Re: What is the diameter and thread on the Pasil.4 polar scope?

Christopher Erickson

Speaking as a hybrid hobbyist/professional machinist, I would add that threading can be one of the trickier machining efforts to get right. Especially when you are going to be making two different threads that you are going to want to have very accurate in reference to each other and with the shoulder that the polar scope is going to seat against. And threading up tight to a shoulder is tricky and usually requires a modified threading tool. Testing the fit of the threads on the mount and the polar scope will also be tricky because you will most likely have to remove the part from the chuck and then replace it and re-register it with the threading tool. If you have a compound slide combined with a big illuminated magnifying glass, that can help to get the threading tool re-registered with the partially-cut threads when re-mounting the part in the chuck. However getting the piece precisely-square in the chuck again (.001") isn't trivial at all. I assume you have a dial-micrometer or something with a magnetic base.

On my (German) Prazi lathe which does not have split carriage nuts, I made a manual drive handle that allows me to manually turn threads at a slow rate, which gives me a very precise feel and ability to stop the tool at precise positions. And no risk of over-driving the cutter and ruining the part or breaking the threading tool. YMMV.

If you get the two threads and the outside shoulder even slightly out of square with each other (more than .001" and .1 degree), you will create problems and inaccuracies for yourself when mounting the polar scope on the mount. You will be able to adjust the PASILL polar scope to compensate for pointing errors in one position but when you rotate the polar scope, your alignment will go out. And the tiny PASILL alignment set screws are in direct contact with the glass reticle. Over-tighten them even a tiny amount and you crack the glass. You don't want any play but even the slightest over-tightening will likely ruin the reticle.

This sounds like a challenging but potentially-fun project however you are still going to want to perform a careful drift alignment with the mount so you can accurately test, align and evaluate your machined adapter. And if your goal is to only use this thing once (or once a season) for a permanent polar alignment of a mount on a pier, I really don't see where you will be gaining much from this project, except honing your budding machining skills.

And no matter which way you go, you are still going to have to perform a careful drift alignment if you want to do some serious astrophotography. Even if you invested in an official AP polar scope bushing adapter.

Depending on your mount threads, I might have 3 or more of these adapters in one of my spare-parts boxes and I could simply mail you one.

I hope this helps.

On Fri, Jun 19, 2020 at 10:31 AM Tom Blahovici <tom.va2fsq@...> wrote:
Sounds like I my have to rethink this...I certainly didn't think about having to try it and reposition it in the lathe.
I guess I will need some practice.

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