Glasspathcorrector 2" / 1.7x for Newtons

Is coma correction at f/3 acceptable? Or maybe better formulation: How fast optical system can be coma corrected which would still have decent correction.

How does the 1.7x GPC for Newtons affect focal ratio? My scope is f4.2, with around 33 mm backfocus. With the GPC, I am expecting a backfocus of 33+80=113 mm. Will that make my scope f4.2x1.7=f7.1 or f4.2x(113/33)=f14.4? Thanks!

Lets say I attach a filter wheel or and Atmospheric dispersion corrector between GPC and binoviewer, i.e. I introduce additional 4 cm of lightpath between GPC and bino. Could you elaborate on two questions: - how much approximately would the 1.7x factor increase; - would that additional lightpath negatively affect the coma-correcting properties?

Is it possible to stack GPC's without losing image quality? For example, a 1,25x or 1,70 GPC on top of the 1,7x newtonian GPC with a Mark V binoviewer. That would give a nice flexibility.

My 1.8x glass path corrector appears out of collimation on f/5 while the 1.7x works perfectly. I have the following: Baader 2" glass path corrector for refractors, SC and other systems. Extension factor 1.8x. I have tested it with various diagonals and in 3 high quality refractors with my Mark V Binoviewer. Everything looks good in my 180mm F7 and 101mm Petzval F5.4 refractors. But in my 140mm Petzval F5 refractor the stars appear out of collimation on regular stars as well as on an artificial star tested indoors. When I replace the 1.8X GPC with the smaller 1.7X GPC or 2.6X GPC everything looks good. The scope has perfect collimation. However, when I use the 1.8X GPC something is out. It doesn't really make sense so I am wondering if you know what might be the problem. Perhaps F5 is too fast for the 1.8X GPC. Is that correct or could it be something else I am overlooking?