This diagonal had been loaned to a friend for several weeks, while he waited for his Takahashi prism to arrive. It's the first time I've tried it with the Sky-Watcher ED120.
Using magnifications from 180x to 450x on a selection of stars (incl. several doubles) showed that it's a very good fit with the ED120. The prism's correction nulls out the tiny bit of red c.a. normally visible with very high magnification at full aperture (120mm). This effect is most noticeable when looking at orangy/reddish stars.
This prism appears to be a nice match to the ED120 lens.
In fact, even slightly better than than a 2" dielectric mirror diagonal, when used at the objective's full aperture. But, it's pretty much a hair-splitting experience.
There's less of a difference in correction when varying between 110mm and 120mm, using the prism, vs. using a mirror diagonal.
The glass in the Baader prism has a higher refractive index than BK7 and is even a little greater than the Tak prism. Whereas BK7 shifts color correction quite noticeably and results in more prominent blue/violet defocus, the Baader prism has a milder effect.
If you already own a dielectric mirror diagonal, the difference in performance between it and the Baader prism likely doesn't justify switching.
Even so, the Baader diagonal compliments the ED120 lens exceptionally well.
