Eric the #7887 works extremely well as a field flattener. The reduction is only 0.85. I have used it on everything from a 80ED, 100ED, 150mm diameter F5 Achro and the TAL200k. There is a thin Aluminium ring between the two sets of elements that you change for different thickness ones depending on the focal lenth of the OTA. This FR/FF works very well on the full frame Canon 5DH with all these scopes.
I now use an Astro Physics FR/FF on the 100ED only because it gives me a focal reduction of 0.75. For this you also need an Astro Physics 2.7" Focuser.
This image was taken with a Skywatcher 150mm F5 Achro with the Hutech #7887 FR/FF so it is now a f/4.5. I also used a Hutech nebula filter. Taken with Canon 5DH.
Large image 1.0MB
http://members.optusnet.com.au/~cheekyfish/ic4628.jpg
As you can see it is good to the corners.
Here is an early thread on the #7887 with an 80ED.
http://www.iceinspace.com.au/forum/s...highlight=7887
Bert