GNU Astronomy Utilities

2.7 Zero point of an image

The “zero point” of an image is astronomical jargon for the calibration factor of its pixel values; allowing us to convert the raw pixel values to physical units. It is therefore a critical step during data reduction. For more on the definition and importance of the zero point magnitude, see Brightness, Flux, Magnitude and Surface brightness and Zero point estimation.

In this tutorial, we will use Gnuastro’s astscript-zeropoint, to estimate the zero point of a single exposure image from the J-PLUS survey, while using an SDSS image as reference (recall that all SDSS images have been calibrated to have a fixed zero point of 22.5). In this case, both images that we are using were taken with the SDSS r filter. See Eskandarlou et al. 2023.

Same filters and SVO filter database: It is very important that both your images are taken with the same filter. When looking at filter names, don’t forget that different filter systems sometimes have the same names for one filter, such as the name “R”; which is used in both the Johnson and SDSS filter systems. Hence if you confront an image in the “R” or “r” filter, double check to see exactly which filter system it corresponds to. If you know which observatory your data came from, you can use the SVO database to confirm the similarity of the transmission curves of the filters of your input and reference images. SVO contains the filter data for many of the observatories world-wide.