Abell 21 object is considered in Telescopius as a planetary nebula (other name: Medusa nebula) located at 7h29 / 13°15 near Gemini constellation :
A search on google over Abell 21 confirm these information, considered as such everywhere (Wikipedia, astro amateur sites, etc.). Well, everywhere… except on the official catalogues !
A search on “Abell 21” in SIMBAD, for instance, gives a Galaxy cluster located at 20h30.8 / 28°37’39 near Pegasus constellation :
The same in NED:
Stellarium locates it also as a galaxy cluster in Pegasus.
That’s quite strange, it seems there were an error made at some point and it has propagated everywhere. On what database is based Telescopius for the objects names and coordinates?