7" f/12 R35 folded iStar refractor. Quick-fit OTA.

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I needed something more substantial to support the folded OTA on the 60cm, 24" saddle of the MkIV Fullerscopes mounting. The present 18mm, 3/4" birch, plywood battens flexed too much. Which tended to amplify any vibrations from focusing or holding a camera to the eyepiece for afocal photography. 

Amongst my collection of "scrap" metals "likely to prove useful one day" I found a plate of 10mm aluminium alloy. There was just enough material to make two new crossbars about 75mm, 3" wide. Providing a far stiffer arrangement than the rather narrow 3/4" plywood battens. The 10mm thickness also saves an extra 8mm, 3/8"" of overhang beyond the saddle compared with the plywood battens. Every little helps where OTA overhang is concerned as it requires less counter-weighting ensuring less moment. Remember that Moment = Mass x Distance from the fulcrum. Many mountings are already, completely inadequate to the task of supporting a large refractor!

 Next I needed a new way to fit and lock the OTA onto these crossbars without the use of tools. The rotating, slotted, plywood crossbars had proved awkward when fitting the OTA onto the saddle. Moreover the wing nuts were too small and hurt my hands when I tightened enough to stop the OTA slipping irrevocably downwards on the crossbars when pointing upwards. Adding through bolts and wingnuts through drilled holes when mounting a heavy OTA is a disaster waiting to happen!

The answer lay in some larger coach screws, some coil springs and much larger wing nuts. The rusty spacer is just something I found handy to illustrate the depth of spacer required. The screw holes will not be offset as they appear in the image.

Imagine there are short slots cut in the 10mm aluminium, crossbars wide enough to clear the springs and square shanks under the coach screw heads. The telescope framework will have four holes drilled to match the rest position of the OTA when the coach screws are safely housed in the bottoms of the slots. 

The underside holes in the framework tubing will be drilled to clear the springs and square screw shanks. The upper holes will just clear the round shanks of the screws. The springs will butt against the insides of the tubes. So that the pressure of the coil springs will push the large screw heads downwards. Thus the screw heads will always safely clear the alloy crossbars to allow the screws to effortlessly enter the location slots provided.

The image shows the 12" x 1/2" x 1.5" alloy crossbars in place on the saddle.[300 x 12.5 x 38mm] The crossbars have to be marked, drilled and slotted for the OTA's projecting [quick release] fixing screws.

A [suitable] spacer above the tubes will allow the wingnuts to clear the Porsa tubing flange. The wing nuts will tension the screws when desired so that, once safely located in the slots in the crossbars, the coach screws can be tightened. Which will pull the underside of the large heads of the coach screws upwards against the underside of  the sturdy alloy crossbar. The coach screws thus act as locating pins in the slots but also lock the OTA firmly into place onto the thick, alloy crossbars by the friction of compression. I have now found some 1/2" x 1.5" alloy bar and will use that for the crossbars.

Without the springs the coach screws have no longitudinal locatory force. So that the screws might rise enough for the heads to jam against the crossbar at an inopportune moment. The springs ensure there will always be clearance under the screw heads until the wing nuts are finally tightened. An equatorial mounting needs the OTA to be safely located and locked into position. Or it may slip out of the location slots under certain pointing angles.

The springs and coach screws in place on the bottom rails of the Porsa tubing, folded OTA are extended by just over 1/2 inch to the undersides of the screw heads.. I have yet to make, or find some spacers, to allow wing nuts to clear the tubing flanges. Next job is to slot the crossbars to clear the springs.

It has been raining all morning but has now brightened up. So I can saw the new crossbars and file everything smooth. Followed by the cutting of the location slots and drilling the crossbars to saddle fixing holes. Once the exact balance point is certain I can mark and drill the framework for the four coach screws to match the bottoms of the locating slots in the crossbars. Adding the crossbars to the saddle avoids having to carry them while attached to the OTA.

When I want to swap the folded OTA for the long tube, perhaps for solar work, I just undo the bolts holding the crossbars to the saddle. Then fix the hinged rings to hold the tubular OTA in place.

Click on any image for an enlargement.    

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