Actually Lewis, it performs well because of its relatively slow f-ratio. Strehl is usually an on-axis measure, and the faster the scope the harder it is to keep under control as you move off axis. That's why the FSQ has 4 elements, and most of the 3 element APOs are a lot faster. Before fluorite glass was available, a flint and crown doublet around 100mm had to be f15 or thereabouts to produce an acceptable fov. That's how vixen killed Unitron!
Cheers,
Andrew.
Quote:
Originally Posted by LewisM
I hate to say it, but the only Tak I ever owned - an FS60 - I disliked immensely. The CA on even medium mag stars was REALLY noticeable, let alone on planets. I was very disappointed, though Tak never claimed it to be super corrected (I found it WORSE than a SW achromat I once had). Beautifully made, superb focus (once I got enough damned doodads and adapters to get it to focus!), but the CA spooked me big time. I found the finder better corrected 
I note NO CA in my FL102S. Nada, zilch, zip. And that's just a high performance fluorite doublet! And it performs beautifully photographically, no doubt because of it's remarkable Strehl ratio.
I have plans for a WO Megrez... be interesting comparisons... |