WCS¶

class gwcs.wcs.WCS(forward_transform=None, input_frame='detector', output_frame=None, name='')[source]¶

Bases: gwcs.api.GWCSAPIMixin

Basic WCS class.

Parameters

forward_transform : Model or a list

The transform between input_frame and output_frame. A list of (frame, transform) tuples where frame is the starting frame and transform is the transform from this frame to the next one or output_frame. The last tuple is (transform, None), where None indicates the end of the pipeline.

input_frame : str, CoordinateFrame

A coordinates object or a string name.

output_frame : str, CoordinateFrame

A coordinates object or a string name.

name : str

a name for this WCS

Attributes Summary

`available_frames`	List all frames in this WCS object.
`backward_transform`	Return the total backward transform if available - from output to input coordinate system.
`bounding_box`	Return the range of acceptable values for each input axis.
`forward_transform`	Return the total forward transform - from input to output coordinate frame.
`input_frame`	Return the input coordinate frame.
`name`	Return the name for this WCS.
`output_frame`	Return the output coordinate frame.
`pipeline`	Return the pipeline structure.
`unit`	The unit of the coordinates in the output coordinate system.

Methods Summary

`__call__`(args, *kwargs)	Executes the forward transform.
`fix_inputs`(fixed)	Return a new unique WCS by fixing inputs to constant values.
`footprint`([bounding_box, center, axis_type])	Return the footprint in world coordinates.
`get_transform`(from_frame, to_frame)	Return a transform between two coordinate frames.
`insert_transform`(frame, transform[, after])	Insert a transform before (default) or after a coordinate frame.
`invert`(args, *kwargs)	Invert coordinates.
`set_transform`(from_frame, to_frame, transform)	Set/replace the transform between two coordinate frames.
`to_fits_sip`([bounding_box, max_pix_error, …])	Construct a SIP-based approximation to the WCS in the form of a FITS header
`transform`(from_frame, to_frame, args, *kwargs)	Transform positions between two frames.

Attributes Documentation

available_frames¶

List all frames in this WCS object.

Returns: available_frames : dict

{frame_name: frame_object or None}

backward_transform¶

Return the total backward transform if available - from output to input coordinate system.

Raises: NotImplementedError :

An analytical inverse does not exist.

bounding_box¶: Return the range of acceptable values for each input axis. The order of the axes is axes_order.

forward_transform¶: Return the total forward transform - from input to output coordinate frame.

input_frame¶: Return the input coordinate frame.

name¶: Return the name for this WCS.

output_frame¶: Return the output coordinate frame.

pipeline¶: Return the pipeline structure.

unit¶: The unit of the coordinates in the output coordinate system.

Methods Documentation

__call__(*args, **kwargs)[source]¶

Executes the forward transform.

argsfloat or array-like: Inputs in the input coordinate system, separate inputs for each dimension.
with_unitsbool: If True returns a SkyCoord or Quantity object, by using the units of the output cooridnate frame. Optional, default=False.
with_bounding_boxbool, optional: If True(default) values in the result which correspond to any of the inputs being outside the bounding_box are set to fill_value.
fill_valuefloat, optional: Output value for inputs outside the bounding_box (default is np.nan).

fix_inputs(fixed)[source]¶

Return a new unique WCS by fixing inputs to constant values.

Parameters: fixed : dict

Keyword arguments with fixed values corresponding to self.selector.
Returns: new_wcs : WCS

A new unique WCS corresponding to the values in fixed.

Examples

>>> w = WCS(pipeline, selector={"spectral_order": [1, 2]}) 
>>> new_wcs = w.set_inputs(spectral_order=2) 
>>> new_wcs.inputs 
    ("x", "y")

footprint(bounding_box=None, center=False, axis_type='all')[source]¶

Return the footprint in world coordinates.

Parameters

bounding_box : tuple of floats: (start, stop)

prop: bounding_box

center : bool

If True use the center of the pixel, otherwise use the corner.

axis_type : str

A supported output_frame.axes_type or “all” (default). One of [‘spatial’, ‘spectral’, ‘temporal’] or a custom type.

