"Did you ever tried to do astrophoto with the 18-135 ?"
Not the lens to use, I have the Samyang (Rockinon) 12mm f/2 for astro photography with the XT1, which incidentally is fantastic for astro work, I tried first with my 14mm, I wasn't entirely happy with the results so bought the Samyang. Sample attached. EXIF 25s @ f/2 ISO 4000, Manual exposure and focus, bulb setting, remote, tripod.
Hope this helps, I have that lens that you used and have had no issue with it for normal photography.