Just to expand a little on my last post...
PAR focal filters are simply equal thickness filters, and will not change the focus of a converging light cone from one filter to the next.
With reflective optics the focal plane remains unchanged across the spectrum.
With refractors however, even APO's, there is a subtle variance in red, green and blue focal planes.
While it would be possible for a refractor manufacturer to design a variable thickness RGB filter set that would correct the focal plane variation in a specific instrument, I am not aware of any....plus a variable thickness filter set would not kind toward reflective or different spec refractive systems.
There is also a secondary effect of having to change focus for RG&B filters. The image scale will change and you may see subtle rainbow fringes around stars after the 3 planes have been colour stacked.
(Hope that wasn't seen as an exercise in "how to suck eggs"
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