Folded refractor: New 10mm front plate.

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As mentioned previously, the objective bayonet system worked but hindered collimation due to clearance problems. There was just not enough room between the plates despite the thick felt pads. So nuts would push or pull against the plates.

My first attempt at cutting a 198mm hole was done by chain drilling a circle of small holes and then opened out with two larger sizes of drill. Each larger drill narrows the metal between the holes. This seemed to go well until the router wanted to take more metal to clear all the previously drill holes. So the hole ended up much too large by the time I had re-cut it twice more using different pilot holes in the router base plate.

So I started again with another scrap aluminium plate. This time I went straight to the router without bothering with drilling all the holes. I took a couple of shaving cuts first and measured the diameter carefully before proceeding. Despite the arcs measuring rather small at first it actually ended up 1mm larger. Which was perfect for my needs in just clearing the back of the iStar objective cell. Which is rather mean in diameter. With barely enough room for the collimation screws to get any purchase in the backplate. At least, not if the mounting hole is made larger than 199mm Ø. 200mm would be too big and the collimation screws would eat into the hole!

During the cutting I had run some screws directly into the workbench in holes provided in the scrap plate. With thick washers sandwiched to lift the plate off the top surface. Otherwise the router bit will take a circular bite out of the top surface of the bench. 

I used a rather tired bit in the router which made for rough cutting if I lowered the cutter by more than a millimeter. Adding a few drops of lamp oil into the groove between cuts helped keep the revs up and the roughness down to a minimum. Then I just kept going round and round with the router cable held above my head with a pole through the hedge.

Now all I need is to mark, drill and tap the plate for the collimation "pull" screws. Followed by sawing the plate to size on the sliding miter saw. With a final going over with a sander to to tidy up all the roughness. Another clear night is forecast. So I had better get on with it!

Click on any image for an enlargement.

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