Beware the WO Flat 3 with the Megrez 90, you'll get unacceptable astigmatism. The Flat 2 has been theorised to work better, however I have not seen a specific test done to confirm this, nor do I know how much vignetting you will get. Flat frames will compensate, but the knowledge that it is there may not be acceptable for some
I use a Borg DGL 0.85x reducer and it does a much better job of correcting the image. Borg manufactures the rings necessary to use it in a 2" focuser, as it uses larger threads to prevent vignetting. I purchased mine from Hutech and they were able to advise all of the adapters necessary for my usage. Be warned, it is twice the price of the WO.
I have heard that there is a Televue that also works, but I think it is a similar price to the DGL.
An extension tube should only be necessary when doing terrestrial photography if I recall correctly, as your focal point will get shifted back. As I always have an extension tube around, I can't remember if it ends up getting used or not.
Regards,
Eric
