A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Alexandre Eremenko
Date: 2005 Nov 21, 13:30 -0500
From: Alexandre Eremenko
Date: 2005 Nov 21, 13:30 -0500
Dear list members, I want to share some test results of my SNO-T. (It was bought in Fall 2004 "new", but made in 1990, has a factory certificate showing correction +10" everywhere on the arc, which is nonsense, of course. In summer 2005 I had it checked and certified by BOTH Freiberger and Cassens-Plath. Freiberger issued a certificate which shows the following corrections: 10d 20d 30d 40d 50d 60d 70d 80d 90d 100d 110d 120d -4" -4" 10" 6" -1" 4" 5" -5" 4" 4" 3" 14" Cassens-Plath wrote only their usual "This instrument is free of errors for practical use". However my multiple careful observations (most of them under perfect conditions, from a balcony) indicate that there is some arc error as well as a visible backlash. (Of course this can always be attributed to insufficient proficiency of the observer, but consistency of the results I obtain seem to indicate that my proficiancy is not that bad). In all examples below the inverting 7 x 40 scope was used, I find it by far superior to the 3.5 x 50 Galileo scope. Sigma stands for mean quadratic error in a series, SD for the visible radius of the Sun. To avoid unconscious bias in selection I include ALL results of the last week of which I have accurate records. IC from Sun, Nov. 17, average of 5 each: clockwise: -0.42' (4SD=64.72) anticlockwise -0.16' (4SD=64.78) sigma=0.16' in each case, true 4SD=64.8 IC from Sun, Nov. 19, average of 5 each: clockwise or anticlockwise the same -0.4' (4SD=64.86', true 4SD=64.8') sigma=0.15', true 4SD=64.8 IC from Sun, Nov. 20, average of 5 each: clockwise: -0.5', anticlockwise -0.4' (4SD=65' in each case, true 4SD=64.8') sigma=0.1' Index from a star (Rasalhague) Nov. 20. clockwise: -0.6', anticlockwise -0.2'. (This is typical for my index observations from stars. Sometimes I get -0.7 clockwise and -0.1 anticlockwise, but the average was always -0.4 or -0.5 during the last two weeks. In most cases I apply the average IC=-0.4 or so.) Deneb-Rasalhague distance, Nov. 20, 00:40 GMT Average of 4 clockwise 51d12.0' (sigma=0.0') corrected for index: 51d11.4' Average of 4 anticlockwise 51d11.6' (sigma=.2') corrected for index: 51d11.4' True distance: 51d11.4' Deneb-Rasalhague again, Nov 20, 01:00 GMT Average of either 4 clockwise or 4 anticlockwise: 51d11.0' Applying the average IC=-0.4' gives 51d10.6' True distance: 51d10.7' Deneb-Altair distance, Nov 20, 01:30 GMT Average of 4 clockwise: 38d00.9' (sigma=0) Average of 5 anticlockwise: 38d00.4' (sigma=0.2) Total average of all 38d00.6'. Corrected for IC=-0.4 gives 38d00.2' True distance: 37d59.8' So we have 0.4' overshot. Deneb-Altair distance, Nov 17, 01:25 GMT Average of 6 clockwise: 38d00.8 (sigma=0) Average of 6 anticlockwise 38d00.3 (sigma=0.3) Total average 38d00.6' Correction for IC=-.044 gives 38d00.2' True distance 37d59.8' So we have overshot 0.4' What can this be is not an arc error? Deneb-Vega distance, Nov 20, 02:40 GMT Average of 4 clockwise: 23d50.1 (sigma=0) Average of 4 anticlockwise: 23d49.9 (sigma=0.1) Total average: 23d50.0' correction for IC=-0.4 gives 23d49.6' True distance 23d49.5' Deneb-Vega dist., Nov 13, 23:40 GMT Average of 4 clockwise: 23d51.3, with IC=-.8 gives 23d50.5' Average of 4 anti-c: 23d51.0, with IC=-.4 gives 23d50.6' True distance: 23d50.4' Sun-Moon distance of 126 degrees, Nov. 20, 14:40 GMT corrected for IC=-0.44 Errors according to Frank's calculator: 0.3', -0.1, 0, 0.1, 0.1. Sun-Moon distance of 137 degrees (maximum SNO arc allows! one can go a bit further, to 141 degrees by detaching the magnifying glass:-) Nov. 19, 14:40 GMT, corrected for IC=-0.4 Errors according to Frank's calculator: -0.2', 0.1', 0.3'. (One blunder discarded, error 0.7') Mars-Moon distance of 40 degrees Nov. 18, 3:30 GMT, corrected for IC=-0.4' Errors according to Frank's calculator: -0.3', 0.8', 0.2', 0.7', 0.7', 0.6' then checked the Frank calculator, and repeated the series, trying to see whether I can match somehow the numbers calculator gives:-) -0.1', 0.5', -0.1', -0.2', -0.5', 0.3', 0.1', -0.1':-) Mars-Moon distance at 16 degrees Nov 13, 1:30 GMT, IC=-0.4' Errors: 1.0', 0.6', 0.8', 0.2', 0.7' Same, with Bill's Astra, same evening, with the SAME inverting scope: Mars-Moon at 16 degrees, Nov 13, 1:30 GMT IC=+0.35 Errors: -0.1', 0.1', -0.1', 0.2', -0.1' Can I conclude that my arc has a huge bump at 16 degrees? However: Venus-Moon November 7, GMT 0:30 at 15 degrees, IC=-0.4 Errors: -0.1', 0', 0'. Mars-Moon distance Nov. 18 at GMT 3:20, at 40 degrees, IC=-0.4 (SNO again) Errors: -0.1', 0.5', -0.1', -0.2', -0.5', 0.3', 0.1', -0.1'. Sun, art horizon, oil over syrup, Nov. 20, 19:20 GMT IC=-0.4, watch exact. Antickockwise, UL: 19:28:20 48d42.8' 19:29:50 48d25.3' 19:31:00 48d10.9' 19:32:32 47d52.9' 19:33.54 47d36.7' Averages: 19:31:07 48d09.7' Error: -0.2 (undershot). But actually this means -0.4 because the measured altitude was divided by 2. Any comments? (I can supply more details on each observation if anyone is interested I would be very interested to see analogous data posted (or sent to me) by other people practicing without real horizon). Disclaimer for George: I understand that the issues discussed here are far from the practical needs of a small-boat navigator:-) Still I think these are proper questions for discussion on this list. My concern is whether I can take reliable Lunars under good conditions from land to 0.2' precision, and whether it is possible at all, and whether it is possible with my SNO-T. Alex.