Adjustable Tangent Mount for a Guidescope


For guiding, I use Cookbook 211 CCD camera attached to a guidescope (cheap 500mm f8 telephoto lens). That attachment needs to by sturdy (to support the weight of autoguider and guidescope), adjustable (to find a suitable guidestar) and stable (should not flex during imaging session). My first mount consisted of a section of PVC pipe strapped to the main scope and holding guidescope inside. Guidescope was adjusted with three setscrews in a manner identical to manmy finderscopes. It worked but frustration was a major experience when hunting for a guidestar. Eventually I had to do something about it...

Below I describe construction of a tangent mount which lets me adjust the guidescope in a controlable manner. All materials were acquired at a local hardware store and. Following tools were used: drill press (hand drill would be fine), hacksaw, metal file and a tap. All screws and bolts used are 1/4 x 20. I built it to fit my particular setup (guidescope mounted below the tube of 8 inch SCT). Hopefully, the general description will help you build similar mounts for other scopes.


Base and RA Tangent Arm Assembly

From the left: A - 2 segments of RA "fork", B - mounting plate / base, C - DEC support clip, D - RA tanget arm

     
Attach RA fork to the base plate - put some grease in between and tighten bolt enough to let the fork rotate with some resistance. Lock the nut with a drop of Crazy Glue.

Also, tightly bolt DEC support clip to one side of RA tangent arm. Use Crazy Glue here, too.

     

Attach RA tangent arm to the base plate. Use nylon washers and tighten the nut enough to allow some movement (with resistance). Lock that nut with Crazy Glue as well.
     
Insert RA driving bolt and tighten locking nut to prevent any wobble. Then attach large knob. Turning it will cause guidescope to move in RA.

     

DEC Movement Assembly

Parts (from the left): E - large clip with 3/8 hole drilled, F - U-clip, G - first guidescope bracket (segment of 2.5 inch square tubing with 4 mounting screws), H - DEC control bolt with knob, I - second guidescope bracket (two pieces of U-channel), J - DEC axis bolt and K - 1 1/4 inch autoguider adapter. 500mm f8 guidescope (telephoto lens) is visible in the back. Holes are marked as in the first Figure.

     

Attach U-clip to the side of the first bracket. Lock nut and bolt with Crazy Glue again. Then slide the first bracket over the middle section of the guidescope and lock it in place with mounting screws (four depressions were drilled in the guidescope sides to keep mounting screws from sliding). Finally, bolt the second bracket to the back end of the guidescope.
     
Connect guidescope to RA tangent arm with DEC axis bolt. Tighten it quite well with a wing nut (use nylon washers here, too).

     

Put large clip over DEC support clip and a bottom part of U-clip. That spring-loads DEC axis. Then tread DEC drive bolt in until it presses firmly against DEC support clip. Turning this bolt will move guidescope in DEC.
     

Attaching Guidescope Assembly

Run two long metal bands through slots cut in the base plate and around telescope tube. Tighten those bands to keep guidescope assembly firmly against scope's tube. Small notches at both ends of mounting plate help to position it against front and back segments of SCT optical tube assembly.

I prefered to have guidescope located below telescopes tube. Of course mounting it on top will put less constraints of the guidescope size, location of knobs etc.


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Jan Wisniewski