Heavy piggyback considerations


Hi all,

Our backyard observatory's current configuration is a TMB-152 on an AP1200GTO, with a light (Borg 90FL) piggybacked. Please see attachment as an illustration. The TMB rides on the mount via rings bolted on to a Losmandy dovetail bar.

I'd like to occasionally piggyback a heavier scope on the TMB. Specifically, a Takahashi FSQ-106 with an STL-series camera. Maybe even an AP130 Gran Turismo.

I believe I've seen a picture on this group (or perhaps the old Yahoo group) of a Mak piggybacked on a refractor (with Roland standing by), so it does appear that such a configuration is feasible (anyone know where that picture is?).

Are there any considerations or concerns about this configuration (a smaller, but heavy, refractor piggybacked on the TMB)? Specifically, should I be concerned (or not at all) about the weight of the piggybacked scope affecting (stressing) the TMB OTA? Or is the structural load actually borne by the rings and I don't have anything to be concerned with?



It would depend on the rings. Thickness, machining, hinge and clamp design, materials, etc.  My 6”f9 has the original AP two-piece hexagonal rings, which are very rigid. I have Parallax rings on another 6”, and they are decent. Then I also have some Orion rings that I would never trust to hold anything valuable. Try mounting the piggyback scope on the rings without the TMB in them, and see how much flexure there is. That will give you a sense of how much the OTA is contributing to the structure.

I prefer a tandem arrangement.  The AP tandem bar is very stiff, especially if you use one of the longer AP saddles that will support most of its length. 


Robert Berta

I run an Officina Stellare 152mm APO  triplet refractor (same optics as your TMB)  with either an Officina Stellare 80mm APO refractor or TV 85 APO on my AP1100 and previously same setup on my AP 900. I use it with a SBIG STL full frame camera with filter wheel so that is heavy also. The smaller scopes get heavy when Ha solar gear attached to it so I over spec'd all the mounting gear including the same overbuilt OS CNC rings so it can remain rigid and solid when used for auto guide duty. The trick is to get a matching STRONG and longer dovetail top and bottom on the 152mm. I use Astro Physics for all dovetails Also get beefy rings for both. I use some CNC cut rings that Officina Stellare made for me. They are strong as hell.  The felt padding between the rings and scope should be thin and the ring clamp ring bolts tight so there is no chance for differential flexure. For counterweights.....try to get them close to the mount. Rather than less weight moved out to the counterweight end, it is better to add more weight closer to the mount to help reduce lever moment.

Don't try to use flimsy Chinese rings or dovetails.


Thanks for the replies! I don't recognize the TMB's rings; they might be APM/TMB rings of the era (the scope is an actual TMB-152 from the early-mid 2000's). The FSQ is using Socal Astro rings (no longer made, but pretty beefy). I definitely have no plans on using flimsy inexpensive rings.

Chip, that's a great idea to mount the FSQ on the TMB's rings without the scope! Yes, I agree that a tandem arrangement is the better approach long-term. This piggyback idea is not meant for occasional use, not long-term (FSQ riding semi-permanently on the TMB).