Coronado PST eyepiece holder disassembly

Following on from my thoughts that I might have to move the blocking filter on my modded PST closer to the camera sensor, I decided to take it apart to see what I might be dealing with.  Though I’ve got rid of the “black box” on mine, the holder is still exactly as it came off:

First step was to split the barrel into its two component parts.  Easier said than done.  Everything is welded together with thread-lock and it just didn’t want to budge.  By coincidence I had some large soft rubber tree ties arrive this afternoon and by wrapping one around each half of the barrel I was eventually able to get them to turn without having to resort to tools.  This is the inside of the bottom half.  Despite the fact that it cannot have been apart since it was assembled at Meade’s factory, the inner face of the ITF was disgustingly dusty.  So bad, in fact, that I think it’s quite clear even in this photo.

And the top half, with the retaining ring for the BF5 blocking filter visible:

I unscrewed the retaining ring to find that the filter is actually a block of material rather than a sheet as I was expecting:

It’s loose, just sitting in a slight circular depression inside the body:

Apologies for the quality of these next few pictures.  I forgot how poor the digital macro is on the camera I was using.  These show three views of the blocking filter.  You can see how raw the back (camera side, rather than objective side) edges of the filter are 🙁

There were some large dust particles on the gold face of the filter, too.  I took the opportunity to clean the faces, but I think perhaps it’s partially scratched (again, how?  It’s never been opened since it left the factory).

It seemed to make sense to have a go at cleaning the ITF too, so I unscrewed the ITF body from the bottom section of the eyepiece holder:

It’s threaded, but also held in with some nasty rubbery adhesive as well.  The “inside” of the ITF unit has a retaining ring in, so I also removed that and dropped out the ITF itself:

Another poor photo, but there’s some writing on the side:

The ITF is “outside up” in this photo.  The upper face goes towards the objective.  I gave the ITF a light clean with a microfibre cloth and left things there.

I estimate that the blocking filter is about 4mm thick.  I measured across the sides with vernier calipers and they were 6mm, but I wasn’t going to touch either optical face.  Allowing 2mm for the retaining ring and another 2mm for the housing, that doesn’t leave me with much to play with if I want to have the filter no more than 11.1mm from the sensor to avoid vignetting.  I’m going to have to give this a fair bit of thought, I think.  Worth it, given that the alternative is probably $500-worth of BF-10 diagonal 🙁