NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: C. Plath Sextants Mirror Adjustment
From: Kieran Kelly
Date: 2003 Sep 16, 18:42 +1000
From: Kieran Kelly
Date: 2003 Sep 16, 18:42 +1000
I recently posted the following request on the sextant list but have had no reply: I am seeking information on the mirror adjustment mechanism on the Plath Navistar Professional which is substantially different from that on the Plath Classic. One reply received said: >First of all, it's not a matter of loosening (and re-fastening) screws, we're only talking >about adjusting screws. These are the ones with the square head for which a special key is >provided with the sextant (at least, it should be). My reply to this was: This is certainly the case with the Plath Classic as it has two square headed adjusting screws on the horizon mirror and a single square headed adjusting screw on the index mirror and is simple to adjust with the special tool provided. The horizon mirror of the Plath Navistar Professional however has two hexagon shaped adjusting screws for which the special tool is used plus a third screw, headed for a screwdriver. All these screws are on the back of the mirror housing. The index mirror on this sextant has one adjusting screw plus a conventional screw all on the back of the housing. The extra screws appear to be designed to hold the mirrors in the frame. What do you do with these screws? Do they have to be loosened while the adjustment process is made then tightened when all is in alignment? This is the process with some of the English sextants. I am frightened if I try the adjustment with this screw done up tight I may break the mirrors. I have checked the exploded view of the Plath Professional sextant which comes with the manual and it shows the conventional screws holding the mirror retaining springs in place however it is silent on what should be done with these screws when adjustment is made. Note that Bauer's The Sextant Handbook cautions on page 55 that the navigator must always loosen one screw when adjusting the mirrors on a sextant before tightening the other otherwise the glass will break. Does anyone know if this applies to the two adjustment screws on the horizon mirrors of the Plath Classic and Professional sextants? Any help would be appreciated. Regards Kieran Kelly