"Can the supplied tube rings be adapted to accept a Vixen dovetail?"
Yes, I believe that they can.
Here's my 395, with what I think are the brackets that you have on your 390.
Where the bottom of the bracket goes thru the Edmund flange, there's a tapped hole in the bracket that has metric threads. The top of the bracket is also tapped but they are for 1/4 -20.
I just ran a long enough bolt thru bottom of the bracket and thru the flange.
I'm sure you can come up with something that will work.
Also, I just purchased these brackets for my 395. These brackets slide onto the tube. I had to unscrew the lens cell. But what I found out was that they do not fit tight! I solved the problem by separating the felt under the top of the bracket where a 1/4-20 bolt would go. I inserted a small piece of flexible thin plastic between the felt and the bracket, and screwed in a 1/4-20 bolt into the top of the bracket. Now, I can tighten the bolt down against the small piece of plastic, and it tightens down onto the tube without marring the tube. I did that to both brackets, and it hold the OTA very good.
One more thing. I found that the bottom of these brackets were not flat or square to the rings. A little filing took care of that.
Edited by Garyth64, 07 November 2019 - 03:18 PM.