I used several Meade Series Super Plossls with the Arcturus BVr and they worked great. I got them since they had no undercut and seemed better in the eyepiece holders. But no noticeable aberrations at all. I had pairs of the 15, 20 and 26mm Super Plossls.
I don't know what to tell you. I recently bought a different well known single eyepiece that should have been a decent quality and the pincushion distortion was terrible. Many times worse than even cheap eyepieces. All I can think is that some of these are arriving with inside lens elements reversed.
Also, try a different nosepiece Barlow. The first binoviewer I got had a bad performing 3x Barlow. I was able to get it replaced with another that worked better. The 1.8x one was always better. Or use a separate Barlow and see what you get.
I spent a couple hours this afternoon testing the Arcturus BV with my new 20mm Meade 4000 super Plossls. I brought my Televue 2x Barlow and my 2.5x Power-mate. This is what I discovered.
When I put the Meades in the BV with the 1.85x nosepiece Barlow, there is extreme distortion in the outer half of FOV. Someone told me it’s called coma, when I explained it to them. The Meades are very sharp without the BVer.
First, I replaced the 1.85x with the native nosepiece and used my 2x Barlow. The Meades were sharp to the edge. Yay! But get this. The 2x Barlow resulted in much more power and less FOV relative to the 1.85x nosepiece Barlow. If I had to guess, it was perhaps twice the power and half the FOV.
Next up was trying the 2.5x Power-mate. The Meades looked great! But this is where it gets weird. The 2.5x Power-Mate resulted in lower power and a larger FOV relative to the 2x Barlow. In fact, the power-mate was very similar to the 1.85x nosepiece. How is this possible?
i was so perplexed, so I had to try the Barlow and Power-mate without the BVer to make sure they are labeled correctly. Yes, they are! Without the BVer, the 2.5x Powermate is a bit more powerful than the 2x Barlow.
Anyone have any idea what’s going on? The 1.85x is going in the trash for sure. It’s horrible.