spaxel

In astrophysics, the term spaxel (short for spectral pixel or spatial pixel) is generally used regarding data cubes that hold observation-information from an imaging spectrometer: a spaxel is that part of a datacube representing a SED (over some wavelength-range) for a single location within the field of view (FOV), which consists of a column of scalar elements within the cube. The data cube can be said to organize the data according to position-position-wavelength (or equivalent, e.g., position-position-frequency) space. The instrument captures these with a sensor (e.g., CCD) that has pixels, but the sensor's individual pixels do not correspond to individual locations in the celestial sphere, the derived spaxels are organized to do so. A single (scalar) element within the data cube is termed a voxel (for volume pixel), which represents one wavelength-interval of the SED of a pixel-like location within the FOV. Note that some data manipulation (resampling) is invariably needed to turn the raw spectrometer output into such a uniform data cube.

Be warned that the term spaxel was also coined for an unrelated pixel-like entity, proposed as a useful concept in computer graphics. References to this other definition may confuse because it is totally unlike the astrophysics usage listed above. The proposed element (spaxel being short for a spatial pixel) is pixel-like, but exists anywhere within a volume (rather than on a plane), and unlike a pixel, it would not be tied to a fixed place in a grid and it would include an indication of a vector representing its motion within the volume.