Ah, I see!
I thougt, the 3x video-crop mode is a thing, cams like my 600D can do, but not a 5D3? That is one of the reasons at the moment, I hesitate to buy a 5D3. Or is this enabled by ML?
For my teleskope, I have a TK called flatfield-converter. It´s very good, but very expensive. With this I reach a real (!) focal lenght of 10m (teleskop 2m, flatfield-converter 3x, 2 rings to extend flatfield-converter to 5x)
Edgar