Sextant dismantling.

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Graham_Wright

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One of my sextants has a group of "hairs" interposed in the optical path. At first I feared lens cracks but that is not the case. Second guess was spider intrusion but the lens tube is sealed.
Closer inspection suggest tiny strands of swarf. I have dismantled the assembly as far as possible (which is not a lot) but it is not obvious how to dismantle further. Do any experts on here know if the inner tube carrying maybe two lenses is pressed into the outer?
 
As AntarcticPilot has said, it will depend upon the make or even model of sextant on how the optical unit is assembled, it does sound as if the optical unit is assembled using the 'burnishing' method, (the glass is centred on the mount and then held in place by 'folding' the edge of the tube around the lens) Sorry not very good description, if assembled in this way it's a very specialised job to work on it, alternatively the glass may be held in by a screw down collar, this should be fairly visible and would make life easier but from the OP this suggests that this is not the case.
 
Unfortunately the precise separation of lens elements often matters, so there may be some level of taking it apart that risks it not giving as good an image after it's put back together.

I agree with Praxinoscope that screw down collars are pretty common, that burnished-in elements certainly exist and are specialist, and that pressed-in collars are rare. I did once encounter a friction fit screw-down in an old camera lens (to undo, you had to spot a hairline crack, then use a bit of rubber on the end of a right-sized tube to push that piece round to unscrew... ugh).

I bet there are a lot of good sextants out there from makers that are no longer around. There are still a few professional sextant repairers but repair costs more than taking a chance on another used sextant :-(
 
Graham_Wright said:
One of my sextants has a group of "hairs" interposed in the optical path. At first I feared lens cracks but that is not the case. Second guess was spider intrusion but the lens tube is sealed.
Closer inspection suggest tiny strands of swarf. I have dismantled the assembly as far as possible (which is not a lot) but it is not obvious how to dismantle further. Do any experts on here know if the inner tube carrying maybe two lenses is pressed into the outer?
Click to expand...
Posting a photo may help?
 
Daydream believer said:
Not always that easy. Mystic Meg used to act as an intermediary, but sadly she has now joined the other side as well. So one needs to find a vicar with a leaky roof & see if crossing his palm with silver would help with contacting the dead :cry:
:rolleyes:
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Sorry - a vicar won't help. Contacting the dead (or attempting it) is strictly verboten (Leviticus 19:31, 20:6 and 20:27 and Deuteronomy 18:11) Saul did it, but it ended badly for him!
 
It's a Hezzanith and otherwise in very good condition. The star telescope is fine. The problem rests with the image inverting telescope. As these are sometimes supplied with cross hairs, I wonder if they may have come adrift although there seem to be too many.
 
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