phoenix wrote:

Strong mass can bend light. Maybe putting heavy neutron star matter
at the periphery of the tube?? But then it's no longer lightweight
unless 4000 A.D. technology has developed antigravity so you can
take it along with you as well as higgs field suppressor to prevent
the heavy neutron star mass from being formed while being carried.

If you have anti-gravity at your disposal, you won't need neutron star
matter. The anti-gravity emitter can bend (unbend?) light all by
itself. That's all you need, an anti-gravity lens. It would work like
the Einstein Cross gravitational lens, only smaller and the other way
around.

http://www.astr.ua.edu/keel/agn/qso2237.html