Well the 5D MkIII is in and I’ve recently been trying it on my refractor. After a few shots of M33 something was obviously amiss.
Previous configuration was a 450d or 7d on a william optics 90mm + a William Optics .8x Flattener/Reducer MkII. Worked pretty well. But there was HUGE vignetting with the full frame. I did expect quite a bit but the amount shocked me. Here’s a flat sample taken against a lit wall. First is with the reducer, second is without.
and here with the flattener introduced into the optical train:
Looking at the flattener the clear window is actually prett
y small. Having checked the WO specs it does say that this is for APS-C applications. Oh well, better start looking for another flattener. Had thought about the variable WO Flattener version IV but apparently that acts like a 1.1-1.2 barlow instead of reducer. Can’t see that being an option as imaging time is limited as is.
Anyway, I’ve since started imaging with the flattener removed. Obviously the stars are distorted as soon as one moves away from the centre of the field but at least it still works. Here’s a version of M33 with that configuration.