Nice solution for the adjustment rings. I should state here that instead of an MPCC I use the Lumicon field flattener which is a 2" screw in optic. See here
http://www.telescopes.com/telescope-...calreducer.cfm. In conjunction with the Lumicon low profile 2" adapter
http://www.lumicon.com/telescope-acc...Low+Profile&hn or here
http://www.myastroshop.com.au/products/cam-adapt.asp 3/4 down the page. The adapter screws into the filter holder of the QHY8 and then the FF screws into that. Simple and a flat field over the entire chip. It gives me better correction than the meade f6.3 focal reducer/FF.