Returns

coord : ndarray

Array of coordinates in the output_frame mapping corners to the output frame. For spatial coordinates the order is clockwise, starting from the bottom left corner.

get_transform(from_frame, to_frame)[source]¶

Return a transform between two coordinate frames.

Parameters

from_frame : str or CoordinateFrame

Initial coordinate frame name of object.

to_frame : str, or instance of CoordinateFrame

End coordinate frame name or object.

Returns

transform : Model

Transform between two frames.

insert_transform(frame, transform, after=False)[source]¶

Insert a transform before (default) or after a coordinate frame.

Append (or prepend) a transform to the transform connected to frame.

Parameters

frame : str or CoordinateFrame

Coordinate frame which sets the point of insertion.

transform : Model

New transform to be inserted in the pipeline

after : bool

If True, the new transform is inserted in the pipeline immediately after frame.

invert(*args, **kwargs)[source]¶

Invert coordinates.

The analytical inverse of the forward transform is used, if available. If not an iterative method is used.

Parameters

args : float, array like, SkyCoord or Unit

coordinates to be inverted

kwargs : dict

keyword arguments to be passed to the iterative invert method.

with_bounding_box : bool, optional

If True(default) values in the result which correspond to any of the inputs being outside the bounding_box are set to fill_value.

fill_value : float, optional

Output value for inputs outside the bounding_box (default is np.nan).

set_transform(from_frame, to_frame, transform)[source]¶

Set/replace the transform between two coordinate frames.

Parameters

from_frame : str or CoordinateFrame

Initial coordinate frame.

to_frame : str, or instance of CoordinateFrame

End coordinate frame.

transform : Model

Transform between from_frame and to_frame.

to_fits_sip(bounding_box=None, max_pix_error=0.25, degree=None, max_inv_pix_error=0.25, inv_degree=None, npoints=32, verbose=False)[source]¶

Construct a SIP-based approximation to the WCS in the form of a FITS header

This assumes a tangent projection.

The default mode in using this attempts to achieve roughly 0.25 pixel accuracy over the whole image.

Parameters

bounding_box : tuple, optional

A pair of tuples, each consisting of two numbers Represents the range of pixel values in both dimensions ((xmin, xmax), (ymin, ymax))

max_pix_error : float, optional

Maximum allowed error over the domain of the pixel array. This error is the equivalent pixel error that corresponds to the maximum error in the output coordinate resulting from the fit based on a nominal plate scale.

degree : int, optional

Degree of the SIP polynomial. If supplied, max_pixel_error is ignored.

max_inv_error : float, optional

Maximum allowed inverse error over the domain of the pixel array in pixel units. If None, no inverse is generated.

inv_degree : int, optional

Degree of the inverse SIP polynomial. If supplied max_inv_pixel_error is ignored.

npoints : int, optional

The number of points in each dimension to sample the bounding box for use in the SIP fit.

verbose : bool, optional

print progress of fits

Returns

FITS header with all SIP WCS keywords

Raises

ValueError

If the WCS is not 2D, an exception will be raised. If the specified accuracy (both forward and inverse, both rms and maximum) is not achieved an exception will be raised.

Notes

Use of this requires a judicious choice of required accuracies. Attempts to use higher degrees (~7 or higher) will typically fail due floating point problems that arise with high powers.

transform(from_frame, to_frame, *args, **kwargs)[source]¶

Transform positions between two frames.

Parameters

from_frame : str or CoordinateFrame

Initial coordinate frame.

to_frame : str, or instance of CoordinateFrame

Coordinate frame into which to transform.

args : float or array-like

Inputs in from_frame, separate inputs for each dimension.

output_with_units : bool

If True - returns a SkyCoord or Quantity object.

with_bounding_box : bool, optional

If True(default) values in the result which correspond to any of the inputs being outside the bounding_box are set to fill_value.

fill_value : float, optional

Output value for inputs outside the bounding_box (default is np.nan).

Navigation

WCS¶