If Newton Rings are a problem and you can't find the AN filter, there is a very cheap and efficient solution. The aim is to place a tiny bit of space between the surfaces of the neg and the condenser. So, what do you do? Well, with a scalpel just cut a hole slightly larger than 24x36mm into the center of a strip of 120 film. You may need to give it a few tries to get a decent cut. Once you have that done, you simply secure the neg with tape on the neg carrier in order that it stays nicely flat; put the perforated strip of 120 film on top and finally lower the condenser. Works great for me. I also have the AN glass, although with bit of muck on it that won't come off, so I prefer this method.