Custom Base Ring for 3.5" FTF 3545B-A focuser.

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My FTF 3.5" base adapter ring has arrived a day early after getting lost with UPS yesterday. I've discovered that I'll have to put the ring in the lathe to bore out the inside.

This is to  make room for the focuser's main body clearance. Which I didn't know anything about before the focuser arrived. The male thread on the ring looks to be the right diameter and the finish and anodizing are both excellent.  So I had better protect it from the jaws of the chuck! Not that any of it will be visible when safely in place on the back plate.

The tubular body of the focuser housing is about 121mm outside diameter. There is an added complication of a draw tube stop screw. This will need a recessed shoulder on the telescope side of the ring. Otherwise the stop screw will limit inward draw tube travel by a few millimetres.

Increasing the bore to 122mm [for FTF body clearance] thins the upper ring to half of its original, radial thickness. So still plenty of material left for strength. The drawing is not to scale but just to show the material in red to be removed. In practice the recess needed to be much deeper. See the next chapter/post.

The actual depth of the increased bore as yet unknown. I shall have to use trial and error until the shoulders meet with the focuser collar [almost] screwed on fully. If I overdo the depth the focuser will wobble on the base ring after collar tightening. It should be locked firmly in place. Only becoming free when the collar is loosened for focuser rotation. I could cut a thin packing ring if I get it wrong but would rather not.

As usual, I am rehearsing it all in my mind to avoid any expensive foul-ups tomorrow. I'm bound to think of something else if I sleep on it.

Further examination of the focuser reminded me that there is a bevel on the outside of the body. This is easily managed in the lathe once the correct depth of boring is achieved. I was going to remove the pinion block/housing but it has odd [US?] sized socket head screws. None of my collection of at least 50 hex keys matches the screws. Ever onwards.

Click on any image for an enlargement.

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