Fernando,
I own the Docter 15x60, and the quality of the optics is about as good as it gets. I am particularly impressed because it is so large and high powered. Perhaps this should not be surprising, since it is closely modeled after the very successful Zeiss Nobilem. And also, since it is one of the most expensive Porros made.
Docter bought the Zeiss Jena plant after the war, and, largely with the original employees, continued production. They have recently moved into a new plant it Eisfeld, and have introduced a line of roof prism binoculars which are said to be only second rate, but something of that tradition continues with their Porros.
On the Cloudy Nights Binocular Forum, you can find several discussions about this binocular for astronomy use. I think the number of users there is up to four, and we are all very happy campers.
I have not used mine very much in the daytime, but I have taken it on several short birding walks. It is a challenge to handle, and also, I find the focus knob difficult to reach after being spoiled to roof prism binoculars. But the focus does work smoothly, although slowly geared, and the external eyepiece carriage is rigid enough to work with no rocking or flexing. I don't really mind the jumpy image at 15x, it really shows more if you can learn not to be irritated by that. The brilliance of the image is unsurpassed, and it is honestly very difficult for me to detect any CA in it, although I see it all the time in smaller roof prism binoculars. The field of view is only moderately wide, but the sharp sweet spot is more than wide enough.
Overall I find the optics superb, the appearance, ergonomics, and mechanics good enough but unrefined.
WARBLERS?? It would be great on warblers, provided they are about half a mile away. Or if they would only be still.
Ron