I got brave this evening and disassembled the eyepiece holder to the point I’d reached as described a few days ago. I then examined the BF5 housing and decided that it appeared to fit into the eyepiece holder from the top. This seemed counter-intuitive given the design of the lower half of the holder which appears to be intended to trap the housing against the upper half. Anyhow, as there was no obvious means to gain purchase on the housing to turn it, I thought I’d give it a bit of a push up through the holder to see if anything happened.
Somewhat unbelievably given that just about everything else that comes apart on the PST is smothered in threadlock, it gave quite easily and I was able to push the housing out of the top of the eyepiece holder, leaving me with this:
There’s a flange on the housing that rests on top of a shoulder inside the eyepiece holder. Interestingly, the flange is quite a snug fit in the bottom of the holder whereas it’s very loose at the top, so the inside of the holder must be tapered.
So, my plan is now to turn a retaining ring with a T2 thread on the outside and either a 1,.25″ nosepiece or a thread to fit the lower half of the eyepiece holder on the inside, with a shoulder to hold the entire BF5 unit central in the ASI120 thus:
The measurements I’ve taken from the camera and BF5 unit are:
ASI120 sensor to flange: 12.5mm
ASI120 flange to case: 7.5mm
ASI120 sensor to case: 5mm
BF5 housing depth: 8.5mm
BF5 retaining ring thickness: 2mm
This would give a “sensor to front of BF5” distance of 11.5mm. That’s 0.4mm more than I think I need to get an image with no vignetting, but perhaps worth a go nonetheless. If I drill the retaining ring out to 5.5mm then it will hopefully mean that any vignetting occurs at the edges of the sensor and doesn’t affect the actual image of the Sun.