Complex sentence: The Guide Star Catalog (GSC), also known as the "Hubble Space Telescope, Guide Catalog" ("HSTGC"), is a star catalog compiled to support the Hubble Space Telescope with targeting off-axis stars.
Split into simpler sentences: The Guide Star Catalog (GSC) is also known as the "Hubble Space Telescope, Guide Catalog" ("HSTGC"). It is a star catalog compiled to support the Hubble Space Telescope with targeting off-axis stars.