We're using a Hoya UV-pass filter which we bought from UQG Optics, but you can buy it on Edmund Optics as well for $26.50. Here are some of its specs:
U-340 12mm Diameter Bandpass Filter Stock #: NT46-096 Thickness (mm) 2.50 Center Wavelength CWL (nm) 340.00 Full Width-Half Max FWHM (nm) 85.00 Density (g/cm3) 2.92 Diameter (mm) 12.0 Dimensional Tolerance (mm) ±0.25 Thickness Tolerance (mm) ±0.2 Surface Quality 80-50
Its spectral profile is shown in this graph:
Though our CMOS camera could probably sense pretty low here, it's limited by the glass lens, which blocks a lot of UV light. We're not sure where the cutoff is.
The camera we used is the Canon SD4000 IS, which is a rear-illuminated CMOS camera. We removed the hot mirror filter, and added the Hoya UV-pass filter to the front. Rear-illuminated CMOS sensors are very sensitive in the UV range, as shown in this spectral sensitivity curve:
Here's a photo showing the image half-blocked with a simple UV filter which I believe is just a sheet of thick glass: