Just picked up one of these used about two weeks ago. Noticed that the glass that came in the drop in filter holder has some scratches and small dings on it. Just wondering if this piece of glass is absolutely necessary. I tried doing some googling on it and the only thing I could is that the optical scheme/formula takes that piece of glass into account whether it's just the glass or an actual filter so it should be left in. Is this accurate? If so, what recommendations do you all have for replacing it? Any assistance would be greatly appreciated.