Interpolating Instrument Error ( Sextant )

[sneuman]

I realize this won't greatly affect the accuracy of my sight reductions, but just curious about whether I should interpolate instrument error. In other words, if my sextant's instrument error is 0 at 15d, but -.2' at 30d, should I use a figure of -.1' for an observed altitude of approximately 22d?

[steve_hendry]

I'm confused. How do you measure the instrument error at those two points?

[sneuman]

Originally Posted by steve_hendry

I'm confused. How do you measure the instrument error at those two points?

Instrument error (I) is the sextant's built-in error recorded on the manufacturer's certificate for measured angles of 15,30,45,60,75,90,105,120degrees. Not to be confused with index error (IC), which is separate (and correctable).

[steve_hendry]

Sorry, victim of a brain f@rt. I need to check, but I suspect those certificates are long gone from the two old MAC sextants I have.

[btrayfors]

The difference is about 600-1200 feet. Do you really care?

Bill

[beetle]

Originally Posted by sneuman

I realize this won't greatly affect the accuracy of my sight reductions, but just curious about whether I should interpolate instrument error. In other words, if my sextant's instrument error is 0 at 15d, but -.2' at 30d, should I use a figure of -.1' for an observed altitude of approximately 22d?

I would check with Celestaire (sextant reseller) to see what they suggest.

My thinking is that if the error is known, then it should be corrected for. Small errors have a way of compounding themselves, so why not back it out?

- rob/beetle

[David M]

A high quality metal sextant will have pretty minimal instrument error. I would not worry too much about it. Chances are it is well below the error you would get from other sources of errors.

[sneuman]

Originally Posted by beetle

I would check with Celestaire (sextant reseller) to see what they suggest.

My thinking is that if the error is known, then it should be corrected for. Small errors have a way of compounding themselves, so why not back it out?

- rob/beetle

I'm with you, there.

I was reducing a practice Sun sight the other day with an observed altitude of 25d and I wondered if I should round up to the -0.2' error on the certificate for 30d or leave it alone. The 15d error is zero, as is the 45d error; but at 30d, there's that slight error of .2 of an arcminute.

[steve_hendry]

Found my two MAC sextants. They do have the error tabulations. It must have been a good day at the MAC factory (the serial numbers differ by 6) since all the errors are zero.

[sneuman]

Originally Posted by steve_hendry

Found my two MAC sextants. They do have the error tabulations. It must have been a good day at the MAC factory (the serial numbers differ by 6) since all the errors are zero.

Apparently, a lot of sextants produced today (and how many could that be??) have zero instrument error. Mine is a Kelvin & Hughes built in the early 70s. It was my father's who gifted it to me a few years ago. While the instrument error on mine is quite minimal, it's still a factor in the sight reduction and does have a slight affect.