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I use the equivalent of an "on axis guider" with the reflective slit in the spectrograph. Used on a C11 with Lodestar guider. No issues, no drama.
A few years ago I sketched out the OAG arrangement to show why in 99% of the cases many guide stars looked crappy! and "seagulls" etc. The problem is that the prism needs to be tilted to bring the optical axis closer to the guide camera optical axis - this is commonly overlooked, hence the bad performance....
OK. Don't waste any more time under the stars. Set it up in daylight.
The attached images (not mine) show a typical set up and comparative FOV's.
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