10.5.1 Invoking astscript-zeropoint ¶

This installed script will calculate the zero point of an input image to calibrate it. A general overview of this script has been published in Eskandarlou et al. 2023; please cite it if this script proves useful in your research. The reference can be an image or catalog (which have been previously calibrated) The executable name is astscript-zeropoint, with the following general template:

## Using a reference image in four apertures.
$ astscript-zeropoint image.fits --hdu=1 \
                      --refimgs=ref-img1.fits,ref-img2.fits \
                      --refimgshdu=1,1 \
                      --refimgszp=22.5,22.5 \
                      --aperarcsec=1.5,2,2.5,3 \
                      --magnituderange=16,18 \
                      --output=output.fits

## Using a reference catalog
$ astscript-zeropoint image.fits --hdu=1 \
                      --refcat=cat.fits \
                      --refcathdu=1 \
                      --aperarcsec=1.5,2,2.5,3 \
                      --magnituderange=16,18 \
                      --output=output.fits

To learn more about the core concepts behind the zero point, please see Brightness, Flux, Magnitude and Surface brightness. For a practical review of how to optimally use this script and ways to interpret its results, we have two tutorials: Zero point tutorial with reference image and Zero point tutorial with reference catalog.

To find the zero point of your input image, this script can use a reference image (that already has a zero point) or a reference catalog (that just has magnitudes). In any case, it is mandatory to identify at least one aperture for aperture photometry over the image (using --aperarcsec). If reference image(s) is(are) given, it is mandatory to specify its(their) zero point(s) using the --refimgszp option (it can take a separate value for each reference image). When a catalog is given, it should already contain the magnitudes of the object (you can specify which column to use).

This script will not estimate the zero point based on all the objects in the reference image or catalog. It will first query Gaia database and only select objects have a significant parallax (because Gaia’s algorithms sometimes confuse galaxies and stars based on pure morphology). You can bypass this step (which needs internet connection and can only be used on real data, not simulations) using the --starcat option described in astscript-zeropoint options. This script will then match the catalog of stars (either through Gaia or --starcat) with the reference catalog and only use them. If the reference is an image, it will simply use the stars catalog to do aperture photometry.

By default, this script will estimate the number of available threads and run all independent steps in parallel on those threads. To control this behavior (and allow it to only run on a certain number of threads), you can use the --numthreads option.

During its operation, this script will build a temporary file in the running directory that will be deleted once it is finished. The --tmpdir option can be used to manually set the temporary directory’s location at any location in your file system. The --keeptmp option can be used to stop the deletion of that directory (useful for when you want to debug the script or better understand what it does).

• astscript-zeropoint output
• astscript-zeropoint options