---

7.4.6.3 Position measurements in WCS ¶

The position of a labeled region within your input dataset (in the World Coordinate System, or WCS) can be measured with the options in this section. As you see below, there are various ways to define the “position” of an object, so read the differences carefully to choose the one that corresponds best to your usage.

The most common WCS coordinates are Right Ascension (RA) and Declination in an equatorial system. Therefore, to simplify their usage, we have special --ra and --dec options. However, the WCS of datasets are in Galactic coordinates, so to be generic, you can use the --w1, --w2 or --w3 (if you have a 3D cube) options. In case your dataset’s WCS is not in your desired system (for example it is Galactic, but you want equatorial 2000), you can use the --wcscoordsys option of Gnuastro’s Fits program on the labeled image before running MakeCatalog (see Keyword inspection and manipulation).

-r

--ra

Flux weighted right ascension of all objects or clumps, see --x. This is just an alias for one of the lower-level --w1 or --w2 options. Using the FITS WCS keywords (CTYPE), MakeCatalog will determine which axis corresponds to the right ascension. If no CTYPE keywords start with RA, an error will be printed when requesting this column and MakeCatalog will abort.

-d

--dec

Flux weighted declination of all objects or clumps, see --x. This is just an alias for one of the lower-level --w1 or --w2 options. Using the FITS WCS keywords (CTYPE), MakeCatalog will determine which axis corresponds to the declination. If no CTYPE keywords start with DEC, an error will be printed when requesting this column and MakeCatalog will abort.

--w1

Flux weighted first WCS axis of all objects or clumps, see --x. The first WCS axis is commonly used as right ascension in images.

--w2

Flux weighted second WCS axis of all objects or clumps, see --x. The second WCS axis is commonly used as declination in images.

--w3

Flux weighted third WCS axis of all objects or clumps, see --x. The third WCS axis is commonly used as wavelength in integral field unit data cubes.

--geo-w1

Geometric center in first WCS axis of all objects or clumps, see --geo-x. The first WCS axis is commonly used as right ascension in images.

--geo-w2

Geometric center in second WCS axis of all objects or clumps, see --geo-x. The second WCS axis is commonly used as declination in images.

--geo-w3

Geometric center in third WCS axis of all objects or clumps, see --geo-x. The third WCS axis is commonly used as wavelength in integral field unit data cubes.

--clumps-w1

[Objects] Flux weighted center in first WCS axis of all clumps in this object, see --x. The first WCS axis is commonly used as right ascension in images.

--clumps-w2

[Objects] Flux weighted declination of all clumps in this object, see --x. The second WCS axis is commonly used as declination in images.

--clumps-w3

[Objects] Flux weighted center in third WCS axis of all clumps in this object, see --x. The third WCS axis is commonly used as wavelength in integral field unit data cubes.

--clumps-geo-w1

[Objects] Geometric center right ascension of all clumps in this object, see --geo-x. The first WCS axis is commonly used as right ascension in images.

--clumps-geo-w2

[Objects] Geometric center declination of all clumps in this object, see --geo-x. The second WCS axis is commonly used as declination in images.

--clumps-geo-w3

[Objects] Geometric center in third WCS axis of all clumps in this object, see --geo-x. The third WCS axis is commonly used as wavelength in integral field unit data cubes.