Unfortunately, I cannot write a first light report yet because of what appears a very serious flaw with the Explore Scientific 3" diagonal that I ordered with the eyepiece. As both arrived before I had a chance to make up a 3" adapter for my 180mm F/7 refractor, I improvised by attaching an Optec 3" focuser to receive the nosepiece of the ES 3" diagonal.
When I inserted the eyepiece and diagonal nosepiece into the 3" focuser and tightened the three set screws, the rest of the diagonal and eyepiece (c. 7 pounds of weight) suddenly rotated sidewards and literally fell off the nosepiece! If I had not reacted quickly enough to catch the eyepiece and rest of the diagonal, it would have dropped to the hard surface of my patio. (Or crushed my foot)
Upon inspection, the threads on the diagonal nosepiece are barely engaged with the rest of the diagonal main body making it impossible to tighten it. I originally thought that the nosepiece might have been overtightened at the factory, stripping the threads, but that does not appear the case. It just seems a poor fit and poor quality control. Currently the nosepiece freely rotates within the diagonal main body and cannot be tightened or used.
This serious flaw aside, my general impression is that the ES 3" diagonal is somewhat underbuilt for the weight and loading it needs to deal with. Just the torque of the weight of 5 pounds of eyepiece, if placed at an angle is going to place considerable strain on the way the nosepiece is threaded into the diagonal main body. Even on a properly threaded version, there is always the risk of it rotating sidewards. I think it needs to be secured along the lines of the diagonal eyepiece holder which has 6 hex heads securing it to a flange on the the main diagonal body. (Pictures in posts below)
One other point. I get that carbon-fibre inserts appear attractive from a marketing POV and superficially promise minor weight savings, but in reality anyone seriously using eyepieces in this weight category is not going to worry about a few ounces of extra weight for solid metal. From an engineering perspective, the thickness of the metal used in the diagonal is on the thin side, and the carbon fibre inserts make an already flimsy diagonal feel flimsier still.
I really wanted this post to be a positive first light review of the ES 30mm 3" eyepiece through a large apochromatic refractor and I still hope to write one soon, but the weak link in the chain represented by the diagonal needs to be resolved. Perhaps I received a poor early example, and I can go back to Opt to see if they will exchange it for another one. However I am also wondering whether I should look at the Siebert 3" diagonals or any other alternatives someone can recommend. (If anyone has any experience?)