Tried mine last night with 80mm F6, and it did remove a good bit of FC in the 21 Ethos.
Saw some astigmatism at the edge, and I wonder if the flattener is adding that, or if it is just a matter of optimizing the distances.
I use the TSFlat2 on my 80mm f/6 and 80mm f/7.5. The rest of my scopes are intrinsically flat field. I have tried it with other scopes down to f/4.3. It is spec'd to be effective f/4 - f/9 and I have no reason to question that range as it corresponds with my own experience. TS offers a detailed grid of working distances relative to telescope focal length and they note that for visual, there is at least 5% error tolerance. This is also consistent with my experiences. Mike found that when he reduced the working distance by screwing the TSFlat2 into the diag in place of the nosepiece rather than as an extension to the nosepiece, it cleaned up his ST120, after initially having a negative reaction on that scope. The working distance matters! But you have some latitude.
I have not seen any astigmatism induced by the TSFlat2 on an 80/480, but then I use eyepieces that are not astigmatic, and I use DiopTrx to tune out my eye's astigmatism at exit pupils above 1mm. Anyway, before you draw definitive conclusions about the flattener, do what you can to optimize working distance within the 5% visual tolerance for error.
Edited by 213Cobra, 01 August 2019 - 08:58 AM.