Most people want to see males down on the ground, feeding. Fine, but those who concentrate on that largely miss out on the real Purple Emperor.
Males mainly seek sustenance from ride surfaces – particularly from made-up hardcore / hogging rides – in reasonable weather during the first two weeks of the season. They rarely descend thereafter.
The best time of day is mid morning, but much depends on temperature. In hot weather breakfast can take place much earlier (from 8am), notably after cloud (or wind) prevented an evening flight the previous day.
Early in the season (effectively the first week) fresh males can be seen on the ground at any time of day, including on warm evenings.
In cool showery weather visits to the rides are rare, even early in the season. Primarily, ride visiting is a feature of warm or hot weather.
Females rarely descend to the rides, and seem to do so only in very hot weather. Matthew Oates has not seen any definite females down on rides since the long hot summer of 1976. Neil Hulme has seen five, all on hot afternoons, probing for moisture not mineral salts, and all very wary and hard to approach.