A couple of H-Alpha queries.

[Merlin]

Query 1. : With normal use, how long can we expect the blocking filter on a PST to last? And how do we tell when a blocker's no longer working? ( that's two queries! ).

Query 2. : When we place an ERF ( Energy Rejection Filter ) in front of the objective of the donor 'scope in a PST mod, is it a good idea to place an optical window in front of the ERF to protect the coating from moisture, etc.?

[JB80]

1. I can't really help you on that, I think it's dependent on what model blocking filter but I can't say for sure.

2. I don't think it would be a great idea to add anything in front, it would just add more variables and the ERF is very reflective so it could cause some odd images being shown. Try it though if you want, there would be no harm in that.

Personally I just use a bit of good paper towel clipped to the lens cap in when not in use and remove it when I want to use it.

[Peter Drew]

I have a PST blocking filter that still works well after several years of hard use, the current PST's have an additional mini ERF just in front of the blocking filter to further prolong its life, suggesting that exposure to solar radiation (use) is a factor. The front ERF in a PST mod can be placed internally behind the objective, this would help protect it and allow the use of a smaller diameter to save cost.