A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Index Correction
From: Fred Hebard
Date: 2007 May 6, 09:20 -0400
From: Fred Hebard
Date: 2007 May 6, 09:20 -0400
Alex, I doubt 15 minutes is enough time for a metal sextant to equilabrate from room to winter air temperature, assuming it's not in the sun, unless it is very windy. Fred On May 6, 2007, at 4:59 AM, email@example.com wrote: > > > Dear Robert, > What you describe sounds unusual and strange to me. > >> This afternoon, I took my sextant >> out for a round of noon sun sites and >> some >> post noon sites. >> After letting the sextant sit outside for about 15 >> minutes >> at minus 3 C, I commenced my IC check. >> IC was determined to be +0.3. Took >> several observations for noon and afterwards, six post noon >> observations. >> Total elapsed time was about 30 minutes. IC check at the end of >> the round >> of >> sights amounted to -0.4. > > It is unlikely that this is due to temperature, because > 15 minutes is enough for temperature to level. > I would try to investigate the reasons of this unusual > behavior of the sextant. There is little doubt that > this is not normal for IC of a metal sextant > to change by .7' in 30 minutes. > > How exactly do you check IC (do you make several sights > for it or just only one sight?) > If you did not do any observations in between do you think > IC would be the same in 30 minites? > > >> For the noon shots, I took +0.3 because the shots were taken >> literally a >> few >> minutes after the index error check. For the remainder, which >> occurred >> about >> 10 minutes after the noon shots, I used the average of the two ICs. >> Results >> were good. >> >> Alex's contention that a well designed sextant should not change >> IC once >> it >> has been allowed to acclimatize for a set period of time is >> something I >> can >> agree with in principle, however, it does not jibe with my own >> observations >> over the decades. I own a C.Plath. You cannot get a better design >> than >> that. >> It has not been dropped, abused or insulted in any way, yet I >> often get a >> slow change of IC over a round of sights. More correctly, side error >> creeps >> into the sextant -- mostly under non-standard conditions -- which >> in turn, >> affects index error. Mind you, the change over time -- at least in my >> sextant -- is less than one minute of arc, so I should not >> complain but it >> is, nonetheless, aggravating. >> >> Further to the above, I can tell you for a fact, that the IC at >> plus 20 C >> will be radically different than the IC at minus 20. No if, ands >> or buts. >> >> Thanks for the response Bill. It is nice to know that at least one >> other >> person out there has faced the same situation. >> >> Robert >> >> ----- Original Message ----- >> From: "Bill"
>> To: >> Sent: Friday, May 04, 2007 4:44 PM >> Subject: [NavList 2819] Re: Index Correction >> >> >>> >>>> From: "Robert Eno" >>> >>>> It is common practice to check for IC before and after a round of >>>> sights. >>>> In >>>> my experience, they are seldom the same particularly when taking >>>> sights >>>> under >>>> non-standard temperatures. >>>> >>>> Question: what correction would you apply? Generally what I do >>>> is a >>>> combination of fudging and black magic but it usually entails >>>> simply >>>> taking >>>> the two ICs and averaging them to arrive at a single IC that can be >>>> applied to >>>> the entire round of sights. >>>> >>>> Please: I am not looking for involved statistical computations. >>>> I am >>>> trying to >>>> determine what other navigators do in the field/at sea. >>> >>> Good question and one I am wrestling with currently. Sextant, Astra >>> IIIB >>> Deluxe. >>> >>> In speaking with Alex, he maintains that a well designed sextant >>> will >>> not >>> change IE based on temperature if you let it acclimate to the >>> ambient >>> temperature. That is, after a 20-minute period (in the shade if >>> doing >>> sun >>> shots) it should have the same IE/IC at 20F as 50F as 90F. The IE >>> should >>> not change over a round of sights. Alex has the silver finish on >>> his >>> SNO-T >>> (tropical). >>> >>> I get mixed results. >>> >>> One day I went out and did IE checks for 25 minutes sitting in >>> the sun >>> (after sextant had achieved ambient temperature and therefore >>> stabile in >>> theory). Over that period the IE changed from 0!0 to 0!6 off the >>> arc, >>> with >>> half the change happening in the first 7 minutes. >>> >>> Temperature was 76F, very clear sky, elevation about 47d, 3 kt >>> breeze. >>> >>> The next day I went out at the same time, same chair, same place, >>> same >>> shirt, temperature 74 F, 12 kt breeze, *very* slight overcast. >>> At most >>> the >>> IE changed (again to off the arc) 0!1 over 27 minutes. >>> >>> I used the Sun's limbs for IE. 4SD was within 0!15 of Frank's >>> site in a >>> given three-minute set, closer if factoring in 1.55" per diameter >>> increase >>> for irradiation. Sigma was less than 0!1 for all off and on the arc >>> averages >>> from a given three-minute set, so I feel confident in that data. >>> >>> My thinking to date: >>> >>> I have noticed the shift off the arc in lengthy sun observations >>> before. >>> I >>> suspect heat from the sun and low ventilation on the first day >>> allowed >>> the >>> black-finished sextant to heat up during the observations and be >>> unstable. >>> The slight reduction in energy due to the haze and wind kept it >>> relatively >>> stable the second day. >>> >>> So tough to predict. Best guess is to do an IE check after >>> letting it >>> come >>> up to ambient temperature, then at the end of the round of >>> sights. If >>> they >>> differ, average the IE and use that for IC. >>> >>> Bill >>> >>> >>> >>> >>> >>> >>>> >>> >> >> >> >>> > > > > --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To unsubscribe, send email to NavListfirstname.lastname@example.org -~----------~----~----~----~------~----~------~--~---