Which Ritchey–Chrétien telescope in the European Southern Observatory has the most observing time available?
select ?ent where { ?ent wdt:P31 wd:Q1062138 . ?ent wdt:P3891 ?obj . ?ent wdt:P137 wd:Q151991 } ORDER BY DESC(?obj)LIMIT 5 
ritchey–chrétien telescope: Q1062138 european southern observatory: Q151991