This eyepiece was designed by two South Africans, in Pretoria, in the mid 80's because fast paraboloids with Dobsonian mounts were becoming popuar amoung amateur astronomers. Big paraboliods with low F ratios results in a lot of
coma and other aberrations developing at the eyepiece so it made sense to correct it at the eyepiece. The design goals were to produce
coma less than 1 arcminute, astigmatism of 0 diopters, field curvature less than 1 dioptre and distortion less than 15% with a apparent field of view of 50 degrees. The logic was that if you could minimise aberrations you would not just see more detail but get excellent contrast.
Only 150 of these were made by University Optics in Japan and although the advent of
coma reducers/correctors like the Tele Vue Paracorr are considered to be the reason for it failing to be a commercial success, I don't agree : the Paracorr were launched after 1990 and the first 150 Pretorias were on the market by 1988 selling for $225 US, none were made again although a 16 and 20mm were also made but in even smaller numbers. There were just not enough people using F4 Newtonian telescopes for visual astronomy that would buy these and then it would not do mutch good in a F10
SCT telescope that were the Most popular type.
I and some club members had the chance to tryout a Pretoria in a 15" Obsession and compair it to a Panoptic 27mm with a Parracorr II. This is 10 glass elements VS 6 and about $700+ VS $225. The Obsession has a 1717mm focal length and a 381mm mirror giving you F4.5 but with the paracorr II it changes to a focal length of 1975mm and F5.2, so the field of view is effectively not much bigger with the Panoptic.
After about 4 hours of viewing and comparing I have to give it to the Panoptic/Paracorr by a small margin, it was only a bit sharper edge to edge, but compairing the Panoptic by itself to the Pretoria, the Pretoria totally destroyed it!
Main Equipment : Tele Vue 27mm Panoptic, 7&13mm Nagler, Big Barlow : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal : Camera Fuji XT100