Cheap coma corrector

[Raph-in-the-sky]

Hey guys,

I would like some recommendations/comparison between cheap coma correctors. In another tread @Louis D mentionned the GSO/Revelation. 

How good is the GSO/Revelation coma corrector? Obviously, I am not expecting Paracorr performance but does it compare favourably to a SW or baader for example.

Thank you in advance for your help!

Clear skies,

Raph

[Louis D]

I can't compare the GSO/Revelation to the competition because it's the only one I've used in my scopes.  At f/6, it nicely cleans up and flattens the field for eyepieces focusing within 5mm of their shoulder with the 25mm extension I use.  Only the 12mm NT4 shows any appreciable coma because it focuses about 19mm from the shoulder.  Even then, it's a major improvement over no CC.  In my two ES-92 eyepieces, I can't see any star bloating at the edge not attributable to minor astigmatism, so it should do well even with 100 degree eyepieces.

The one shortcoming I've noticed is at high power.  It contributes a bit of spherical aberration on axis, so planetary images are sharper without it at small exit pupils.  It's my understanding that the 5 element Paracorr II does not exhibit this behavior, so it can stay in the focuser permanently.

The GSO/Revelation magnifies 1.1x vs. 1.15x for the Paracorr and 1.06x for the ES.  I know nothing of the SW and Baader CCs.  The GSO/Revelation requires about 10mm to 15mm of in-focus (I'll have to measure it exactly sometime).  The Paracorr is around 19mm and the ES is about 38mm.  Since I have so little in-focus available, the ES is a non-starter for me.

The GSO/Revelation does so well, I have never felt  a need to upgrade it.  At f/4.7, the Paracorr II might show a noticeable improvement, but at what cost?

I went CC-less for over a decade of observing and then saw a used GSO CC for sale and thought I'd give it a try.  After getting it setup and using it for a few night, I couldn't believe I had been living with so much outer field degradation due to coma and field curvature and never noticed it.  Once you see it, you can't unsee it.