Robert,
As you have read in the link Steve provided, Pincushion Distortion is a form of rectilinear distortion. The other form is barrell distortion where the lines bend inwards at the EOF instead of outwards like with pincushion distortion. As I mentioned in
a previous post all eyepieces regardless of price have some aberrations, this is inevitable as the designer cannot correct for everything. In the case of Naglers and Panoptics a little distortion is left in the design to allow for correction of off-axis astigmatism and other EOF aberrations. The remaining distortion is not usually detectable to an observer viewing pinpoints of light or extended objects at night. This is what makes them such a good choice for astronomical viewing. You can detect the distortion at night by getting an eyepiece (eg: a Panoptic) in a dobsonian and dragging a middle magnitude star in a straight line in azimuth from one edge of the field to the other so the star basically traces a line across the FOV. If there is any rectilinear distortion in the eyepiece it will tend to scribe a slight curve rather than a dead straight line. This rectilinear distortion does cause issues to some observers using these eyepieces in a dobsonian while panning the sky at night. Rectilinear distortion is easy to detect in the day by observing reasonably distant targets, known to have straight edges. Example targets could be, a telegraph pole, bridge pylons or staunchions, the corners of tall buildings etc. Another test, is to observe the waterline of a distant ship at sea, on a calm day, and see if Neptune is sucking the sea down or pushing it up in the middle of the ship.
CS-John B