A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Mark Coady
Date: 2015 Dec 19, 10:10 -0800
Been experimenting with setup and index correction on my davis plastic mark 25. Mark 25 of unknown pedigree bought used at consignment shop a long time ago....
I lubricated mine at the start with clock fulcrum oil. This cured a sticking problem awhile back. THe index arm would seem to observadly hang with friction and twist things just a whisker...so errors were increased. I cannot promise lubrication is a smart thing (do at own risk), the davis website still has the documentation, with no mention of lubrication. I am sure worry about negative effects on the plastic with the wrong product is a concern. "Plasticizers" used in plastic manufacture are oils....so the wrong oil or additive can attack plastic. I have lubed mine for several years now with no deletorius effects using very pure oils.
Nice thing is if it gets dusty/dirty....I pop the mirrors off and "dawn" detergent and a bucket work wonders..(rinse well). (LOL if "dawn" works on oily pelicans, why not a plastic sextant....not that to many similarities exist except for the oil...). My sensibilities would (perhaps unfairly) recoil in horror at my metal sextant treated so horribly.
I have learned a gentle and consistant lighter grip on the handle than my usual heavy handed from brass. I found I tend to add distortion of several tenths.
I am not great at index correction anyway, even on metal sextants, so I am enchanted with result.
Its cold and windy on dock. Sights today were taken out the window of my boat pilothouse. which is heated. So temperature variation and solar heating effects are minimized.
I have proved to myself how critical consistancy in direction is paramount to premium plastic accuracy.
TO wit: I set side and index error screw adjustmets several times alternately by direction of always increasing sextant angle. CCW drum, body descends, arm moves away from you. index error final set .1 "on the arc"
Result index error consistant over many sights within +/- .1 or so and often closer to +/- .5, if I always bring body down same direction. I was impressed.
Reversing the direction, CW drum, I found index error to be .4 to .5 "on the arc", leading to a conclusion of increasing vs decreasing backlash error of about .4 - .5 with some consistancy. Not horrible, and the rack might be worn somewhat.
When I get a warmer day without a howling wind, I am going to use Frank's spotting scope trick and see how much more lash I can get out. Some of the slop here is clearly my uncalibrated eyeballs.
Suffice it to say...the results were quite good. I have a good metal sextant, but like to use the plastic playing around in adverse conditions.
I also use this learning to build good habits on metal sextants. Always going the same way makes perfect sense.