GNU Astronomy Utilities manual

7.3 MakeCatalog

Detecting and segmenting signal over an image results in labeled images where each pixel is labeled with the ID (an integer) that is specified by the detector program. But this labeled image by its self can hardly be of any scientific use. The job of MakeCatalog is to combine the input image, the noise properties and the labels of pixels into a catalog (a text file table) which can easily be used for high-level scientific interpretations.

NoiseChisel (Gnuastro’s signal detection tool, see NoiseChisel) does not produce any catalog of the detected objects by its self. Only a labeled FITS image is output, see NoiseChisel output. The output of NoiseChisel can be directly fed into MakeCatalog to generate the catalog. Some of the reasons for making the catalog in a separate program⁸⁰ are listed below:

• Complexity: Adding in a catalog functionality to the detector program will add several more steps (and options) to its processings that can equally well be done outside of it. This makes following the code harder for a curious reader and also potentially adds bugs.

  Another advantage of less complexity is when 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. However, if making a catalog was part of NoiseChisel, it would require a lot of energy to understand all the steps in order to add desired parameter.

• Low level nature of Gnuastro: Making catalogs is a separate process from labeling (detecting and segmenting) the pixels. A user might want to do certain operations on the labed regions before creating a catalog for them. Another user might want the properties of the same pixels in another image (possibly from another broadband filter) for measuring the colors or SEDs for example.

  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.

  Therefore NoiseChisel can be run on the reference image and ImageWarp (ImageWarp) can be applied to the labeled images to find the pixels to use in the other image. Then MakeCatalog can generate the final catalog for both targets. It is currently customary to warp the images to the same pixel grid, however, this is very harmful for the data and creates correlated noise. It is much more accurate to do the transformations on the labeled image.

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GNU Astronomy Utilities manual, November 2015.