Many of these GPC would give you lower power than your current 4.2x. There are 1.25x, 1.7x, and 2.6x GPC from Baader. So if you are interested in lower powers, wider fields, etc. then a GPC would do that. The 1.25x GPC still needs much in-focus travel (about 92mm for straight-through viewing), as does the 1.7x GPC (about 77mm), so it might not work for you, if you are already having focus travel problems. There is also the Baader 2" 1.8x GPC for refractors 2456300, which requires the Maxbright diagonal mirror to get spacings right.
The GPC are also designed to correct chromatic aberration and spherical aberration in the bino prisms. But spherical aberration will be a small effect in your F/9 scope. I'm thinking the ED100 F9 already has some chromatic aberration (though a small amount), so adding the GPC wont suddenly give you color-free images. Maybe if your scope had perfect color correction, it would make sense to get the GPC for this reason. But I don't see a strong case for it here based on color correction.
Edited by ngc7319_20, 25 March 2019 - 04:38 AM.