A lovely image, Philippe!
Using a 10mm lens certainly reduces the star trails. :-)
Have you considered/tried Sequator?
"Sequator is free software which can track stars on multiple images, align
stars and stack them. If you don't have an equatorial mount, by stacking
non-blur star images in short-time exposures, the result will be almost similar
to long-exposure on an equatorial mount. "
Which means you should be able to use longer focal lengths. It's sort of like
a panorama stitcher for star shots.
More info & download, here:
https://sites.google.com/site/sequatorglobal/download
I've downloaded it, but not yet had the chance to try it.
Has anyone given it a go?
David.
Tested long exposure on the sky last night.------
http://gallery.leica-users.org/v/Phileica/Playground/Stars-1354-
2.jpg.html
May be viewed larger.
Amities
Philippe