The Nautical Sextant

This post is preceded by “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  “Refilling Mark V/AN5851 bubble  chambers” and “Overhaul of MkV/An5851 bubble chamber”

I have been asked about the carrousel that holds the various shade glasses in the AN5851 bubble sextant. This is one of my least favourite bubble sextants, heavy (2.5 kg), awkward to hold and the optical path complicated by the inclusion of a horizon prism. The near indestructability of the case was bought at the expense of more weight (6.4 kg!).

My correspondent reported that his carrousel was jammed and wondered whether its associated spring could be the cause. Here’s how you get at it.

1) Remove 4 screws (circled in white, below) from the black plastic cover and remove it. Take the opportunity to clean the copper electrical contacts inside it, that carry current to the scales-illumination lamp.

2) Loosen, but do not remove, the two screws that hold the black shade cover over the carrousel. Then dislocate it upwards and pull it outwards.

 This reveals the shouldered screw that holds the carrousel on to a complicated little alloy casting that also mounts the astigmatiser. There is a brass spring washer between the head of the screw and the alloy carrousel body. Removing the black shade cover also loosens the leaf spring’s attachment, and in the unlikely event that a disordered spring is the cause of stiffness, this should allow the carrousel to revolve freely.

3) Corrosion between the central screw and the carrousel is much the most likely cause of stiffness. It is not easy to get at the screw slot without removing the index prism (best left alone) or improvising a bent screw driver from a piece of steel strip or grasping the head of the screw in a narrow pair of pliers. However, if you swing the astigmatiser out of the way, you will be able to get at the two screws that hold the casting into which it is inserted, to the frame of the sextant. If there is corrosion, and you need to use a lot of force, you may break off the screw where it enters the casting or damage the casting. It is much safer to apply a little thin oil between the screw head and the carrousel. Allow a few hours for it to penetrate and then gently work the carrousel free. Be prepared patiently to repeat the process several times. Remove the central screw only if it loosens easily.

Replacement is the reverse of dis-assembly.