[Stefan Buda]

A couple of days ago I received Paul's troublesome corrector and we agreed to make public the findings of the autopsy, here on IIS.

The very first thing I wanted to do was to turn up a fixture on the lathe so that I could screw on the corrector and measure the external runout of the housing relative to the tread that is supposed to be concentric with the internals.

In order to measure the internal thread properly, I had to unscrew the retaining ring that keeps the internals from falling out.
Well, I had low expectations for the workmanship and I was not surprised to find that the retaining ring was so poorly made that it would be considered bad even on agricultural equipment. Someone just turned up a threaded pipe and then parted off 4mm wide rings without any attempt of chamfering the thread or further finishing cuts. The internal edges are manually and unevenly chamfered. The sides of the ring have the rough finish characteristic of parting off operations. Frankly this is butchery not even general engineering practice, not to mention optical equipment standards.
Anyway, I measured the thread to be M71.7 x 0.75 If you ask what the heck is that, I can't answer because I've never seen such a thread.
I did turn up a matching fixture and proceeded to measure the runout of the external cylindrical part. The end furthest from the jig had a runout of 0.12mm which is not good but not a disaster. However near the jig it exceeded the range of my dial indicator and had to use a coarser one which measured it to be 0.44mm That is very bad and I completely fail to understand how someone can produce something that bad. You don't need a dial indicator to see how bad it is spinning. So what was the culprit thinking?
Next I looked through the spinning corrector to find that everything - all the lens surfaces - were wobbling madly.

On the next episode we'll take a closer look at the internals.