Inside the Skalensextant

4 04 2010

In a post about a year ago, I gave an introductory description of this unusual sextant, that uses a glass scale moving past the optics of a micrometer microscope to give a rapid readout in degrees and minutes. Recently, a correspondent in France met one of the same problems as I had, that of being unable to view the degrees scale, and asked me how he could get at the optical system to clean it. Sometimes I am guilty of thinking that what is clear to me is clear to everyone else, so here is a blow by blow account of how to get at the hidden insides of the Skalensextant. The mirrors, shades and telescope are conventional and need no special description.

First, remove the handle by undoing two screws as shown in the photo that follows:

This allows access to the back of the index arm bearing and “index” arm. I’m putting index in inverted commas just this once, to recall that it is not strictly speaking an index arm, since it carries the scale that moves past a fixed index in the viewing microscope. You need to remove the index arm by removing a grub screw…

…and then unscrew what we have to call a bolt (since it is a screw with a shaped head), although the term seems out of place in the context of instruments.  If you now operate the release catch, it allows you to wriggle off  the index arm from the index arm shaft, given its proper name of journal in the photo. Notice that the grub screw has rather a ragged slot. If you find a grub screw in this state, either replace it or file it smooth and cut a new slot. If it loses its head at the bottom of a hole, it can be very difficult to remove without drilling it out and possibly damaging other parts in the process.

The rear cover can now be removed by undoing six screws as circled in the next photo. You do not need to remove the horizon mirror. Note at this point the little cover held on with two screws, just below and to the right of the upper middle cover screw in the photo.

You can now take a look inside the sextant, but I won’t repeat the description given in the previous post.

Next to come off is the little dovetail slide to which is attached the lighting system. This allow access to the two cheese headed screw that hold the objective  assembly to the frame. Remove these two screws…

…and lift out the assembly.

The prism carrier is attached to the objective assembly body by two screws, only one of which has been removed in the next photo because the other one defied all my non-destructive efforts to loosen it. Fortunately, the two right-angle faces are now accessible for cleaning, and traces of rouge that I used to clean up the very heavily soiled lens faces can still be seen on the carrier. Since I could not detach the prism carrier from the rest of the assembly, I was not able to release the prism to get at the hypotenuse face, by my normal method of slowly heating in water from cold until the shellac melts. I give an account in the previous post of why it was necessary to remove what was left of the silvering on the hypotenuse face and how I did it. If after cleaning the other faces, it is possible to get a clear view through the prism, there is no need to touch the third face.

Re-assembly is the reverse of dis-assembly.

Returning now to the little round cover noted above, its removal allows access to the focus adjusting screw. To bring the main scale into focus, first adjust the micrometer eyepiece until the minutes scale is sharply in focus. Then loosen the screw a little until it and the attached objective lens can slide in its slot. Move it back and forth until the degrees scale come into sharp focus. Only one number will be in coincidence with the minutes scale at a time. This reduces ambiguity, but makes it a little difficult to find an out-of-focus degrees number. When you think you have both scales in focus, check that the degrees marker does not move at all over the minutes scale when you move your head a little from side to side (the parallax test), and when you are satisfied, tighten the screw and check again.

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