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Accessories: Crayford Focuser: Collimation Procedure

Rich Wood, an amateur astronomer in Nevada, experimented with the Burgess Crayford Focuser and was able to collimate the focuser to his refractor tube. The procedure he used was reported in the BurgessRefractors and the Chinese Refractors groups in Yahoo. Burgess Optical thanks Rich for providing this collimation procedure.

The William Optics and Burgess refractor Crayford focusers are quite similar. One difference is in the tube adapter. The Burgess version has an adapter sized for short tube 80 and 90mm tubes while the WO's adapter is intended to fit Synta made scopes such as the 80 and 100 ED's, 120 and 150 achros. The WO version also has a focuser rotation mechanism on the adapter.

The tube adapter, shown above and to the right, has three screwholes for securing the adapter to the telescope tube. The three setscrews in the focuser body engage the reverse taper at the outboard end of the adapter. Visible in the picture of the adapter, above the tube screwhole, is one of the marks left by the setscrews securing the adapter to the focuser body. As the three setscrews are tightened, the focuser body and the adapter are pulled together.

This construction method allows the focuser body to be adjusted in relation to the tube adapter for focuser collimation capabilities. By placing a shim between the end of the focuser body and the edge of the adapter, the focuser body can be slightly tilted relative to the adapter and the telescope tube, thereby allowing the focuser body to be collimated to account for any misalignment of the mounting holes in the telescope tube.

To collimate the focuser, loosen the three setscrews and tilt the focuser on the adapter until the optical axis of the focuser tube is aligned with the center of the objective lens. The picture at the right illustrates the gap when the setscrews are loosened. The setscrews have a recessed hex head and can be adjusted with a 2mm hex wrench (Allen wrench). The picture also illustrates the 4mm plastic filler screws that are in the focuser body. These plastic screws can be removed and a finder bracket can be attached to the focuser body with new screws.

Returning to the collimation procedure, note where the gap is between the focuser and adapter and how wide it is. Place a piece of shim material in this gap at the widest point. Tighten the set screws and double check the result. Adjust shim thickness and radial position until collimation is acceptable.

Rich Wood used brass shim material because it is easily trimmed flush after the focuser is tightened down, as long as it is thin. In collimating his telescope, he only needed 0.005" thickness of shim material.

Rich Wood also used a laser collimator in the focusing tube with a center dotted target on white paper carefully positioned in front of the lens. He also used the simultaneous reflection of the laser off of the multiple surfaces of the lens and adjusted until all lens reflections were concentric on the laser collimator's return target window. Both methods of checking collimation agreed very closely.