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Is this the vernier you have?
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Looks like it.
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Originally Posted by Poita
So what is your process for making sure it is centred?
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I briefly thought about writing it up for an IIS article, but it's not rocket science - it's finicky, very time consuming and error prone, and I figured I'd get many complaints from people expecting miracle cures or instant results. My method works for me, but I don't guarantee it will work for anyone else.
It took many hours trial and error and painstaking care.
I first measured the machined secondary holder outer diameter and found it to be "round" to within the limit of measurement error. All other measurements were for the distance from the inner edge of the corrector cell to the outer edge of the secondary holder.
The key points are (IIRC):
- Have the OTA in a stable, comfortable working position - I had it in my EQ6 Pro, oriented to point vertically so that the corrector cell was at chest height, and levelled.
- Take many measurements and take mean and RMS error (ignoring obvious outliers), or I suppose you could calculate standard deviation in lieu of RMS error - this is, IMHO, the best way to reduce method, observational and instrumentation error, though the Vernier was quite reliable - I think it lives up to it's stated single measurement error margin of +/- 0.01mm (<50mm) or +/- 0.03mm (>50mm).
- For roundness, use measurements at 30 deg. intervals around the cell - add opposite measurements to calculate a "diameter".
- Complete each entire process more than once to improve/verify technique.
- Measure "roundness"; calculate centre position; centre corrector assembly; re-measure centring; re-centre corrector; re-measure centring ... until satisfied that no better precision can be achieved.
Physically, I loosened corrector cell retaining screws and lightly tapped the secondary holder to achieve movement.
After tightening the corrector cell screws, I took a final set of measurements to confirm no unintentional movement.
Of note, I found that the secondary holder did not measurably move relative to the corrector.