SkyImage

class stcal.skymatch.skyimage.SkyImage(image, wcs_fwd, wcs_inv, pix_area=1.0, convf=1.0, mask=None, sky_id=None, skystat=None, stepsize=None, meta=None, reduce_memory_usage=True)[source]

Bases: object

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, NDArrayDataAccessor) – A 2D array of image data or a NDArrayDataAccessor.
wcs_fwd (collections.abc.Callable) – “forward” pixel-to-world transformation function.
wcs_inv (collections.abc.Callable) – “inverse” world-to-pixel 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, NDArrayDataAccessor) – A 2D array or NDArrayDataAccessor of a 2D array that indicates which pixels in the input image should be used for sky computations (1) and which pixels should not be used for sky computations (0).
sky_id (Any) – The value of this parameter is simple stored within the SkyImage object. While it can be of any type, it is preferable that id be of a type with nice string representation.
skystat (collections.abc.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 use SkyStats 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.
reduce_memory_usage (bool, optional) – Indicates whether to attempt to minimize memory usage by attaching input image and/or mask numpy.ndarray arrays to file-mapped accessor. This has no effect when input parameters image and/or mask are already of NDArrayDataAccessor objects.

Attributes Summary

`image`	Set or get SkyImage's `image` data array.
`image_shape`	Get SkyImage's `image` data shape.
`mask`	Set or get SkyImage's `mask` data array or None.
`poly_area`	Get bounding polygon area in srad units.
`polygon`	Get image's bounding polygon.
`radec`	Get RA and DEC of the vertices of the bounding polygon as a ~numpy.ndarray of shape (N, 2) where N is the number of vertices + 1.

Methods Summary

`calc_bounding_polygon`([stepsize])	Compute image's bounding polygon.
`calc_sky`([overlap, delta])	Compute sky background value.
`copy`()	Return a shallow copy of the SkyImage object.
`intersection`(skyimage)	Compute intersection of this SkyImage object and another SkyImage, SkyGroup, or `SphericalPolygon` object.
`set_builtin_skystat`([skystat, lower, upper, ...])	Replace already set skystat with a "built-in" version of a statistics callable object used to measure sky background.

Attributes Documentation

image: Set or get SkyImage’s image data array.

image_shape: Get SkyImage’s image data shape.

mask: Set or get SkyImage’s mask data array or None.

poly_area: Get bounding polygon area in srad units.

polygon: Get image’s bounding polygon.

radec: Get RA and DEC of the vertices of the bounding polygon as a ~numpy.ndarray of shape (N, 2) where N is the number of vertices + 1.

Methods Documentation

calc_bounding_polygon(stepsize=None)[source]

Compute image’s bounding polygon.

Parameters:: stepsize (int, None, optional) – Indicates the maximum separation between two adjacent vertices of the bounding polygon along each side of the image. Corners of the image are included automatically. If stepsize is None, bounding polygon will contain only vertices of the image.

Notes

The bounding polygon is defined from corners of pixels whereas the pixel coordinates refer to their centers and therefore the lower-left corner is located at (-0.5, -0.5)

calc_sky(overlap=None, delta=True)[source]

Compute sky background value.

Parameters:

overlap (SkyImage, SkyGroup, SphericalPolygon, list[tuple[typing.Any]], None, optional) – Another SkyImage, SkyGroup, spherical_geometry.polygon.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 by overlap. When overlap is None, 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 by overlap), skyval will be set to None.
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.

Notes

Due to a bug in the sphere package, see https://github.com/spacetelescope/sphere/issues/74 intersections with polygons formed as union does not work. For this reason I re-implement ‘calc_sky’ below with a workaround for the bug. The original implementation should be used when the bug is fixed.

copy()[source]: Return a shallow copy of the SkyImage object.

intersection(skyimage)[source]

Compute intersection of this SkyImage object and another SkyImage, SkyGroup, or SphericalPolygon object.

Parameters:: skyimage (SkyImage, SkyGroup, spherical_geometry.polygon.SphericalPolygon) – Another object that should be intersected with this SkyImage.
Returns:: polygon – A SphericalPolygon that is the intersection of this SkyImage and skyimage.
Return type:: SphericalPolygon

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 “built-in” version of a statistics callable object used to measure sky background.

See SkyStats for the parameter description.