Enter positions, times, missions, ... to query the
HEASARC database.
Try ROSAT 3c273 1d to get ROSAT data within one degree of 3c273 or chanmaster bii>80 status='archived' to get archived Chandra Observations data near the north galactic pole.
Note: For more than one target or when using any qualifier other than a mission name, use quotes around targets that have embedded white space.(e.g., 'ar lac').