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» Online Classroom   »   » Public Discussion of Cel Nav   » index correction question

   
Author Topic: index correction question
Plante


 - posted October 08, 2008 02:48 PM      Profile for Plante           Edit/Delete Post 
My question is about index correction related to the sextant when I bring down right in line the two pictures.
If I fixe a target at 5 feeth from me the index correction is 1 minutes.
If I fixe a target at 1000 feeth from me the index correction is 1 degre 45 minutes.
How come? What is the probleme? It is normal?

Thank you for your attention
Regards
Paul-E. Plante

From: Ottawa
David Burch


 - posted October 08, 2008 02:59 PM      Profile for David Burch           Edit/Delete Post 
This point is discussed in Chapter 2 of our book on cel nav, included in our online course or now separately. The object used for index correction must be a long distance off, at lease several miles to be good. Up close you are essentially seeing the parallax determined by the horizontal distance to the object and the vertical separation between the center of the scope and the center of the index mirror, typically about 2 inches.

I will come back shortly and compute these to see if the magnitudes match what you observe.

From: Starpath, Seattle, WA
David Burch


 - posted October 08, 2008 03:06 PM      Profile for David Burch           Edit/Delete Post 
at 5 feet the error is 1° 54', ie arc tan 2/5x12

whereas at 1000 ft the error is only 2.5'
from arc tan 2/12000.

thus the difference is 1° 51.5' which is roughly what you observe.

However, the fact that you see these reversed means you do have an index error to be removed. if there if it were set correctly, the error would be smaller at farther distances.

Thus find something that is farther away to use. try for at least one mile that is 6,000 ft and the error is down to arc tan 2 /72000 = 0.1'... ie even one mile is too close for best work

From: Starpath, Seattle, WA


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