A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Davis and Astra head to head
From: Hewitt Schlereth
Date: 2013 Mar 5, 09:11 -0800
From: Hewitt Schlereth
Date: 2013 Mar 5, 09:11 -0800
Right, Lu. Most yachts sail in summer's generally sunny conditions. Until I got my C. Plath (huge mirrors) it was DR, sun lines, noon sights; moon only when it made good crossings with sun lines. If I happened to have the helm, my first and last of the day were often horizon sights. Hewitt Sent from my iPad On Mar 5, 2013, at 7:47 AM, "Lu Abel"
wrote: > I will totally agree -- there is no comparison between Davis optics and > those found even on an Astra. When I was just learning celestial I > tried a Polaris shot with my Davis. Very difficult. > > Interestingly, I've heard of research that shows that less than 10% of > offshore sailors (ie, pleasure boaters, not ships) practice celestial > any more and 90% of them do only Sun and Moon shots -- no planets, no stars. > > > On 3/4/2013 1:44 PM, Hewitt Schlereth wrote: >> ------------------------------------------------------------------------ >> Lu - >> >> I've had experience with both the Davis 15 and 25 and found no difference. In 2010, three students and I shared a brand-new 15 for five days aboard a 45-foot sailboat in the western Caribbean. Conditions were perfect and I was able to check each sight against GPS noted at the time of the sight. The ranges for the 5 days were 0.1' to 3.1' from the GPS. Index errors ran from -1' to + 3'. >> >> The main thing we had difficulty with was stars. Best approach turned out to be to take only the brightest and remove the scope. >> >> When I began navigating at sea in 1972, I had a new Ebbco plastic sextant (you can see me using it aboard a 40' yawl on the cover of Commonsense Celestial Navigation). In those days the only check was the DR, but judging from our landfalls I'd say the Ebbco and Davis were equivalent. >> >> I now have an old 15 that a friend found in the bilge of a boat he bought. The scope was missing and the mirrors were shot, but I cleaned the sextant with hot water and soap, and replaced the mirrors and shades. Now I get pretty much the same results from land here in San Diego as the four of us did in the Caribbean. The main thing I've noticed is the index errors on this old 15 range from -8' to +12' which I suspect has a lot to do with wear of the mechanism over years of use - not to say abuse. :-) >> >> Hewitt >> >> >> Sent from my iPad >> >> On Mar 4, 2013, at 8:55 AM, "Lu Abel" wrote: >> >>> Randy: >>> >>> First of all, congratulations on 3,000 sights. More than I've ever taken in my life, likely more than most ship navigators will take in their lifetimes these days. >>> >>> I don't think there's anyone on this list who would argue that there's any comparison between a Davis and an Astra. I think, however, that there's a question of "good enough" >>> >>> If I'm going to take sights off the pitching deck of a small boat in the middle of an ocean, do I need a sextant that is going to give hs's to tenth-of-a-minute accuracy, when it's doubtful that I can take a sight to anywhere near that accuracy? >>> >>> Also, is your Davis a Mark 15 or a Mark 25? They seem to be made out of very different plastic and it would not at all surprise me to see a difference in variation in run if IE measurements between the two. >>> >>> Also, you describe measuring IE by alternately bringing the horizon up and bringing it down. In my experience, IE can differ by a tenth or two on a Davis because of some lash in the micrometer. My advice when I'm teaching celestial to anyone using a Davis is to measure IE both ways and then use whichever one matches the way the body is brought up/down. >>> >>> Thanks for the comments on scope distortion. The next time I have my hands on a Davis I'll look for that. >>> >>> Again, not trying to say that a Davis is anywhere near as good as an Astra. At the same time, a good navigator understands the limits and weak spots of his instruments and tries to work around them. I think there are some techniques to squeeze a bit more accuracy out of a Davis than you observed. >>> >>> Lu >>> >>> >>> >>> >>> >>>> ________________________________ >>>> From: Randall Morrow >>>> To: luabel---com >>>> Sent: Monday, March 4, 2013 7:29 AM >>>> Subject: [NavList 22615] Davis and Astra head to head >>>> >>>> >>>> >>>> ________________________________ >>>> The topic of metal versus plastic comes up form time to time so I did a head to head comparison to satisfy my curiosity abotu how much diffrence there was. Though I am the only tester I have taken over 3,300 observations in the last 2-3 years so it is probably as good as any. >>>> Index corrections checks for the Davis were done the night before with a star, using 6 pairs of sights, each pair turning in opposite directions,and then averaged. The result was >>>> -8.4. The next day 3 sun sight IC checks showed and average of -8.0. The Astra IC was done witgh three sun sights that averaged +0.4. I have always found index correction sights to be inconsistent with the Davis so extra care was taken. >>>> Fifteen consectuive LL sun sights were done with the Davis using a mirror artificial horizon. The average intercept was 6.5 nm. The range was 3.6 minutes, from a low of 4.6' to a high of 8.2'. Every sight was "away". >>>> Only 12 sights were done with the Astra because I expected good results. Six were taken with the standard 3X scope with and average intercept of 0.4 nm. Another six with the 7X scope averaged 0.6 nm. The reange was 0.9', from 0.1' to 1.0'. There was only one "away". I was surprised that the 3X scope results wre pooere than the 7x, but thia is not typical. >>>> The preponderance of "away" sights is liekly a personal bias toward how I percieve tangency throgh the scopes. The Davis always appears to me to have optical distortion of the shape of the sun disc as you approach tangency. >>>> It appears the Davis is very consitent, though clearly not as accurate as the Astra. The reslults are certanly good enough to take you anywhere you might want to sail to. The metal frame was an order of magnitude better and though the stability of the metal frame and arc is a factor, I suspect that the quality of the optics may also be important. The more sights I take the more I am convinced that it is the observation, or the perception of what you see or think you see through the scope that is the dominant factor in accuracy. >>>> If anyone is interested I can post the sight data some other time. Randy >>>> ---------------------------------------------------------------- >>>> NavList message boards and member settings: www.fer3.com/NavList >>>> Members may optionally receive posts by email. >>>> To cancel email delivery, send a message to NoMail[at]fer3.com >>>> ---------------------------------------------------------------- >>>> View and reply to this message: http://fer3.com/arc/m2.aspx?i=122615 >>>> >>>> >>> >>> >>> >>> View and reply to this message: http://fer3.com/arc/m2.aspx?i=122618 >>> >>> >> >> >> >> View and reply to this message: http://fer3.com/arc/m2.aspx?i=122627 >> > > > > > > View and reply to this message: http://fer3.com/arc/m2.aspx?i=122635 > >