Planewave

You can use every 1.25" eyepiece where the position of the field stop is known. You can find the data in this PDF: https://www.baader-planetarium.com/en/downloads/dl/file/id/357/product/3117/hyperion_universal_zoom_mark_iv_specifications_drawing_and_technical_data.pdf
It depends on the configuration of the Mark IV, how the MPCC must be fonfigurated to be set to 55mm.

For the Canon Protective T-Ring, you need Baader M48 Spacer Ring for MPCC III / Protective EOS T-Ring # 2458405 as described on page 6 of the manual: https://www.baader-planetarium.com/en/downloads/dl/file/id/483/product/3004/mpcc_mark_iii_instruction_manual.pdf

The outer diameter of our 1.25" filters (including the cell mount) is 31 mm.

Please take a look at our Digiscoping-PDF at https://www.baader-planetarium.com/en/downloads/dl/file/id/444/product/1210/digiscoping_camera_adaptations_for_afocal_photography_and_eyepiece_projection.pdf where we describe several options to attach a camera or video module.

You need some distance between camera and eyepiece to avoid distortions at the edges; but with a smaller sensor, you can get closer. You'll usually achieve higher magnifications than with a Barlow lens; the formulas for calculating the effective focal lengths are inside of the PDF.

For large sensor and really high magnifications, we recommend the FFC # 2458200

Probably...

If you want to equip any 2" star diagonal with a ClickLock, you can always use
Baader 2" ClickLock 37 mm extension #2956237
or
Baader 2" ClickLock 47 mm extension #2956247

If you want to replace the nosepiece of a star diagonal with this nosepiece, it depends on the star diagonal. If you can remove the original nosepiece and if you can then access a female SC-thread (2" 24 GZ 60°), then you can replace one nosepiece with another - although this shouldn't make too much of a difference?

If you can remove the clamp of the star diagonal and can then see a male 2" SC thread, you can use the Baader 2" ClickLock SC/HD (2") clamp (C5" - C9¼") #2956220 to replace the original clamp.