GNU Astronomy Utilities Measurements per slice (spectra)

When the input is a 3D data cube, MakeCatalog has the following multi-valued measurements per label. For a tutorial on how to use these options and interpret their values, see Detecting lines and extracting spectra in 3D data.

These options will do measurements on each 2D slice of the input 3D cube; hence the common the format of --*-in-slice. Each slice usually corresponds to a certain wavelength, you can also think of these measurements as spectra.

For each row (input label), each of the columns described here will contain multiple values as a vector column. The number of measurements in each column is the number of slices in the cube, or the size of the cube along the third dimension. To learn more about vector columns and how to manipulate them, see Vector columns. For example usage of these columns in the tutorial above, see 3D measurements and spectra and Extracting a single spectrum and plotting it.

There are two ways to do each measurement on a slice for each label:

Only label

The measurement will only be done on the voxels in the slice that are associated to that label. These types of per-slice measurement therefore have the following properties:

  • This will only be a measurement of that label and will not be affected by any other label.
  • The number of voxels used in each slice can be different (usually only one or two voxels at the two extremes of the label (along the third dimension), and many in the middle.
  • Since most labels are localized along the third dimension (maybe only covering 20 slices out of thousands!), many of the measurements (on slices where the label doesn’t exist) will be NaN (for the sum measurements for example) or 0 (for the area measurements).
Projected label

MakeCatalog will first project the 3D label into a 2D surface (along the third dimension) to get its 2D footprint. Afterwards, all the voxels in that 2D footprint will be measured all slices. All these measurements will have a -proj- component in their name. These types of per-slice measurement therefore has the following properties:

  • A measurement will be done on each slice of the cube.
  • All measurements will be done on the same surface area.
  • Labels can overlap when they are projected onto the first two FITS dimensions (the spatial coordinates, not spectral). As a result, other emission lines or objects may contaminate the resulting spectrum for each label.

To help separate other labels, MakeCatalog can do a third type of measurement on each slice: measurements on the voxels that belong to other labels but overlap with the 2D projection. This can be used to see how much your projected measurement is affected by other emission sources (on the projected spectra) and also if multiple lines (labeled regions) belong to the same physical object. These measurements contain -other- in their name.


[Only label] Sum of values in each slice.


[Only label] Error in ’–sum-in-slice’.


[Only label] Number of labeled in each slice.


[Projected label] Sum of projected area in each slice.


[Projected label] Number of voxels that are used in --sum-proj-in-slice.


[Projected label] Error of --sum-proj-in-slice.


[Projected label] Area of other label in projected area on each slice.


[Projected label] Sum of other label in projected area on each slice.


[Projected label] Area in --sum-other-in-slice.