When I got it, I found the screw adjustment to be tight in one place with some light friction and resistance, but running it up and down about 20 times cleared that.They say:
This 1 1/4", 90 degree erecting prism is the same as used on our top rated Stellarvue finder scopes. This is a correct image erecting prism. The helical focuser has a rubber grip and provides about .6" of travel. Many purchase these to be able to do fine focusing adjustments on telescopes that suffer from image shift when also needing an image that is right side up and correctly oriented.
The erecting prism is fully multi-coated providing extremely high light transmission (greater than 99%). Compared to prisms using Mgfl2 coatings, the contrast is much higher and the stars appear brighter.
Side plates are carbon fiber. The insert tube is threaded for filters. Plastic dust caps are provided.
https://www.stellarvue.com/90-degree-er ... h-focuser/
I bought this to use with my Explore First Light 102-mm refractor which could use a fine focus. Granted that last night was not easy, the diagonal worked as expected and I was able to get somewhat sharper views of the binaries. I would like to try it on planets and the Moon, of course, but I will have to wait.
One thing is that as a correcting prism, my Right-Left were corrected opposite to how my hands have learned to direct the telescope, but it is not a big deal.