The output will be a multi-extension FITS table.
The first table in the output gives the zero point and its standard deviation for all the requested apertures.
This gives you the ability to inspect them and select the best.
The other table(s) give the exact measurements for each star that was used (if you use `--keepzpap`, it will be for all your apertures, if not, only for the aperture with the smallest standard deviation).
For a full tutorial on how to interpret the output of this script, see Zero point tutorial with reference image

If you just want the estimated zero point with the least standard deviation, this script will write it as a FITS keyword in the first table of the output.

`ZPAPER`

Read as “Zero Point APERture”. This shows the aperture radius (in arcseconds) that had the smallest standard deviation in the estimated zero points.

`ZPVALUE`

The zero point estimation for the aperture of

`ZPAPER`

.`ZPSTD`

The standard deviation of the zero point (for all the stars used, within the aperture of

`ZPAPER`

).`ZPMAGMIN`

The minimum (brightest) magnitude used to estimate the zero point.

`ZPMAGMAX`

The maximum (faintest) magnitude used to estimate the zero point.

A simple way to see these keywords, or read the value of one is shown below. For more on viewing and manipulating FITS keywords, see Keyword inspection and manipulation.

## See all the keywords written by this script (that start with 'ZP') $ astfits out.fits -h1 | grep ^ZP ## If you just want the zero point $ astfits jplus-zeropoint.fits -h1 --keyvalue=ZPVALUE

GNU Astronomy Utilities 0.20 manual, April 2023.