There are many ways to attach a telescope to a mount available to amateur astronomers. Most of them perform well enough for a visual astronomer to be very satisfactory. Astrophotographers have a very different set of needs than visual astronomers. Most telescopes are attached to the mount using either tube rings or dovetail plates and radius blocks (common for SCT systems). These tube rings are either bolted to the mount directly, bolted to a dovetail plate that attaches to the mount or bolted to a clamp mechanism that attaches to a dovetail on the mount. Some tube rings are very high quality and some are not. For an astrophotographer, every attachment point becomes a spot where flexure can occur. When telescopes are "stacked" on a mount the possibility of flexure is compounded.
Most tube rings are of a hinged design and have a hand knob to tighten them to the scope tube. A quantity of felt or rubber lining the rings provides grip on the scope tube. These are usually designed to not run out of travel when the scope is "tight". The scope tube is then part of the support structure for anything mounted above it - like a second imaging scope or a guide scope. If not checked constantly these types of rings can loosen up, as the rubber or felt takes a set, and allow the tube to shift slightly as the mount tracks or slew's from east to west. Even worse, if multiple scopes are mounted each scope can shift differently. This is differential flexure - and a bad thing to an astrophotographer.
A more extreme example of making the scope part of the support structure is mounting of the common SCT. An SCT, when removed from its fork mount, typically uses a dovetail plate and a pair of radius blocks to attach to an equatorial mount. The dovetail plate attaches to the radius blocks with 4¼-20 screws. The radius blocks attach to the SCT front and rear castings using 4 existing screws, usually #10-32 in size. If a secondary imaging scope or a guide scope is desired a second dovetail plate and radius block setup is mounted to the top of the SCT. As you can see the entire weight of the imaging system is dependent on 4 #10 size screws and a pair of radius blocks. Any slight movement here will be magnified through the imaging system to the camera! The front and rear castings of the OTA become an integral part of the support system. These castings are designed to support the optical train - not other telescopes!
Did you get a carbon fiber tube to reduce focus shift from temperature changes? Think about what happens to the ends of the scope when those aluminum bars shrink and grow as the temp changes!
Our system is designed to provide a secure support system independent of the telescopes. Each telescope in the imaging system is independently gripped. The tube rings fasten together into a solid connection using stainless steel socket cap screws. The rings are calculated to have a 1/32" clearance on the telescope tube when closed. We apply 1/16" of felt to the inside of the rings. This provides a firm grip on the tube without scratching the tube finish. Dovetail plates and clamp blocks are attached using two ¼" stainless socket cap screws and two #10 stainless low head socket cap screws. We have supported telescopes as large as a C14 for several years using this design with no issues.
Our large tube rings are not inexpensive. They are, however, precision machined from solid 6061-T651 aircraft aluminum plates and bars. Material is removed in pockets to reduce the weight on our already overtaxed mounts without compromising strength. We have not seen a tube ring of comparable quality on the market. To achieve this level of quality takes the best materials, solid carbide tooling and time on state of the art CNC machining centers. The cost is necessary but it results in VALUE!
Take a good look at the system that we have designed. We feel it is a valuable tool in the pursuit of excellence in astrophotography.