Arithmetic

Operations along one or more axes

In Cube objects, the sum, mean and median methods return the sum, mean and median values along a given axis or axes of the cube:

• If axis=0, the operation is performed along the wavelength axis and an Image is returned,

• If axis=(1, 2), the operation is performed over the two spatial axes and a Spectrum is returned.

In [1]: import matplotlib.pyplot as plt

In [2]: from mpdaf.obj import Cube

In [3]: cube = Cube('sdetect/minicube.fits')

In [4]: cube.info()
[INFO] 3681 x 40 x 40 Cube (sdetect/minicube.fits)
[INFO] .data(3681 x 40 x 40) (1e-20 erg / (Angstrom s cm2)), .var(3681 x 40 x 40)
[INFO] center:(10:27:56.39623411,04:13:25.35875899) size:(8.000",8.000") step:(0.200",0.200") rot:-0.0 deg frame:FK5
[INFO] wavelength: min:4749.89 max:9349.89 step:1.25 Angstrom

In [5]: spe = cube.mean(axis=(1,2))

In [6]: spe.info()
[INFO] 3681 Spectrum (sdetect/minicube.fits)
[INFO] .data(3681) (1e-20 erg / (Angstrom s cm2)), .var(3681)
[INFO] wavelength: min:4749.89 max:9349.89 step:1.25 Angstrom

In [7]: spe.plot()

# white image
In [8]: ima = cube.sum(axis=0)

In [9]: ima.info()
[INFO] 40 x 40 Image (sdetect/minicube.fits)
[INFO] .data(40 x 40) (1e-20 erg / (Angstrom s cm2)), .var(40 x 40)
[INFO] center:(10:27:56.39623411,04:13:25.35875899) size:(8.000",8.000") step:(0.200",0.200") rot:-0.0 deg frame:FK5

In [10]: plt.figure()
Out[10]: <Figure size 640x480 with 0 Axes>

In [11]: ima.plot()
Out[11]: <matplotlib.image.AxesImage at 0x7fa598823940>

In [12]: spe.sum(7100,7200)
Out[12]: (3344.121536058322, 1.6975161139416672)

In [13]: spe.integrate(7100, 7200)
Out[13]: 
(<Quantity 4436.30591051 1e-20 erg / (s cm2)>,
 <Quantity 2.11729934 1e-20 erg / (s cm2)>)

The Spectrum.mean and Spectrum.sum methods compute the mean and total flux value over a given wavelength range. Similarly, Spectrum.integrate integrates the flux value over a given wavelength range.

Arithmetic Operations between objects

Arithmetic operations can be performed between MPDAF objects and scalar numbers, numpy arrays, masked arrays or other MPDAF objects. When an operation is performed between an MPDAF object and an array, their dimensions must either match, or be broadcastable to the same dimensions via the usual numpy rules. When an operation is performed between two MPDAF objects, it requires the objects to have identical WCS properties, equivalent units, and identical shapes. Variances are propagated, although correlated errors are not supported. If data are detected to be correlated, a warning is issued. Values masked in either object before the operation are masked in the resulting object. The broadcasting rules make it possible to perform arithmetic operations between a Cube and an Image or a Spectrum, assuming that they have compatible spatial and spectral world-coordinates.

The following demonstration shows a cube being multiplied by an image that has the same spatial dimensions and world-coordinates as the cube, and also being divided by a spectrum that has the same spectral dimensions and wavelengths as the spectral axis of the cube.

In [14]: cube2 = cube * ima / spe

In [15]: cube2.info()
[INFO] 3681 x 40 x 40 Cube (sdetect/minicube.fits)
[INFO] .data(3681 x 40 x 40) (1e-20 erg / (Angstrom cm2 s)), .var(3681 x 40 x 40)
[INFO] center:(10:27:56.3962,04:13:25.3588) size in arcsec:(8.000,8.000) step in arcsec:(0.200,0.200) rot:-0.0 deg
[INFO] wavelength: min:4749.89 max:9349.89 step:1.25 Angstrom

Generic object arithmetic:

Cube, Image and Spectrum objects are all derived from a base class called DataArray. This class implements a couple of arithmetic functions that operate on the data and variance arrays of these objects:

• sqrt returns a new object with positive data square-rooted and negative data masked.

• abs returns a new object containing the absolute values of the data.

In [16]: ima2 = ima.sqrt()

In [17]: ima2.plot()
Out[17]: <matplotlib.image.AxesImage at 0x7fa595f82070>