NavList:

George, you wrote:

" when the two images are zooming around the field of view, it seems to me it would be necessary to see simultaneous views, at least part of the time"

 

That has exactly been my experience.  Using a 10x telescope and holding the sextant steady enough to be able to see the direct (horizon) object is

hard, especially as it is an inverting telescope.  Right means left, left means right, up means down and my head is swimming!  Add to that the

difficulty of aligning the arc to the objects and getting the two object images to coincide within the very narrow view of the telescope.  However,

once this is achieved, the two objects bobble around in the view, leaving no doubt as to which on is which.  Since my 10x scope has those observational

wires demanded by Bowditch, I attempt to see the coincidence on both wires.  Naturally, this is challenging enough without trying to adjust

the index to the arc.  Hence my reason for pre-setting the sextant to the observational value prior to observations.

 

I do believe that this offers just the advantage you are looking for here.  Exactly because the stars separate, come together, separate, dance around each

other & etc, you can see if the objects "pass through one another".  That is, if the objects do "pass through one another", at the correct angle, then one

can easily state that the arc is accurate, for that setting. 

 

If they don't "pass through one another",  and they will not for second order sextants, adjustment will be needed.  If adjustment is required, I expect to

set the sextant into a stand or clamped arrangement.  I will use the method I used when setting up interferometric laser paths for machinery.  Literally,

go to a photocopy machine, increase the contrast to maximum, lift the lid and press copy.  You will get the 'blackest' paper possible.  By alternately

sticking this paper in one path or the other, it will be possible to separate the images even when coincident.  Far easier than using the shades and quickly sorted.

Then adjustment of the index arm to the arc can occur with the other hand.

 

One disappointment yesterday was the conclusion that the Skyscout does not provide the degree of accuracy and that I must then retreat back to the

Nautical Almanac.  This leaves me with 57 navigational stars (and 4 planets) distributed throughout the entire celestial sphere, or simply not enough.

After a good long think, it occurred to me that I could use the Astronomical Almanac Online, Bright Stars table.  In this table, about 4500 of the brightest

stars (magnitude greater than 4.5) are present, sorted in ascending order of Right Ascension.  The thought was to use the Skyscout to point at some random

star.  It will determine the most likely candidate, providing me with the scientific name, RA, declination and magnitude.  Ah ha!  Assuming the star does

have a magnitude greater than 4.5, then it will be in the table.  In some experiments last night, it was quite clear that the concept is workable.  That is,

I could readily pick out the star in the table, given the Skyscout inputs.  The table provides right ascension in hours, minutes and seconds to the nearest

10th of a second.  Ditto the declination.  Converting 1/10 second of RA (time) yields 1.5 seconds of arc, about 4x better than the NA.   

 

Question: I expect that the Bright Stars table provides true (uncorrected) positional data.  Will this star data require correction for aberration?

 

Best Regards

Brad