He made several observations of eclipses, both of the sun and moon, at Gresham College and in other places; and he was known particularly for inventing and improving planetary instruments.
[2] On the death of Henry Gellibrand, he was elected Gresham Professor of Astronomy on 2 March 1636, but resigned later in the year and was succeeded by Mungo Murray.
During the civil war and Commonwealth he was one of the society of gentlemen who met in London for cultivating the 'new philosophy,' in the group around Charles Scarburgh.
[3] Wallis's retrospective account of the origins of the Royal Society made Foster's lectures a rendezvous of the London-based Scarburgh-Jonathan Goddard group; but it is disputed to what extent this connection was with Gresham College and its tradition, rather than simply the location.
He published little himself, but many treatises written by him were printed after his death, though John Twysden and Edmund Wingate, his editors, state that long illness caused them to be left very imperfect, and Twysden complains that some people had taken advantage of his liberality by publishing his works as their own (Preface to Foster's Miscellanies).