I had a T-S 1.6x glass path correcter with Barlow in the post. Due to the distance to the eyepiece the resulting power will lie between 2.6x and 2.8x. So a 32mm EP will effectively become a 12mm. Whether the PST filter stack adds further power, due to path length, is still an unknown. More on this later.
Firstly, the GPC is smaller in outside diameter than the 1.25" fitting. Had it been the same size it would have provided valuable support for times when the binoviewer cannot be pushed right into a fitting.
Secondly, the S-T binoviewer has an undersized body thread compared with a standard filter thread. Just imagine if they had the imagination and daring to have fitted a standard 1.25" sized filter thread in the binoviewer body. Now anything and everything would fit the body. But no!
The empty binoviewer nose piece could be simply unscrewed and the GPC or Barlow[s] with 1.25" OD fitted directly to the body. Assuming, of course, it was the correct 1.25" diameter. Instead of which the daft GPC will not even fit into a standard 1.25" star diagonal when attached to the binoviewer! How crazy is that! Nor will it fit into the blind end of the PST eyepiece holder without pushing the binoviewer too high to clamp with the thumbscrew! Now what?!!?
How utterly futile! They chose a non-standard, undersized thread for the body which cripples all the huge potential had it been a standard 1.25" filter thread. Argh!! Ooh, it does make you cross. Well it makes me angry, anyway.
I fitted the binoviewer, fitted with 26mm Meade 4000 Eps without GPC/Barlow in the Vixen 90mm with a Baader Solar film filter in place. The Sun was now nicely large, sharp and 3D but completely unblemished by any of the typical features. Unfortunately it was a day plagued by cloud. With only rare thinning and almost no blue patches at all. It was slightly amusing to see the clouds crossing the 3D sun but visibly behind it.
When the GPC arrived mid-afternoon I added that to the mix and enjoyed much dimmer but larger views of a sun behind clouds. There was almost no free focusing movement left [at all] without the GPC and 65mm left with it fitted. I couldn't use the 1.25" star diagonal without the GPC. The 45° Baader diagonal used up far too much light path to work at all with the binoviewer regardless of fitting the GPC.
The major mechanical problem is the Vixen's single thumbscrew and very loose [oversized] eyepiece holder. It will not support the binoviewer without cosmetic damage and a general floppiness.