The advent of large chip CCD cameras for amateur use has put more requirements on the optical quality of the imaging systems. In the case of a type Ritchey-Chrétien Cassegrain telescope, which is inherently coma free and has "the largest aberration-free field of view of any reflecting telescope made", the ever-present field curvature and off-axis astigmatism will be noticeable in the corners of a 36mm x 24mm chip, such as the one used in the SBIG STL-11000 camera.
To counter these effects, RCOS offers a dedicated Field Flattener/Corrector (FFC), consisting of a 2 lens optical assembly in a black anodized aluminum screw-in cell. See: http://www.rcopticalsystems.com/fieldflattener.html . This FFC was designed specifically for the Star Instruments optics, used on most RCOS telescopes.
For a long time, I was unable / unwilling to install the FFC on my 14.5" RCOS because there was insufficient data on its Aries optics to ensure that adequate correction could be achieved for the f/9 configuration shown below. Note how the Precision Instrument Rotator (PIR) - that's the bright red item - is attached to the scope's back plate by means of a black anodized base plate and 6 screws.