Actually the RFOV of the 13 mm Ethos can be calculated, since TeleVue gives us the field stop as being 22.3 mm.

RFOV = FS/Eyepiece_FL * 180/Pi
RFOV = 22.3/13 * 57.296
RFOV = 98.28 degrees

This tremendous result would indicate that pincushion distortion (rectilinear distortion, or the bending of straight lines) is quite well controlled for the Ethos, and this means that by default it must have predominantly angular magnification distortion instead (whereby objects do not retain a common magnification [size] across the field if view). With a field that wide, one would likely never notice that Saturn for example is perhaps 5% or 6% different in size from center to edge, and since stars effectively do not change in size (having no effective size to magnify by virtue of being "point sources"), this effect would never be noticed for them.