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Ed Popko you said: Geoffrey, You commented: No cross hairs to center bubble!! No, because the instrument is tilt invarient. If you tilt it, the bubble follows the image, so you do not need cross hairs...

If I tilt my A-12, the bubble will, of course, move opposite the tilt. AND the body will go opposite that.  The bubble doesn't follow the image at all. What am I missing? It would be wonderful if the A-12 was tilt invarient. Perhaps I'm using it wrong or something in the optics is missing. Ed

Cleverer men than me have said that for the star to move with the bubble, the radius of the curvature (the convexness) of the top of the bubble chamber must equal the focal length of the collimating lens.  I find my A12, or indeed my MkIXs as well, work after a fashion, but you can only tilt them so far before the bubble and the star start to diverge.  The MkIXs seem less affected than the A12.  Maybe it’s the result of years of amateur attempts at ‘renovation??’, using a substitute fluid, or the bubble chamber incorrectly aligned.  Don’t be tempted to do what I tried.  I thought I’d see how I got on with a black spot stuck onto top of the chamber as a kind of fiduciary mark.  It doesn’t work.  It just mucks up your bubble chamber.  Don’t be tempted to put a touch of varnish around the filler hole either to reduce evaporation.  If it gets onto the tiny ball bearing, you’ll never get it out of its socket again.  That said, the A12 is a real traveller's sextant, light and easily stored or carried.  The box isn't constantly trying to cripple you like the MkIX box.  Dave