Re: Spotting scope technique, success wow!!...
From: Greg Rudzinski
Date: 2015 Dec 21, 18:48 -0800
Another big reason not to use a round of stars to determine personal error is the inconsistency of horizon quality. The horizon may appear anywhere from razor sharp to fat and lumpy. Even when the horizon is razor sharp abnormal atmospheric conditions can lead to errors in dip. There have been many posts on this topic over the years.
How many minutes of arc of personal error do you suspect in your observations ? If your talking tenths of a minute then arc error needs to be taken into account. Most metal sextants are in the +/- 0.2' arc error range. I wouldn't worry about the tenths unless you're planning to observe lunars.
From: Greg Rudzinski
Date: 2015 Dec 21, 15:20 -0800
I wouldn't do the round of stars to determine residual personal error. There are too many different inconsistent variables to account for.
Here are few influences to be aware of that can cause small random and systematic errors not normally corrected for:
1. Filter refractions.
2. Fogged or dirty filters, mirrors, or optics.
3. Centering of body in scope.
4. Focus sharpness.
5. contrast of body vs. horizon.
7. Arm and eye fatigue.
8. Rate of body movement.
9. Perpendicularity to horizon.
From: Robin Stuart
Date: 2015 Dec 21, 13:21 -0800
I gave the spotting scope method a try using the top of the Ritz-Carlton in White Plains, New York, visible at 3.5 n.m. distance and 20x80mm binoculars. I left a bit of side error to be able to distinguish the two images. Correct alignment is discernible to within a very small fractional turn off the index screw.
When I now go back and use the solar SD method to check the index error it doesn’t come out to zero. In some ways that’s not that surprising giving the level of accuracy being sought. Accuracy of 0.1’ requires that adjustments be made to within 1/300 th of the solar diameter in the sextant scope which seems quite a daunting challenge.
How reproducible do people find the results of solar SD method for determining the index error?
Typically when I do rounds of sights after all the usual adjustments and corrections have been taken into account there remains some systematic bias (see for example the observations in http://fer3.com/arc/m2.aspx/Noon-Sun-sights-Jamestown-RI-this-Sunday-Aug-25-Stuart-aug-2013-g25027 ). This could be a misestimate of the height of the eye or similar but may have a component of personal error in it. I have begun to think that the best approach might be do a round of sights from the vessel in a known location to determine a personal correction which would also account for residual misadjustments. Thoughts?