R. Lattimore's translation is neat and accurate:
There is a rocky island there in the middle channel halfway between Ithaka and towering Samos,
called Asteris, not large, but it has a double anchorage
where ships can lie hidden. There the Achaeans waited in ambush.
I assume that Homeric Ithaca is the island now called Ithaki and that Samos is Kephallinia. The channel will then be the Ithaca Channel, and here there is only one island, now called Daskalio (FIG. 1). So Daskalio = Asteris. So far, so good; Homer has deftly pinpointed the location of the ambush by associating it with the only small island off the west coast of Ithaca.
Daskalio is certainly πετρήεσσα; indeed it is nothing but a narrow shelf of rock about 200 yards long, and rising only about 15 feet above water level (Plate IIIa). No one can deny that it is οὐ μεγάλη; the phrase may well be a litotes. It is not strictly in ‘mid-channel’, being 3,000 yards from Ithaca and only 800 yards from Kephallinia, but this may pass in a poetic description. It is with the ‘double anchorage’ that Homer's description appears to lose touch with reality.