Martin,
The 85mm f/1.8 USM is an excellent portrait photography lens, however, it's not designed for astrophotography.
The longer focal length L-series lenses such as the 200mm f/2.8L II USM, or 300m f/2.8L IS USM, and even the 300mm f/4L USM are better suited to flat fields.
And, yes, you must stop down these lenses, unless you wish to use an external aperture mask -- the benefit of the external mask is that it prevents diffraction spikes caused by lens diaphragm.
Here's a field that I did with the 200mm f/2.8L II USM on my old 40D (now sold; same sensor size as your 550D):
http://members.optusnet.com.au/mrozy...elt_Orion.html
H