I have long known that my 12.5" telescope suffers from contrast issues. I would constantly compare my scope to others and see a noticeable difference. The 16" also suffers from this malady. Contrast issues can be one of two things, overall contrast (i.e., grey background skies) and small contrast (i.e., fine planetary detail). My problem is with overall contrast where the faint wisps of galaxies can be lost in the grey background sky. This type of contrast problem boils down to unwanted light sneaking somehow into the optical path.
I went searching on the net on ways to solve it and I found a post on www.cloudynights.com that discusses how you can identify where that light is coming from. Mike Jones wrote an article in Astronomy Technology today that describes that technique. Essentially, you use a long focal length eyepiece and a another eyepiece stacked on top of it. Mike says in his forum post: "The second eyepiece must have a real focal plane such that you can use it like a loupe, like an orthoscopic, Plossl, etc."