The current reference standard UV-pass filter for UV imaging is the 2″ Baader U. It is costly, but well worth the investment as there is no visible or IR-leak with this filter.
For those on a budget, a filter stack comprising a UV-pass filter and an IR-block filter will also perform well when the rest of the equipment are up to the task.
UV-pass filter – the best ones are the Schott UG11 or Hoya U340 filters in 1 or 1.5mm thickness. Bear in mind that the Baader U is a UG11 filter with multiple dichroic coatings applied to both surfaces.
IR-block filter – the S8612 passes the most UV and is the most efficient IR-block, especially at 1.5 to 2mm thickness. Other suitable filters are the BG38, BG39 and BG40, but these do not perform as well as the S8612. These filters should all be kept dry as humid conditions will result in damage i.e. needs to be kept in a dry box with silica gel.
Here are two images of Rudbeckia taken with different UV-pass filters:
To compare the prices:
Baader U 2″ £220+
U340 + S8612 in 25.5mm filter – less than half of the price of Baader U 2″. Do remember that the smaller the filters, the less expensive they will be.
How to use 25.5mm filters?
As you can see, the 25.5mm filters fit into the c-mount female threads of this M42 (c-mount) to Micro 4/3rds adapter (bought from eBay), while the M42 focusing helicoid fits into the M42 female threads. All you need now is an M42 lens.
I would recommend looking here if you are interested in filter stacks for UV-imaging, as this is whom I bought my filters from: