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7.4.1 Detection and catalog production ¶

Most existing common tools in low-level astronomical data-analysis (for example, SExtractor²⁰⁸) merge the two processes of detection and measurement (catalog production) in one program. However, in light of Gnuastro’s modularized approach (modeled on the Unix system) detection is separated from measurements and catalog production. This modularity 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 2016.

As discussed in the introduction of MakeCatalog, detection (identifying which pixels to do measurements on) can be done with different programs. Their outputs (a labeled dataset) can be directly fed into MakeCatalog to do the measurements and write the result as a catalog/table. Beyond that, Gnuastro’s modular approach 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:

• Simplicity/robustness of independent, modular tools: making a catalog is a logically separate process from labeling (detection, segmentation, or aperture production). A user might want to do certain operations on the labeled regions before creating a catalog for them. Another user might want the properties of the same pixels/objects in another image (another filter for example) to measure the colors or SED fittings.

  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. Afterwards, 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.

• Complexity of a monolith: Adding in a catalog functionality to the detector program will add several more steps (and many more options) to its processing that can equally well be done outside of it. This makes following what the program does harder for the users and developers, it can also potentially add many bugs.

  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).