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It is reflector system, and only element that could contribute to CA is corrector plate.
Being relatively thin. it's effect is negligeable (also according to my experince).
However, if you have other optical elements in a train (like focal reducer, field flattener) then you may need IR filter, because they may not be corrected for IR (at least cheaper models).
As to improvement you noticed with your Newt, the problem was in those additional correctors. IR filter can help there, but if you are using Newt in prime focus, IR is not needed.
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