This is something I've noticed with all SCTs. Don't get me wrong,
some SCTs deliver excellent images, provide terrific platforms for
planetary photography, keep out wind currents, etc. But, when you
look through an SCT and compare it directly to a pure mirror system
of similar size (for uniformity's sake) you do see a kind of dulling
down or veiling of contrast. Now, I have no idea why this is seen,
I suspect it is the corrector, even though the companies that produce
them are now applying about the best coatings they've used.
I've owned around 45 SCTs and most have produced good or excellent images,
but I have always noticed the effect I mentioned.
I have not noticed this with top-flight Maks.
I'm no fan of open systems owing to the problems with dust, tube currents,
etc and therefore will stick to SCTs as my scopes.
-Rich