Re: F. Barker & Son Box Sextant
From: Brad Morris
Date: 2016 Apr 9, 06:38 -0400
Any competent machine shop can make just about any screw to any thread. I've had it done. It is not cheap. One manufacturer here in the US was fond of making oddball threads in the 1800's to make sure you could only buy parts from themselves. It is a nightmare because you simply can not source those screws today. That is, without getting them made.
As to the resilvering, I've had that done too. That's actually not so expensive at all. I had both the index and horizon mirrors done. Came out beautifully.
As to your phantom, you may have missed my point. One of the lenses may be slightly off axis. Wouldn't take much. That's why you should take the lenses out (clean them) and put them back. It's to reseat them and get them back on axis. It could be nothing more than an eyelash at the edge of a lens seat.
One other venue for parts is to simply (ha!) buy another box sextant of the same type for parts. Use two to make one.
I actually did muster up the courage to remove the mirrors and clean them. I held them with a micro-fiber cloth and wiped them with some disposable lens cloths. Turns out that what I thought was the retaining clip was actually the mount and vice versa. Unfortunately, the cleaning had no effect on the distortion. I think perhaps the silvering on one or both of the mirrors has become too degraded over time. The backs of the mirrors were moderately corroded. When I look at reflections of nearby objects in each of the mirrors individually, the images appear nice and sharp. But I realize that when looking at objects that are several AU away, what would otherwise be imperceptable defects have an exponential effect.
I am now considering having the mirrors resilvered, or having new mirrors made altogether. I'm just not entirely sure that my local glass shops are up to the task of making such small, precise mirrors. Especially ones that need to be silvered only in very specific areas. (Both mirrors lack silvering around the edges, where the glass contacts the mount/retaining clip.) But, maybe I'm underestimating them.
I've also discovered that the square head of one of the side error adjusting screws is starting to become stripped. This might be an even bigger concern than the mirrors, because I have no idea where I would get such a tiny, unique new screw made. I'm going to try to ensure that this sextant doesn't become nothing more than a really fancy paper weight. But with limited funds and resources, it's going to be a challenge. :o
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