Eduard Ender's painting is the centrepiece of this exhibition. But as an image of 'The Noble Dane' it is not without problems and in many ways the painting poses more questions than it answers.

The title and thus the subject of the painting seem well attested by contemporary reports of Ender's work. We might expect a scene of Tycho and Rudolph II to show a strongly typecast figure of the astronomer and here we indeed have that figure, hand on sphere, with astronomical instruments and cosmological and astrological diagrams to signal the identification.

The depiction of Rudolph as a painter, however, comes as more of a surprise. Is the image of the emperor with palette in hand and large (but hidden) canvas merely a painterly conceit? Perhaps, but Rudolph is now probably best remembered as a patron of the arts, moreover one who took a very close personal interest in the work of such principal court artists as Arcimboldo and Spranger.

Nor was he content just to observe and supervise. One of Rudolph's contemporaries recorded that as well as making daily visits to view the work of artists and craftsmen, Rudolph sometimes took up the brush himself. Ender may have been exploiting this historical fact to celebrate the imperial status of his art.

Whatever we may think of such issues, the painting does pose a more fundamental iconographic challenge. Is the figure with his hand on the globe really Tycho? Risking the subversion of the entire rationale of the exhibition, it is worth reconsidering the identity of Ender's astronomer.

The images in The Noble Dane show that Tycho's iconography was well established. He is typically shown bearded and with a prominent moustache, his head compact and rounded and with some indication of his notoriously disfigured nose.

Of these elements, the only distinctive one present in Ender's painting is the moustache, whose characteristic shape has been lengthened beyond typical bounds. The face is rather elongated and Tycho himself easily exceeds the stature of the figure behind him.

Yet the older engravings suggest that Tycho might have cut a more squat figure, lacking the elegance of pose which Ender has adopted. Even more unsettling is the slender and finely formed nose which Ender has given to his astronomer. Although fastidious in his depiction of instruments, Ender has nevertheless chosen to ignore perhaps the most distinctive feature of his subject.

Did Ender deliberately attempt some cosmetic adjustments to improve Tycho's appearance and give him a more 'noble' aspect? Or did he simply use alternative iconographic sources? If it was not for the moustache, which seems a definitive mark of Tycho, it would be tempting to suggest that Kepler rather than Tycho had served as a model for Ender. Doubtless, other real or imagined similarities could be summoned. At first glance the sprightly hair might even recall a little of Galileo's pugnacious crop in the portrait frontispiece to his Il saggiatore.

But even if Ender had straightforwardly 'got it wrong' in his painting he would certainly not have been alone amongst nineteenth-century commemorators of the scientific past. A more prominent - and more embarrassing - slip was made in France at the end of the century. Some years after a public statue of Lavoisier had been erected in the 1890s, it was discovered that the sculptor had copied the face of Condorcet, Lavoisier's philosopher colleague in the Académie des Sciences, rather than Lavoisier himself. Lavoisier's literal loss of face continued until the Second World War, when the statue was melted down.

Whoever served as a model for Ender - whether Tycho alone, or a Tycho 'improved' by borrowing from elsewhere - the result has at least survived and the restoration of Ender's work has revealed an image which, irrespective of its often pedestrian genre, remains enigmatic not only in its setting and narrative but even in the identity of one of its principal characters.