# skystatistics¶

The skystatistics module contains various statistical functions used by skymatch.

skystatistics module provides statistics computation class used by match() and SkyImage.

Authors

Mihai Cara (contact: help@stsci.edu)

class jwst.skymatch.skystatistics.SkyStats(skystat='mean', lower=None, upper=None, nclip=5, lsig=4.0, usig=4.0, binwidth=0.1, **kwargs)[source]

This is a superclass build on top of stsci.imagestats.ImageStats. Compared to stsci.imagestats.ImageStats, SkyStats has “persistent settings” in the sense that object’s parameters need to be set once and these settings will be applied to all subsequent computations on different data.

Initializes the SkyStats object.

Parameters
• skystat ({'mode', 'median', 'mode', 'midpt'}, optional) – Sets the statistics that will be returned by calc_sky. The following statistics are supported: ‘mean’, ‘mode’, ‘midpt’, and ‘median’. First three statistics have the same meaning as in stsdas.toolbox.imgtools.gstatistics while ‘median’ will compute the median of the distribution.

• lower (float, None, optional) – Lower limit of usable pixel values for computing the sky. This value should be specified in the units of the input image(s).

• upper (float, None, optional) – Upper limit of usable pixel values for computing the sky. This value should be specified in the units of the input image(s).

• nclip (int, optional) – A non-negative number of clipping iterations to use when computing the sky value.

• lsig (float, optional) – Lower clipping limit, in sigma, used when computing the sky value.

• usig (float, optional) – Upper clipping limit, in sigma, used when computing the sky value.

• binwidth (float, optional) – Bin width, in sigma, used to sample the distribution of pixel brightness values in order to compute the sky background statistics.

• kwargs (dict) – A dictionary of optional arguments to be passed to ImageStats.

calc_sky(data)[source]

Computes statistics on data.

Parameters

data (numpy.ndarray) – A numpy array of values for which the statistics needs to be computed.

Returns

statistics – A tuple of two values: (skyvalue, npix), where skyvalue is the statistics specified by the skystat parameter during the initialization of the SkyStats object and npix is the number of pixels used in comuting the statistics reported in skyvalue.

Return type

tuple