[NTG-context] Align the chapter-title to the middle of the header

Dear list, how can I align the text of the header in the double-page layout to the middle? I want something like that: \setupheadertexts [{\startalignment[middle] \definedfont[name:Gentium_Basic at 9pt]\getmarking[chapter] \stopalignment}][] [][] Of course, that does not work. By default, there are only the four places to set the text in the header: left and right in the header for both pages. How to place the chapter-title (or whatever) into the middle of the header? Thanks. Huseyin