NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Vernier sextant adjustment?
From: Doug Royer
Date: 2003 May 8, 10:13 -0700
From: Doug Royer
Date: 2003 May 8, 10:13 -0700
I was looking through an old navigation book last night and came to a discriptive paragraph of telescope adjustment for a sextant.This book was written in 1918 and all the illustrations of the sextants were of the vernier type.I was hopeing some of you who are familiar with older instruments can understand what the instructions mean and can explain them.I will write it directly from the book. From "Elements of Navigation and Pilotting" by Lt. W.J. Henderson, A.M. : section IV. The line of sight of the telescope must be parallel to the plane of the instrument."Screw in the telescope containing the two parallel wires,and see that they are turned untill parallel with the plane of the sextant;then select 2 stars,at least 90* apart,and make an exact contact at the wire nearest the plane of the instrument,and read the measured angle.Move the sextant so as to throw the objects on the other wire,and if the contact is still perfect,the axis of the telescope is in its right situation and the telescope adjustment is correct.If the images have seperated,it shows that the object end of the telescope droops toward the plane of the sextant,and if the images overlap, it proves that the object end of the telescope points away from the plane of the instrument.This will be rectified by the screws in the collar of the sextant.A defect in the telescope adjustment always makes angles too great"(Patterson) What are the wires he talks about?What is accomplished by reading the measured angle?