[NTG-context] How to use charter?

Hi all, I am trying to use the Bitstream Charter as a font. Unluckily, all the umlauts fail to appear. Here is how I do it: \definefont[bchr][bchr8t at 12pt] \starttext \bchr \"a \"o \"u \SS{} \stoptext Any idea why that happens? Could someone explain to me (or point me to some documentation) on what exactly happens when I do something like the above? Or is there a better way to do it (I guess so...)? Greetings, Alex