Hi Fred, Hi Eric,
According to a report by Christian Buil, the front glass on the 40d has basic UV/IR filter properties. Check the before/after response curves in fig.6 on his website
http://www.astrosurf.com/buil/eos40d2/filter.htm
It seems you can almost get away without a UV/IR filter although adding one would give you tighter stars.
Yes... I'm looking at doing the same as Fred, simply remove the filter glass, keep the front glass and transform the 40d into a dedicated astro camera. Keep my 400d for daylight.
Cheers,
Rob