Most other common tools in low-level astronomical data-analysis (for example SExtractor135) merge the two processes of detection and measurement into one. Gnuastro’s modularized methodology to separating detection from measurements is therefore new to many experienced astronomers and deserves a short review here. Further discussion on the benefits of this methodology can be seen in Akhlaghi .
The raw output of any of the programs mentioned in MakeCatalog can be directly fed into MakeCatalog to get to a fast catalog. This is good when no further customization is necessary and you want a fast/simple catalog. But the modular approach taken by Gnuastro has many benefits that will become clear as you get more experienced in astronomical data analysis and want to be more creative in using your valuable data for the exciting scientific project you are working on. In short the reasons for this modularity can be classified as below:
Here is an example of doing both: suppose you have images in various broad band filters at various resolutions and orientations. The image of one color will thus not lie exactly on another or even be in the same scale. However, it is imperative that the same pixels be used in measuring the colors of galaxies.
To solve the problem, NoiseChisel can be run on the reference image to generate the labeled detection image. After wards, the labeled image can be warped into the grid of the other color (using Warp). MakeCatalog will then generate the same catalog for both colors (with the different labeled images). It is currently customary to warp the images to the same pixel grid, however, modification of the scientific dataset is very harmful for the data and creates correlated noise. It is much more accurate to do the transformations on the labeled image.
As an example, if the parameter you want to measure over one profile is not provided by the developers of MakeCatalog. You can simply open this tiny little program and add your desired calculation easily. This process is discussed in Adding new columns to MakeCatalog. However, if making a catalog was part of NoiseChisel for example, adding a new column/measurement would require a lot of energy to understand all the steps and internal structures of that huge program. It might even be so intertwined with its processing, that adding new columns might cause problems/bugs in its primary job (detection).