What is the “Back-focus”? / バックフォーカスとは?

Back focus is the length of the tail of light cone out of the drawtube.

At planning your binoscope with some OTA, your first task is to decide the D in the diagram below and check the back-focus if it will accommodate the EMS or not.

When your planning OTA has less back-focus for your plan, you can replace the end adapter, or the focuser for more low-profiled ones.
Cutting short the OTA tube is the last resort when you know all other measures are not enough.
昨今の高級鏡筒は昔よりもバックフォーカスが長い物が多く、少しバックフォーカスが足りない場合も、大抵は、ドローチューブ末端のアダプターを短い物に交換/改造するか、それでも足りない場合はフォーカサーをlow-profileな物と交換することで解決することがほとんどです。 鏡筒の切断短縮は最後の手段です。