Thank you. I've just tried that and I get the following output
tim@dell-desktop:~$ sudo apt list |grep cfits WARNING: apt does not have a stable CLI interface. Use with caution in scripts. libastro-fits-cfitsio-perl/focal,now 1.12-1build2 amd64 [installed] libccfits-dev/focal 2.5+dfsg-2 amd64 libccfits-doc/focal,focal 2.5+dfsg-2 all libccfits0v5/focal 2.5+dfsg-2 amd64 libcfitsio-bin/focal,now 3.470-3 amd64 [installed] libcfitsio-dev/focal,now 3.470-3 amd64 [installed] libcfitsio-doc/focal,focal,now 3.470-3 all [installed,automatic] libcfitsio8/focal,now 3.470-3 amd64 [installed,automatic] slang-cfitsio/focal 0.3.8+nosvn-5build4 amd64