This thread was very useful. After a few years in storage I very recently took my Lunt LS60T out and discovered it had this problem. Spent a whole day thinking it was just hazy cloud! Finally had a look and saw the issue.
My scope is 10 years old and I am the original owner. Lunt quickly put me contact with Brewer Germany and my replacement filter has just arrived - no cost to me. I am sure I saw one on the Lunt site for about $7.
I am very happy with this level of aftercare!!
Lunt were able to tell me that the blue glass is a UV Cut filter.
The newest version has a coating to inhibit this build up.
They also confirmed it was OK to try and clean the old BG filter and suggested CLR.
I used stuff lying around the house which didn’t do much so I polished it up with a soft wheel at low speed on the Dremel and whatever the red compound is that came with the Dremel. I’m not suggesting that anyone do this of course. I did it knowing that I could completely ruin it.
I was careful not to scratch the glass!
Finished the process with a little bit of car polish.
Result was very effective, not perfect, but a massive difference. Had a few minutes with a decent view and ran off a few AVIs.
It’s been cloudy or I have been busy ever since!