4. Technical specifications¶
In this page we summarize the assumptions that are made in galario and the specifications for the input quantities.
4.1. Image specifications¶
the matrix axes \([i, j]\) mapping the pixel coordinates, running from
Nxyis the number of pixels on each axis).
the physical axes \((R.A., Dec.)\) mapping Right Ascension and Declination coordinates.
[i, j] = [0, 0] of the matrix axes can be put either in the upper left or in the lower left corner of the matrix.
By default galario assumes the origin is in the upper left corner of the matrix, but it can be changed to the
lower corner by specifying the optional parameter
origin='lower' in the
The origin \((R.A., Dec.)=(0,0)\) of the physical axes is always in the center of the
[Nxy/2, Nxy/2] pixel
(grey pixel in the Figure below) for any value of
origin. The \(R.A.\) axis always increases leftwards, following the usual convention
of having East to the left and West to the right: therefore, the \(R.A.\) axis always decreases with increasing
Note that, with
origin='upper', both the \((R.A., Dec.)\) axes decrease with increasing
j index (see Figure below).
galario assumes that the values of the input image are evaluated in the pixel centers (not in the pixel edges).
For instructions on how to compute the correct \((R.A., Dec.)\) coordinate meshgrid to create the image, see the Cookbook.
The left and right panels of the Figure below show the relative orientations of matrix and physical axes
origin='lower' cases, respectively (click on the images for a larger version).
origin parameter in galario follows the same definition as in the
imshow commands of the
To get the desired orientation of the Declination axis in galario, use the same
origin parameter that produces
the desired image orientation with