Probably a cultivar or hybrid of Rhododendron indicum Sweet. Galle lists two azaleas named ‘Optima’.  The first is a late flowering amoena hybrid with large, deep pink flowers which may correspond the plant named Optima figured in Paxton's Magazine of Botany [MB p.55/1845].  The flowers are bright scarlet with deeper spots.  English bred, its parents are unknown.  This is similar to Azalea optima described in the Floricultural Cabinet: ‘Bright rosy-scarlet; flower large and showy.  the plant six feet by four.’  [FC p.137/1848], and: ‘Fine orange, with darker blotch on upper segment; flower large, good form, and very showy.’  [FC p.148/1850].  This is probably Macarthur’s plant.  Wilson & Rehder provide some information on the probable origins of this plant.  ‘Azalea “Optima” with dark scarlet flowers [and] Azalea “Prince Albert” […] were undoubtedly derivatives of R. simsii and representatives of the “Indian Azaleas” of to-day, and the first introductions into America [in 1847] as far as I can discover.’  [Wilson & Rehder p.48].

Galle’s second azalea is an old Ghent Hybrid with yellowish-white flowers, edged reddish-orange, and with a yellow blotch.  This is unlikely to be Macarthur’s plant.