It was found that the PST etalon will work at f/7.22 but not at f/6.86, and previously, another sample was found to work at f/7.14 as well.
Followup experiment should be conducted to add a barlows in front of the PST etalon to see if it will make it work with the Ranger.
Corrently possibilities are Tele Vue 2x and 3x barlows, as well as a 2.5x binoviewer corrector which should work at around 1.29x and 1.51x without and with extension tube respectivity. That would mean f/8.85 and f/10.35 respectively. There should be better than using the 2x and 3x barlows.
If it ever works, it will be the most elegant way to upsize, instead of going for a Cheapy-ERF with my C5, tilting will be a bit easier to do with a smaller ERF.