Originally posted by m8o "Sharp, well-defined images — particularly with distant subjects — thanks to the adoption of optics used in astronomical telescopes"
... sure sounds like a Mirror lens to me. [!] Are there any excellent optics used in astronomical telescopes that are not mirror base in the last century?
Amateur astronomers rank photo tele lenses highest, refractive scopes second and reflective scopes (mirrors) last. So, Pentax making refrence to astronomical telescopes is not a good sign. Astronomical telescopes have too few lens elements to really compete with photo tele lenses.
Sure, the difference is minimized by small nominal apertures (around f/8 refractive, f/11 reflective) where scopes are almost diffraction limited only. But photo tele lenses can be sharper at wider nominal apertures, at f/5.6 and below; requiring more lens elements, 10 or more.
I would be very disappointed to learn the DA 560 f/5.6 is crippled, with only 4-8 lens elements and no or only one ED element. Such a lens can be replaced by a DA* 300 with a sensor with small enough pixel pitch.