To elaborate the point made by Andrea:
blocking IR contamination is the key to successful UV photography.
Using a dandelion as example, capturing with the UV-Nikkor lens on a D70 camera. This camera already has pretty good IR blocking as it was not modified at all. Even the stock model of D70 is able to record UV, but compared to modified cameras its sensitivity is of course much lower.
Visible light as a reference:
Using the Nikon FF filter that was delivered with the UV-Nikkor. As everything at that time was film-based and normal film stocks lack IR response, nobody talked about IR contamination back then.
Apparently the dandelion mop has no clear "bull's eye" pattern to boast of?
However, that
conclusion is entirely false, just look what happens if a better UV bandpass filter is deployed on the lens. Now IR contributions are eliminated and the real UV signature can be seen.
The full article is found here:
http://www.ultravioletphotography.com/content/index.php/topic/107-how-to-use-filters-for-uv-photography/