  Recent theoretical work suggested upper bounds on the operating bandwidths of
flat lenses. Here, we show how these bounds can be circumvented via a
multi-level diffractive lens (MDL) of diameter = 100 mm, focal length = 200 mm,
device thickness = 2.4{\mu} m and operating bandwidth from ? = 400 nm to 800
nm. We further combine the MDL with a refractive lens to demonstrate a hybrid
telescope. By appealing to coherence theory, we show that the upper bound on
relative bandwidth is surprisingly independent of lens diameter or numerical
aperture, but is only limited by the bandwidth of the image sensor. Since
large-area achromatic flat lenses produce significant reductions in weight over
their refractive counterparts, these calculations and experiments open up
opportunities for very large scale diffractive and diffractive-refractive
telescopes.
