This is (inside) the Mihrimah Sultan Mosque, also called the Iskele Camii (iskele is quay or jetty, it's the mosque facing the spot where the ferries to Istanbul make land). In was built in 1547-8 by Sinan for Mihrimah Sultan, daughter of Süleyman the Magnificent and wife of the Grand Vezir Rüstem Pasha (between the two of them they built some marvellous mosques).

On the picture: The upper half of the ‘mihrab’ (prayer niche) with its muqarnas (stalactites) decoration.
The painted decor, the result of a recent restoration, was executed in a plain Classical Ottoman style, close to the original 16th century murals; they are called ‘kalem işi’ (= work of the pen/paintbrush), because they are inspired on the geometric or stylised ornamentation used by Ottoman calligraphers to illuminate books.

Correspondent: J.M.Criel, Antwerpen.
Source: ‘Islamic Architecture: Ottoman Turkey’ (Godfrey Goodwin) – London 1977 .