skyimage¶
The skyimage
module contains algorithms that are used by
skymatch
to manage all of the information for footprints (image outlines)
on the sky as well as perform useful operations on these outlines such as
computing intersections and statistics in the overlap regions.
 Authors
Mihai Cara (contact: help@stsci.edu)

class
jwst.skymatch.skyimage.
SkyGroup
(images, id=None, sky=0.0)[source]¶ Holds multiple
SkyImage
objects whose sky background values must be adjusted together.SkyGroup
provides methods for obtaining bounding polygon of the group ofSkyImage
objects and to compute sky value of the group.
calc_sky
(overlap=None, delta=True)[source]¶ Compute sky background value.
 Parameters
overlap (SkyImage, SkyGroup, SphericalPolygon, list of tuples, None, optional) – Another
SkyImage
,SkyGroup
,spherical_geometry.polygons.SphericalPolygon
, or a list of tuples of (RA, DEC) of vertices of a spherical polygon. This parameter is used to indicate that sky statistics should computed only in the region of intersection of this image with the polygon indicated byoverlap
. Whenoverlap
isNone
, sky statistics will be computed over the entire image.delta (bool, optional) – Should this function return absolute sky value or the difference between the computed value and the value of the sky stored in the
sky
property.
 Returns
skyval (float, None) – Computed sky value (absolute or relative to the
sky
attribute). If there are no valid data to perform this computations (e.g., because this image does not overlap with the image indicated byoverlap
),skyval
will be set toNone
.npix (int) – Number of pixels used to compute sky statistics.
polyarea (float) – Area (in srad) of the polygon that bounds data used to compute sky statistics.

property
id
¶ 
While
id
can be of any type, it is prefereble thatid
be of a type with nice string representation.

intersection
(skyimage)[source]¶ Compute intersection of this
SkyImage
object and anotherSkyImage
,SkyGroup
, orSphericalPolygon
object.

property
polygon
¶ Get image’s bounding polygon.


class
jwst.skymatch.skyimage.
SkyImage
(image, wcs_fwd, wcs_inv, pix_area=1.0, convf=1.0, mask=None, id=None, skystat=None, stepsize=None, meta=None)[source]¶ Container that holds information about properties of a single image such as:
image data;
WCS of the chip image;
bounding spherical polygon;
id;
pixel area;
sky background value;
sky statistics parameters;
mask associated image data indicating “good” (1) data.
Initializes the SkyImage object.
 Parameters
image (numpy.ndarray) – A 2D array of image data.
wcs_fwd (function) – “forward” pixeltoworld transformation function.
wcs_inv (function) – “inverse” worldtopixel transformation function.
pix_area (float, optional) – Average pixel’s sky area.
convf (float, optional) –
Conversion factor that when multiplied to
image
data converts the data to “uniform” (across multiple images) surface brightness units.Note
The functionality to support this conversion is not yet implemented and at this moment
convf
is ignored.mask (numpy.ndarray) – A 2D array that indicates what pixels in the input
image
should be used for sky computations (1
) and which pixels should not be used for sky computations (0
).id (anything) – The value of this parameter is simple stored within the
SkyImage
object. While it can be of any type, it is prefereble thatid
be of a type with nice string representation.skystat (callable, None, optional) –
A callable object that takes a either a 2D image (2D
numpy.ndarray
) or a list of pixel values (a Nx1 array) and returns a tuple of two values: some statistics (e.g., mean, median, etc.) and number of pixels/values from the input image used in computing that statistics.When
skystat
is not set,SkyImage
will useSkyStats
object to perform sky statistics on image data.stepsize (int, None, optional) – Spacing between vertices of the image’s bounding polygon. Default value of
None
creates bounding polygons with four vertices corresponding to the corners of the image.meta (dict, None, optional) – A dictionary of various items to be stored within the
SkyImage
object.

calc_sky
(overlap=None, delta=True)[source]¶ Compute sky background value.
 Parameters
overlap (SkyImage, SkyGroup, SphericalPolygon, list of tuples, None, optional) – Another
SkyImage
,SkyGroup
,spherical_geometry.polygons.SphericalPolygon
, or a list of tuples of (RA, DEC) of vertices of a spherical polygon. This parameter is used to indicate that sky statistics should computed only in the region of intersection of this image with the polygon indicated byoverlap
. Whenoverlap
isNone
, sky statistics will be computed over the entire image.delta (bool, optional) – Should this function return absolute sky value or the difference between the computed value and the value of the sky stored in the
sky
property.
 Returns
skyval (float, None) – Computed sky value (absolute or relative to the
sky
attribute). If there are no valid data to perform this computations (e.g., because this image does not overlap with the image indicated byoverlap
),skyval
will be set toNone
.npix (int) – Number of pixels used to compute sky statistics.
polyarea (float) – Area (in srad) of the polygon that bounds data used to compute sky statistics.

property
id
¶ 
While
id
can be of any type, it is prefereble thatid
be of a type with nice string representation.

intersection
(skyimage)[source]¶ Compute intersection of this
SkyImage
object and anotherSkyImage
,SkyGroup
, orSphericalPolygon
object.

property
is_sky_valid
¶ Indicates whether sky value was successfully computed. Must be set externally.

property
pix_area
¶ Set or get mean pixel area.

property
poly_area
¶ Get bounding polygon area in srad units.

property
polygon
¶ Get image’s bounding polygon.

property
radec
¶ Get RA and DEC of the verteces of the bounding polygon as a
ndarray
of shape (N, 2) where N is the number of verteces + 1.

set_builtin_skystat
(skystat='median', lower=None, upper=None, nclip=5, lsigma=4.0, usigma=4.0, binwidth=0.1)[source]¶ Replace already set
skystat
with a “builtin” version of a statistics callable object used to measure sky background.See
SkyStats
for the parameter description.

property
skystat
¶ Stores/retrieves a callable object that takes a either a 2D image (2D
numpy.ndarray
) or a list of pixel values (a Nx1 array) and returns a tuple of two values: some statistics (e.g., mean, median, etc.) and number of pixels/values from the input image used in computing that statistics.When
skystat
is not set,SkyImage
will useSkyStats
object to perform sky statistics on image data.