Just got the Skywatcher Quattro 4-element Aplanatic coma corrector and though I would compare it to my Baader MPCC Mark III. I was not thrilled with the MPCC, which is why I decided to go for the Skywatcher which set me back $285. The MPCC is a 2-element design that runs about $215 while the Skywatcher is a 4-element design made by GPU Optics in Hungary and also sold by TS. I used my Orion 8" f3.9 reflector with a full-spectrum modded EOS 500D and a Baader UV/IR cut filter. Done with my Celestron AVX mount, unguided 15 sec shots at ISO 1600 in the vicinity of Vega. I took great care to get the best collimation I could with a combination of laser, cheshire and DIY autocollimator. I achieved best focus for each test with BYEOS looking at Vega centered on the sensor minimizing FWHM and checking with a Bahtnikov mask.
The top row of images are crops roughly 160 pixels square from a region near the center of the senor.
The bottom row are crops near the edge of the sensor (roughly 11 mm right of center at image plane).
My suspicions about the MPCC were confirmed that while it does significantly correct coma it does introduce substantial abberations over the entire field. From what I understand, these are spherical abberations. I did take great care to get the back spacing just right with the MPCC at 57.5 mm from the edge of the M48 mounting surface. The backspacing for the Skywatcher was at 54.3 mm (recommended is 53.66 mm for f=800 mm). Also note the double diffraction spikes from nearby Vega with the MPCC. I saw this type of thing (double or sometimes "fat" diffraction spikes) on several nights of imaging with the MPCC having tweaked collimation and coma corrector mounting each night so it seems this really is an artifact of the MPCC and not something specific to my setup on any particular night.
The Skywatcher produced nice stars across the field and doesn't seem to introduce any other abberations. There was some apparent elongation in the vertical direction near the edge of the field and this might be due to the camera/corrector not being perfectly square to the optical axis of the scope. I need to test this theory out.
I have been able to reproduce the MPCC results showing spherical abberation with OSLO using measured surface curvatures and lens separations. I'm a bit surprised to see all of the positive feedback on the MPCC here on CN, but maybe it works just fine for visual observing.
If anyone has the prescription data for the Skywatcher/GPU, I would love to plug this into OSLO as well. It would also be interesting to see a comparison of the Skywatcher with the Paracorr Type II.