Binocular Mirror

I am trying to build a binocular "mirror box" to ease my aching neck. My binos are Oberwerk 20X80's. Does anyone know where you can get a affordable mirror for binos of this power?

One source states: Our PREMIUM mirrors have a wavelength error of ~/= 1 lambda.... While some purchasers have successfully used this mirror with binoculars up to 20 Power, 18x is the recommended limit.

So, what surface accuracy will I need for 20X binos, and where can I get one? I've been searching the web and emailing vendors without much success.

Hi Paul,
I had one of the mirrors that you mention and it was pretty astigmatic. I think you need better for a really nice image.

I have a mirror still that is not as large as the other I had, but is the same quality. It is coated the same as a optical mirror. Not sure if it is as large as you would want. Contact me by PM and I will give you the size and specs.