This is spelled out in the README and LICENSE files, eg, the README says
Parts of the code written by the Astrometry.net Team are licensed
under a 3-clause BSD-style license. See the file LICENSE for the full
license text. However, since this code uses libraries licensed under
the GNU GPL, the whole work must be distributed under the GPL version
3 or later.
and the LICENSE tells you what you have to do, eg
Assuming you're including the GPL'd parts, then also must obey the terms of the GPL v3. This will probably be helpful: