Collimatable focuser back plate.

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Having finished the base ring/adapter I could then proceed with turning the collimatable back plate. 

It started as a 20mm thick slice from a length of 180mm / 7" round aluminium stock. I removed a core disk from the centre of the disk [from both sides of the disk] using a parting tool. 

Not ideal, on a disk, because of the depth of the tool jamming in the curved cut. So I swapped tools midway for a dovetail shaped tool to allow more room for the deep, parting tool blade. 

A 10mm deep groove, from each side, was not too time consuming using manual feed. I had imagined myself chain drilling to remove the centre disk but it would have been far too slow on 20mm thick aluminium. Chain drilling is probably more sensible on steel which would resent a parting tool being abused this way.

Once the central disk had fallen out [image above] I could continue with a boring bar to smooth the hole and open it up to the correct diameter. I was careful to allow clearance for the focuser drawtube. 

Then I made a 10mm deep recess, or counterbore, to the diameter of the focuser base ring. This will ensure firm support aided by a few small, hidden screws from the telescope side.

The image left shows the [black anodized] base ring fully embedded in the collimation plate. The 5.125" x 24tpi thread for the focuser collar is projecting towards the camera.

There is still some tidying up to do on the raw edge of the disk. I may bevel or even curve the edge for decoration and a little extra weight loss. I have a home-made, ball turning tool which could be pressed into service here to achieve a concave radius.

The back side of the disk also needs facing off and perhaps some judicious thinning. Four collimation screws will be fitted in the plate to allow accurate collimation of the focuser with the optical axis of the 180mm/ 7" objective. [More to follow.]

Thursday: I tried to put a curve on the collimation plate but my home-made, ball turning toll wasn't up to turning such a large diameter. So I turned a series of steps and bevels just to see what it might look like. I realised afterwards that I could use one of my very long, wood turning chisels. To scrape a curve with a round nosed chisel on a tool rest. I have no idea if it will work until I try.

Don't you just wish that once in your life you has an original idea? As soon as I turned to today's master reference of all things practical there were hordes of aluminium turners on wood lathes wielding long series bowl gouges. No wonder. It is dead easy and provides fast material removal if you avoid chatter.

I went back out before dinner and used three of my Marples bowl turning tools. A rub with maroon Scotchbrite fibre with the lathe running smoothed any remaining tool marks. Very satisfying work too. If you don't mind the huge quantities of woolly swarf. I imagine the original brass tail plates on classical telescopes were either spun or cast.

Friday: I cut back the curve on the back plate slightly more.

Click on any image for an enlargement.

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