Camera-
My experience is only with Nikon.
If you plan on doing only UV work (no IR) with a D70 you do not need to modify it, in fact it may be beneficial to keep it as it is from the factory. The reason I say this is because if you do remove the internal filter pack of a D70 you will gain only about 1/2 stop sensitivity in the UV range but 6-7 stops sensitivity in the IR range if I remember correctly. Your challenge (depending on your filter) would then be keeping IR contamination out of your photograph (see below). The D70 does quite well in the UV spectrum straight from the factory. All nikon DSLR's after the D70 have a stronger internal filter and would need removal of the internal filter pack to work efficiently in the UV spectrum (D40, D80, D200 cameras). Camera's with CMOS sensors (D2x, D300, D700 for example) are not as sensitive in the UV spectrum as CCD sensor cameras from what I understand, that is not to say you can not do UV with CMOS.
If you do want to remove the internal filter from your D70 it is quite simple. There is a tutorial to follow at lifepixel.
http://www.lifepixel.com/IR.htm
I removed the internal filter pack and left the sensor bare. I do not recommend doing this however. Lifepixel makes a UV/VIS/IR pass clear filter that you may want to consider, though it is rather expensive. There may be other suppliers such as maxmax but I am not certain.
http://www.lifepixel.com/shop/cart.php?m=product_detail&p=21
Reason being is that the removal of all filters in front of the sensor is in effect moving the register distance of the lens further away which will can make achieving infinity focus a more difficult feat. F mount Nikkor lenses seem to do fine but some of the enlarger lenses and various other lenses I use for UV that are not designed for F mount don't quite reach infinity focus. If I had the clear filter in place they would.
Filter-
Hands down with out a doubt the Baader-U filter is the best out there. All other filters that I know of, such as the B+W 403, need an additional filter to block IR contamination, such as he B+W bg38, And still I dom not believe they are as efficient. Baader made a 1.25" diameter filter named the Baader-U "venus" filter and since then have made a more efficient 2" diameter that better meets the needs of photographers. The 6-7 stop increase in IR sensitivity of a converted camera is fully mitigated with the Baader-U filter, in my experience.
http://www.company7.com/baader/options/u-filter_bpu2.html
For example here is a picture using the B+W 403 in direct sunlight. What you see is mainly an IR capture.
And here is the same scene with the Baader-U
I hope it is ok for me to post a link to another forum with a wealth of information on this topic. The people that frequent this forum are far more knowledgeable on the subject than myself. They will get down to the science of UV photography where I prefer to stay at the pictorial value of it for the most part. You will find the author of the link above (
http://www.naturfotograf.com/uvstart.html) Bjørn Rørslett (nfoto) a frequent contributor here as well.
http://nikongear.com/smf/index.php?board=57.0
and a plethora of information
http://nikongear.com/smf/index.php?topic=22838.0
Best of luck
