calculate_moments#

irispy.utils.moments.calculate_moments( cube, *, rest_wavelength=None, wings=None, integrated=False, min_intensity=None, saturation_limit=None, )[source]#

Calculate the 0th, 1st, and 2nd spectral moments of a data cube.

The moments are computed along the spectral (wavelength) axis for each spatial pixel:

0th moment: total intensity, \(\sum I(\lambda_i)\) (or \(\int I(\lambda) \, d\lambda\) when integrated=True)
1st moment: centroid wavelength, \(\sum \lambda_i I(\lambda_i) / \sum I(\lambda_i)\)
2nd moment: standard deviation, \(\sqrt{\sum (\lambda_i - \lambda_0)^2 I(\lambda_i) / \sum I(\lambda_i)}\)

Parameters:

cube (irispy.spectrograph.SpectrogramCube) – The input data cube. Must have a spectral (wavelength) axis.
rest_wavelength (astropy.units.Quantity, optional) – The rest wavelength of the spectral line.
wings (astropy.units.Quantity, optional) – The spectral range around rest_wavelength to include in the calculation. Must be an Quantity with appropriate units (e.g., nm or Angstrom). If a scalar Quantity, it is applied symmetrically. If a tuple of two Quantities, they are the lower and upper offsets respectively.
integrated (bool, optional) – If True, the 0th moment is computed as \(\int I(\lambda) \, d\lambda\) with units of DN·nm. If False (default), it is computed as \(\sum I(\lambda)\) with units of DN (i.e., per-pixel sum, matching the convention used in Gaussian fitting).
min_intensity (float or astropy.units.Quantity, optional) – Minimum integrated (or per-pixel) intensity required for a pixel to be considered valid. Pixels below this value have all moments set to NaN.
saturation_limit (float or astropy.units.Quantity, optional) – Maximum allowed peak intensity in any spectral bin. Pixels exceeding this value have all moments set to NaN.

Returns:

A collection containing 2D SpectrogramCube objects with the spatial WCS preserved from the input cube.

Always present:

"intensity" — 0th moment (total intensity)
"centroid" — 1st moment (centroid wavelength)
"width" — 2nd moment (standard deviation)

Additionally, if rest_wavelength is known:

"velocity" — Doppler shift from the centroid in km/s
"velocity_width" — line width converted to velocity units in km/s

Return type:

irispy.spectrograph.RasterCollection

Notes

Negative and non-finite data values are set to zero before computing moments.
Wavelength coordinates are converted to nm internally, so centroid and width are always returned in nm.
For a uniform spectral grid, the 1st and 2nd moments are identical regardless of the integrated setting because the pixel spacing cancels out in the ratio.

References