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The several postings with respect to the measurement of interstellar
distances by sextant, or whatever, have been most interesting and
instructive. Unless I have misunderstood the intent, the purpose thereof,
however, from a purely navigational point of view, appears somewhat
elusive.

Interstellar distances, to the best of my somewhat antiquated knowledge,
were considered a second line method of determining centering error,
which was usually done at intervals of altitude and  increased therewith.
At one time, an instrument could be sent off for testing by a system of
collimators, whereby the centering error was quickly and accurately
determined at given, usually 15-degree, intervals and tabulated as a
permanent error. As all are undoubtedly aware, index and centering error
are entirely different animals - an error determined by interstellar
distance will include both and, theoretically at least, will be valid
only at the distance measured, unless an independently determined index
error be negated.

Hopefully as a matter of interest, the writer offers the following
discussion on the determination of index error. No effort is made to
compare the accuracy of the various available methods, however, the
opinion is advanced that the immediate following, utilizing the sun, is
the more simple, sensitive and comprehensive approach, as well as being
the best suited to practical usage.

Having set the index to zero, mounted the telescope, and turned down
shades as necessary, or affixed the telescope screen, to counter the
glare, sight the sun and move the index forward (on the arc) until the
direct and reflected rim edges of the sun be in contact - now read off
minutes and seconds "on the arc".

Next, move the index back (off the arc) until the contrary rims of the
sun are brought into contact, and read off the minutes and seconds "off
the arc".

Half the difference between the foregoing two readings will be the index
error, independent of any tabular values or other calculations. If the
sun's diameter be greater on the arc than off the error is subtractive,
or vice versa. Remember always the old and reliable adage - "when it's
on, it's off", and "when it's off, it's on".

Further advantages of this simple method are ....

1) As the direct and reflected images are passed one over the other, any
horizontal separation of limbs immediately signals a lack of
perpendicularity, in either the horizon or index mirrors, to the plane of
the instrument and demands further investigation of each mirror
separately.

2) By alternating the screens, both at horizon and index mirrors, the
error (due to a lack in parallelism of the screen surfaces) induced by
various combinations may be tabulated.

Otherwise ...

I have for years used the method of bringing second and third magnitude
stars into coincidence, also to determine index error. As far as my
experience is concerned, there has always been an apparent range of
uncertainty in establishing exact coincidence, necessitating an averaging
of both on and off the arc measurements in an effort to negate, to the
extent possible, any error induced by this uncertainty; I have not found
this uncertainty of contact applicable when using the sun's limbs. Of
course, any separation of direct and reflected star images in the
horizontal plane again signals perpendicularity problems. Also in this
method, a coincident assessment of shade error is not so conveniently
provided, as when utilizing the sun.

The sea horizon provides probably the most convenient and least
complicated method of establishing index error, as well as checking
perpendicularity but, depending on conditions, may also be the least
accurate. Regardless, a careful navigator should check his instrument by
the sea horizon before each sight, or set of sights, as well as perhaps
thereafter, to insure no significant erroror change thereof, through
accident or otherwise, since the previous use.

   

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