Clyde W. Tombaugh discovered Pluto on the 18 February 1930 about 23:00 UT in Flagstaff, USA - the chart below. The plates were photographed on the 23 and 29 January 1930 - the chart for the first plate above.

Robert von Heeren's article of his search for Pluto's discovery charts and about the connections between Chiron's and Pluto's discoveries http://www.kentauren.info/plutchir.htm