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C11 Mechanical Alignment & Collimation

Focusing Arm Modifications

The focusing arm was modified to include a push/pull tilt mechanism for the primary. The original tube was also machined down to allow for mirror tilt. The small annulus between the focusing arm tube and the stainless steel flange is needed so the mirror has enough room to tilt. This gap is only 3mm but that is all that is needed. The machined depression stops at the beginning of the retaining ring thread.

The three main holes are for the primary anchor bolts (pull screws). These bolts are threaded directly into a stainless steel ring that is bonded on the mirror glass.
The other ring visible on the focusing arm here is made out of aluminium and was machined then glued onto the original arm, then tapped for the lock screws and push screws.

So the original focusing arm is now 'sandwiched' in between the back aluminium ring and the front stainless steel flange. The anchor bolts run through the two rings. The aluminium ring was machined to 'encase' the original arm so that the whole face would be flush with the stainless steel flange. In practice the gap difference in between the two rings when the primary mirror is tilted is in the order of 1mm to 1.5mm

*note: It is important to understand that the whole weight load of the primary mirror is now transfered via the three anchor bolts to those two rings and the push/lock screws at the back of the focusing arm.
The mirror does not touch the focusing tube any more. It is free from it. The only thing holding it on the stainless steel flange now is the RTV bonding.

The original retaining ring in the front is only a safety measure should the bond fail. In no way should it be pressing the primary mirror surface. Light contact, then finger tight. That is all. Like the corrector ring in the front cell. No pressure at all.

*note: You'll notice that the holes in the aluminium ring are a little wider and oblong towards the center of the tube. This is so the 'pass-through' anchor bolts have a little play and room to move when they go out of square as the primary mirror tilts forward.

The back of the primary casting cell was drilled to make room for access holes for both push/pull and locking screws.

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Focusing Arm Modifications