci1.dat',
    ...                      colnames=['c3','c4'], output="radec_sci1.dat")
drizzlepac.pixtosky.xy2rd(input, x=None, y=None, coords=None, coordfile=None, colnames=None, separator=None, hms=True, precision=6, output=None, verbose=True)

Primary interface to perform coordinate transformations from pixel to sky coordinates using STWCS and full distortion models read from the input image header.