coords_chandra

Context: contrib

Synopsis

Utility routines for working with Chandra Coordinates

Syntax

from coords.chandra import cel_to_chandra
from coords.chandra import sky_to_chandra
from coords.chandra import get_coord_keywords

keyword_list = get_coord_keywords(filename)
cel_to_chandra(keyword_list, ra_vals, dec_vals)
sky_to_chandra(keyword_list, x_vals, y_vals)

Description

The coords.chandra module contains two routines to convert coordinates - in SKY or Celestial (Right Ascension, Declination) systems - to the most common Chandra coordinate systems:

detector (detx, dety),
mirror spherical (off-axis angles theta and phi),
or chip coordinates.

See ahelp coords for more information on the Chandra coordinate systema.

It also contains a helper routine to retrieve the header keywords these routines need from either a file or crate.

Parameter values

The first input to each routine is a dictionary of header keywords.

>>> from coords.chandra import get_coord_keywords
>>> keywords = get_coord_keywords(filename)

The coordinates can either be given as scalars or lists.

Examples

Example 1

>>> from coords.chandra import sky_to_chandra
>>> sky_to_chandra(keywords, 4096.5, 4096.5)
{'dety': [4096.5], 'detx': [4096.5], 'phi': [0.0], 'ra':
[246.82474880852999], 'chipy': [996.6745464316374], 'chip_id': [3],
'theta': [0.0], 'chipx': [975.2463325198958], 'dec':
[-24.573378630717002], 'pixsize': 0.492}

In this example a single set of sky X,Y values is input and a dictionary listing the coordinates in a variety of systems are returned.

Example 2

>>> from coords.chandra import cel_to_chandra, get_coord_keywords
>>> from pycrates import read_file
>>> srclist = read_file("wavedetect.src")
>>> keywords = get_coord_keywords(srclist)
>>> ra = srclist.get_column("RA").values
>>> dec = srclist.get_column("DEC").values
>>> coords = cel_to_chandra(keywords, ra, dec)
>>> print(sorted(coords.keys()))
['chip_id', 'chipx', 'chipy', 'detx', 'dety', 'phi', 'pixsize',
'theta', 'x', 'y']

In this longer example the RA and DEC values for each source in a wavdetect source list is read in using pycrates. The Chandra coordinates are computed for all the source positions.

The pixsize value is the only value not returned as a list. It is in units of arcsec/pixel.

The following code will plot up the SKY coordinates using Matplotlib (loading it if necessary):

>>> import matplotlib.pyplot as plt
>>> plt.scatter(coords["x"], coords["y"])
>>> plt.xlabel('X')
>>> plt.ylabel('Y')
>>> plt.show()

Changes in scripts 4.17.2 (September 2025) release

Added new get_coord_keywords routine. Note: ahelp file was updated in 4.18.0.

Changes in scripts 4.11.4 (August 2019) release

Improvements to the docstrings to identify the required keywords values in the input keyword dictionary. Improved error checking to verify that the keyword list contains valid values for the required keywords.

About Contributed Software

This module is not an official part of the CIAO release but is made available as "contributed" software via the CIAO scripts page. Please see this page for installation instructions.