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10.5 Zero point estimation ¶

Through the “zero point”, we are able to give physical units to the pixel values of an image (often in units of “counts” or ADUs) and thus compare them with other images (as well as measurements that are done on them). The zero point is therefore an important calibration of pixel values (as astromerty is a calibration of the pixel positions). The fundamental concepts behind the zero point are described in Brightness, Flux, Magnitude and Surface brightness. We will therefore not go deeper into the basics here and stick to the practical aspects of it.

The purpose of Gnuastro’s astscript-zeropoint script is to obtain the zero point of an image using a reference image, or a reference catalog. The reference image must have a known zero point, and the reference should contain known positions and magnitudes of stars; both should of course also contain some spatial overlap (on the sky) with the input image. The reference data (catalog or image) serve as the anchor point to which your input image will be calibrated (a zero point will be found for it). By comparing the instrumental magnitudes of the stars with their cataloged (standard) magnitudes, the differences can be calculated and used to determine the zero point. For example, when using a reference image, the script will take the following steps:

1. Download the Gaia catalog that overlaps with the input image using Gnuastro’s Query program (see Query). This is done to determine the stars within the image²⁴⁸.
2. Perform aperture photometry²⁴⁹ with MakeProfiles MakeCatalog. We will assume a zero point of 0 for the input image. If the reference is an image, then we should perform aperture photometry also in that image.
3. Match the two catalogs²⁵⁰ with Match.
4. The difference between the input and reference magnitudes should be independent of the magnitude of the stars. This does not hold when the stars are saturated in one/both the images (giving us a bright-limit for the magnitude range to use) or for stars fainter than a certain magnitude, where the signal-to-noise ratio drops significantly in one/both images (giving us a faint limit for the magnitude range to use).
5. Since the input image was assigned a zero point of zero, the magnitude difference shown above (within the reliable magnitude range) corresponds to the zero point of the input image.

In the “Tutorials” chapter of this Gnuastro book, there are two tutorials dedicated to the usage of this script. The first uses an image as a reference (Zero point tutorial with reference image) and the second uses a catalog (Zero point tutorial with reference catalog). For the full set of options an a detailed description of each, see Invoking astscript-zeropoint.

• Invoking astscript-zeropoint