Artist mainly in etching and aquatint but also relief, watercolour and oil, teacher, born in London. He studied at Slade School of Fine Art with Randolph Schwabe as principal, 1944–7. Taught at Folkestone Art School, 1966–70; Medway School of Design, 1970–5; New Metropole Arts Centre, 1975–6; then part-time at Medway College, 1976–91. Koster was a founder-member of SWLA and among influences on his work were Thomas Bewick, Eric Ravilious and Allen W Seaby for birds. Koster aimed to produce work “with a wide appeal but also with authenticity”. He created a mural for Royal Veterinary College and completed wood engravings for John Stewart Collis’s Down to Earth. Participated in many group shows, including RA Summer Exhibitions from 1960; New Metropole Arts Centre from 1961; Bear Lane Gallery, Oxford, from 1964; John Nevill Gallery, Canterbury, from 1975; and Daler Gallery, Bournemouth, 1990.