NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: "Vernier acuity" of horizon IC tests
From: Bill B
Date: 2009 Jul 07, 02:54 -0400
From: Bill B
Date: 2009 Jul 07, 02:54 -0400
> Close to the eye the line between the silvered and unsilvered protions of the > horizon mirror is out of focus. At arms length, it is better focussed, as in, > say, the foresight of a rifle. A sighting tube, by increasing depth of focus > by means of the eye stop, has the same effect. Some excellent points. I might also question differences in scopes, their effective depth of field may vary based on the diameter of the objective lens and the exit. My 3.5x scope has a mask at the exit. A trick I borrowed from Ken G's catalog with my first cardboard sextant (sight tube) was adding a mask with a small opening at the eye end to increase depth of field, especially when using the practice bubble horizon as I could not focus on all the elements without an increase. > Frank also wrote: > > ?By contrast, when a telescope is attached or when the instrument is held > close to the eye, the horizon on the direct side of the field of view fades > away slowly and merges with the reflected view on the other side of the > field of view. We align these horizon images by superimposing them. This is > not a hyperacuity task, and so the results are limited by normal > resolution.? > > and you responded: > > ?It seems logical to me, that if a 7x telescope makes no improvement in the > eye's ability to align two images of a horizon, that's because either the > telescope is a very bad one, or, more likely, that the horizon is an unsharp > one (in which case, magnification will make it no sharper).? > > A x 7 telescope is likely to be a Keplerian rather than a Galilean. ... > > If we accept the reality of vernier acuity, and I think we must, we would > expect vernier acuity using the naked eye to be at least as good as merging > images using a telescope, as Frank has postulated earlier; and this is what an > appeal to experiment might show. A question based on depth of field. If we double the magnification, does the "blending" area between the glass and mirror automatically double as well? Could it get proportionately smaller (good) or proportionately larger (bad) depending on the scope(s). If hand held, performance can deteriorate as magnification increases. Try a pair of 10X or greater binoculars on the water in a 30-40 ft foot boat with significant seas or swells. Another point is the target itself. The crispness of a horizon against a sky is a fuzzy variable (groan). Recalling visual perception classes from the psychological vantage point (some 40 years ago) and photographic lens resolution tests, it strikes me contrast between the line and the field is of some importance. Some of the tests with my "real" sextant (a bit more weather resistant my cardboard unit) were done employing David Burch's/Starpath "touch and leave" method. Record when you first perceive alignment, and when you perceive alignment is "broken." I used a distant phone line against the sky, and tripod mounted my sextant. Most of my belongings are in storage, so I have to work from memory, but my limits were burned into my memory. Over several days I did perhaps 100+ observations (split between turning the drum CW for one set then CCW for the next set to test for slack as well) with my Astra IIIB and the 3.5x scope. The mean was about 0.2 minutes between touch and leave, with a n-1 standard deviation much less than 0.1 minute in a given drum-rotation direction. I do not recall the difference between CW and CCW rotation of the micrometer drum, but it was nothing to be concerned with if one follows SOP (always rotate the drum in the same direction for IE/IC and observations). My conclusion was that since I was doing touch and leave (a range of alignment with one end being *not* aligned, *my* man/machine limit was likely 0.2' or better under ideal conditions. Bill B. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---