It's been mentioned here before, but I thought I'd freshen up the discussion.
Lunt Ha solar scopes have a small blue disc of glass as part of the blocking filter. If you take the eyepiece off your scope and peer down the whole in the diagonal, you will see it. For some reason, this filter crusts over after a while and many telescopes have terrible optical performance as a result.
It is seemingly impossible to shift, but had been discovered that the product CLR works and it was suspected that the key ingredient was Phosphoric Acid - the rust eating component. I just happen to have a bottle of pure Phosphoric Acid which is used in the fish keeping business as a safe way of lowering pH in aquariums and then I discovered a spare and cruddy blue disk...
It took about 60 seconds to go from pathetic to pristine!
