Rings and Plates

We recommend 30mm distance between camera and eyepiece for APS-C-cameras, as described in this PDF: https://www.baader-planetarium.com/en/downloads/dl/file/id/444/product/1211/digiscoping_camera_adaptations_for_afocal_photography_and_eyepiece_projection.pdf

The focus position with the camera should be quite close to that of the eyepiece, when you look at it with your eye. If you can't reach focus for visual observations without the star diagonal, then you should use it also for photography (or some extension tubes between focuser and eyepiece).

We do not have a set of instructions, but there are tools available for opening camera lenses (basically two screw driver tips where you can set the distance in between, which you can also use to unscrew the locking ring inside of the filter cell. We recommend that you remember which side of the filter faces the telescope, or put the more shiny side towards the telescope to avoid reflections.

The original Alan Gee was designed for Schmidt-Cassegrains and will not work with an EdgeHD for photographic use - it can't be installed inside of the baffle.
The UAG-II can be used with a binoviewer at an EdgeHD, because it still works fine with the relatively small image circle of a binoviewer (1.25") even on an EdgeHD. But we do not recommend to use it so far away from its optimum position with a camera, where you will notice any aberrations further away from the center of the field of view when you look at the image. Even with a binoviewer, you shouldn't add the additional light path of an Off-Axis-Guider.
For photography with an EdgeHD, you should use the original Celestron reducers specifically designed for the EdgeHD - then you have a system which is optimised to work.

Blocking starts at 420nm; 400nm are completely blocked, as seen on the transmission graph.

Yes, of course. They are available individually with safety kerfs (included with all Baader 2" prisms/mirrors as standard since about one year):
https://www.baader-planetarium.com/en/baader-2%22-safety-kerf-nosepiece-with-2%22-filter-thread.html
or also as traditional nosepiece with undercut (included with previous versions):
https://www.baader-planetarium.com/en/baader-2%22-nosepiece-with-2%22-filter-thread.html