The Schuler UV filter
Once you have acquired a DSLR and a lens that perform acceptably in the near-UV, you are only half-way to the goal of UV photography. There are a few choices of UV-pass filters available, like the U-360. However, all the normally available ones (including the venerable Nikon FF) let through also substantial amounts of near-IR. Since most DSLRs are far more sensitive to IR than to UV, and sunlight contains copious amounts of IR, using these filters with natural illumination means that you are, in effects, taking IR pictures containing just a small tinge of UV. Some UV photographers have solved this problem by stacking together UV-pass and IR-blocking filters (mostly of the hot-mirror type). One problem with this solution is that many hot-mirror filters are not designed for UV transmission, so you loose much of it.
I decided to try another way, and to look into the use of filters designed for astronomic observation. Astronomers often need to isolate narrow spectral bands and to cut out everything else. In the context of a UV-pass, cut-everything-else filter, two or three types are repeatedly mentioned. One is the Baader U filter (also called Venus filter because it allows the observation of cloud bands on this planet). The second type is the Schuler UV filter, which is built in an entirely different way. It is a thick (over 3 mm) cemented stack of a UV-pass glass filter (with the typical IR leak of these materials) and a cyan-coloured red- and IR-cut filter (probably a Hoya BG-38 or BG-40). Therefore, virtually no IR passes through the stack (see astrosurf.com). The Schuler UV filter is significantly cheaper than the prices normally asked for the Baader U, and also cheaper than attempting to build your own stack of UV-pass and IR-cut filters. It may even be cheaper than the traditional solution of stacking an ordinary UV-pass filter and a hot mirror.
The following discussion is based on a 2" Schuler UV mounted in a metal cell with 48 mm thread. The Schuler UV filter transmits roughly 60% of near-UV, as opposed to 80% for the Baader U/U2. The spectral properties of the two filters are also not identical, with the Schuler peaking at 360 nm and transmitting even a little visible indigo, and the Baader U/U2 extending farther into the UV. However, these are not very large differences for UV photography, and practical results may be counterintuitive (e.g., astronomers have reported 30% brighter images with the Schuler). If you want pure UV, you should use the Baader. If you are interested in non-monochromatic results, or simply want to spend less and still obtain good results, you may use the Schuler.
My choice between Baader and Shuler filters at the time was based on the desire to keep some colour information, and doing so while paying a lower price did have its attractiveness. Therefore, the Schuler UV filter was my natural choice (I now have both alternatives). I found no source in the EU and a couple of US-based Internet shops could not ship it abroad because of commercial agreements, but Adirondack Astronomy did, and a 48mm Schuler UV is now sitting in my bag of tricks (mounted on an adapter to use it as a standard 52mm lens filter, and on a Nikon AF-1 holder to swing it out of the way when focusing).
There is also a 1.25" Schuler UV filter available, which could be useful in macrophotography because of its small diameter. However, my specimen of this filter has very different optical properties than the 2" one discussed above. Like its big brother, it is a stack of two cemented filters mounted in a metal cell, but the IR-cut one, instead of being cyan-tinted, is transparent to the eye and, unfortunately, is also transparent in the near-IR that it is supposed to block. As a result, this filter leaks massive amounts of near-IR and is completely useless for UV photography with digital cameras. It must also be noted that the 2" Schuler UV filter in my possession is almost completely opaque to the eye, while the 1.25" filter transmits enough deep-red to allow me to see rather easily the details of a sun-lit panorama. To put it simply, they are quite different filters.
I don't know whether this 1.25" Schuler UV filter I received is an old stock or a newer model - I obtained it from the same shop that sold me the "good" 2" one a year earlier, and is packaged in the same type of plastic box without any shrink-wrap or seal. In any case, watch out for this batch of unusable 1.25" Schuler UV filters that is currently available on the market.
Both the 2" and the 1.25" Schuler filters are identified by ugly adhesive labels of ordinary paper, apparently printed on an ordinary laser printer and stuck by hand, slightly askew, onto the mount. They wear out quickly while handling the filter and leave a sticky glue residue on the mount. Filters that retail for 89-300 US$ (based on web prices) probably should have better labels.
The pictures at the left, of the same subject, were taken with the U-360 (top) and Schuler UV filters (bottom). The two images were too different from each other to post-process in the same way, so I was forced to process each picture separately to provide the best visual result possible with the information present in each picture. The results speak for themselves. While the U-360 is in practice an acceptable (if expensive) IR-pass filter, the bottom picture is the only one that can be called a UV image, and the only one that shows the characteristic UV pattern used by insects to home-in on the centre of the flower (i.e., the darker areas of the petals near the centre of the flower).
These pictures were taken with a UV fluorescent tube and an incandescent lamp (for framing and focusing), a D70s modified for full-spectrum photography and a UV Rodagon 60mm at f/11 on ultra-short bellows. Exposure was manual (1 second with U-360, 8 seconds with Schuler UV).
There is also a Schuler SP 470 astronomical filter that transmits UV, indigo and blue, roughly in a range between 350 and 470 nm, and apparently has no leak in the IR range. It might or might not be useful in photography (UV information usually is recorded at least in part in the blue channel, so it would superimpose the blue information, probably with no way to separate them).
If you shoot UV macro photography and are worried that the thickness of the Schuler UV filter may cause a focus-shift, there are sets of coloured Schuler filters that include a transparent one designed for parfocality. Use it to compose and focus, then swap it with the UV filter. So far, I did not have this problem with reproduction ratios as high as 1:2.
The Schuler UV and Baader U filters are more different from each other than I initially thought. A side-by-side comparison of results is available here.