I put a 16" (f8) RC into my backyard observatory that was originally designed for a Meade 10" SCT. Needless to say, the RC barely fit within the dimensions of the building. My biggest concern, however, was that the open dome shutters only provided a width of 24". Not only would I have to frequently rotate the dome during an imaging session, but the placement of the piggyback guide scope would frequently need to be moved as the RC position rotated - otherwise, the guide scope FOV would eventually be obstructed by the dome, as it approaches the edge of the 24" viewing window.
I tried off-axis guiding, but that proved to be inconsistent and unreliable in being able to capture adequate guide stars while operating at such a long focal length. I came up with a novel solution that seems to work very well. It struck me that a nice place for a guide scope would be at the very front of the telescope and within the area occupied by the secondary mirror cell.
My first challenge was that I didn't want to attach anything directly to the mirror cell, or the spider, given the delicate nature of maintaining RC optics collimation. I also didn't have room between the front of the RC and the observatory wall for much of a guide scope. So, I found a little 30mm mini guide scope at Orion and purchased it. I mounted it on a platform that attaches to the front of my telescope – it sits in front of my secondary mirror holder so it doesn’t obstruct the primary mirror (see photo). The horizontal plate lies in the plane of my spider vane, so it too doesn’t add any obstruction. The question was – can thus little cigar-sized scope adequately autoguide my 16 inch Ritchey-Chretien telescope? The answer appears to be yes, as testing has proved it to work very well.