GNU Astronomy Utilities astscript-zeropoint options

All the operating phases of the this script can be customized through the options below.


The HDU/extension of the input image to use.

-o STR

The name of the output file produced by this script. See astscript-zeropoint output for the format of its contents.


The number of threads to use. By default this script will attempt to find the number of available threads at run-time and will use them.

-a FLT,[FLT]

The radius/radii (in arc seconds) of aperture(s) used in aperture photometry of the input image. This option can take many values (to check different apertures and find the best for a more accurate zero point estimation). If a reference image is used, the same aperture radii will be used for aperture photometry there.


Range of the magnitude for finding the best aperture and zero point. Very bright stars get saturated and fainter stars are affected too much by noise. Therefore, it is important to limit the range of magnitudes used in estimating the zero point. A full tutorial is given in Zero point tutorial with reference image.


Name of catalog containing the RA and Dec of positions for aperture photometry in the input image and reference (catalog or image). If not given, the Gaia database will be queried for all stars that overlap with the input image (see Available databases).

This option is therefore useful in the following scenarios (among others):

  • No internet connection.
  • Many images having a major overlap in the sky, making it inefficient to query Gaia for every image separately: you can query the larger area (containing all the images) once, and directly give the downloaded table to all the separate runs of this script. Especially if the field is wide, the download time can be the slowest part of this script.
  • In simulations (where you have a pre-defined list of stars).

Through the --starcathdu, --starcatra and --starcatdec options described below, you can specify the HDU, RA column and Dec Column within this file.

The reference image or catalog probably have many objects that are not stars. But it is only stars that have the same shape (the PSF) across the image233. Therefore


The HDU name or number in file given to --starcat (described above) that contains the table of RA and Dec positions for aperture photometry. If not given, it is assumed that the table is in HDU number 1 (counting from 0).


The column name or number (in the table given to --starcat) that contains the Right Ascension.


The column name or number (in the table given to --starcat) that contains the Declination.

-c STR

Reference catalog used to estimate the zero point of the input image. This option is mutually exclusive with (cannot be given at the same time as) --refimgs. This catalog should have RA, Dec and Magnitude of the stars (that match with Gaia or --starcat).


The HDU/extension of the reference catalog will be calculated.

-r STR

Right Ascension column name of the reference catalog.

-d STR

Declination column name of the reference catalog.

-m STR

Magnitude column name of the reference catalog.

-s FLT

Matching radius of stars (in arc seconds) and reference catalog in arc-seconds. By default it is 0.2 arc seconds.


Reference image(s) for estimating the zero point. This option can take any number of separate file names, separated by a comma. The HDUs of each reference image should be given to the refimgshdu option. In case the images are in separate HDUs of the same file, you need to repeat the file name here. This option is mutually exclusive with (cannot be given at the same time as) --refimgs.


HDU/Extension name of number of the reference files. The number of values given to this option should be the same as the number of reference image(s).

-z FLT,[FLT]

Zero point of the reference image(s). The number of values given to this should be the same as the number of names given to --refimgs.


Keep the table of separate zero points found for each star for all apertures. By default, this table is only present for the aperture that had the least standard deviation in the estimated zero point.


Directory to keep temporary files during the execution of the script. If the directory does not exist at run$-$time, this script will create it. By default, upon completion of the script, this directory will be deleted. However, if you would like to keep the intermediate files, you can use the --keeptmp option.


Its recommended to not remove the temporary directory (see description of --keeptmp). This option is useful for debugging and checking the outputs of internal steps.



The PSF itself can vary across the field of view; but that is second-order for this analysis.