Next message: Frank Reed: "Re: Question to Frank"
Bill you wrote:
"My Astra IIIB has an aprrox. vertical distance of 2.25" between the center
of the horizon mirror and the index mirror. Using a plane right triangle,
and knowing the distance to the object, I should be able to calculate a
useable correction for parallax. For 100 yards approx. 0d 2' 9", for 0.1 nm
approx. 0d 1' 4", for .5 nm approx. 13".
Distance must be known to calculate the above, and distance is what we want
to solve for using the tables. Is it reasonable to hold the sextant
horizontally, align the sides of the object between the horizon glass and
mirror, and use that angle (off the arc so, add to sextant measurment)
plus/minus IC to correct to the actual angle?"
If I've understood you right, yes.
I've been thinking along similar lines for a "laboratory" test of arc
errors. The parallax within the instrument (because of that 2.25" distance between
the line of sight through the horizon glass and the index mirror that you're
describing) is something that we can calculate exactly and correct for. For
example, I could do an index correction by looking at a an index card across
the room. If the top and bottom of the card are aligned (assuming that's the
same as the distance between the line of sight through the horizon glass and
the index mirror), then my sextant should reading should be identical to the
I.C. based on objects observed at great distance (the standard method for
checking index correction). For other angles, I am guessing that the parallax goes
as d*cos(h) where h is the angle read off the sextant and d is that distance
(2.25" in your case).
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
