OK, just seeing if I understand this correctly (ie so this is a vague notion rather than a project yet) ... would mounting the objectives out of two achro doublets in line make a Petzval?
http://www.andrewscom.com.au/site-co...skywatcher.htm has some candidates: The 150 f6 skywatcher could perhaps be followed by the 120 f5, costing $1348 together (if those prices haven't changed), with the aim to get a fast, relatively cheap 6" that was shorter and (perhaps) had a flatter field & less false colour.
Is it a possibility?
Thanks, Mike