Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 18-01-2011, 08:28   #1
Senior Cruiser
 
sneuman's Avatar

Cruisers Forum Supporter

Join Date: May 2003
Location: Jamaica
Boat: Tayana 37 Cutter
Posts: 3,167
Images: 37
Interpolating Instrument Error ( Sextant )

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?
__________________

__________________
Voyage of Symbiosis: http://svsymbiosis.blogspot.com/
sneuman is offline   Reply With Quote
Old 18-01-2011, 08:58   #2
Registered User

Join Date: Sep 2009
Location: Baltimore, USA
Boat: Irwin Citation 39 'Chesagansett'
Posts: 159
I'm confused. How do you measure the instrument error at those two points?
__________________

__________________
steve_hendry is offline   Reply With Quote
Old 18-01-2011, 09:03   #3
Senior Cruiser
 
sneuman's Avatar

Cruisers Forum Supporter

Join Date: May 2003
Location: Jamaica
Boat: Tayana 37 Cutter
Posts: 3,167
Images: 37
Quote:
Originally Posted by steve_hendry View Post
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).
__________________
Voyage of Symbiosis: http://svsymbiosis.blogspot.com/
sneuman is offline   Reply With Quote
Old 18-01-2011, 09:22   #4
Registered User

Join Date: Sep 2009
Location: Baltimore, USA
Boat: Irwin Citation 39 'Chesagansett'
Posts: 159
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.
__________________
steve_hendry is offline   Reply With Quote
Old 18-01-2011, 09:39   #5
Senior Cruiser

Cruisers Forum Supporter

Join Date: Jul 2006
Location: Chesapeake Region and Maine
Boat: 42' Bob Perry sloop
Posts: 4,038
Images: 4
The difference is about 600-1200 feet. Do you really care?

Bill
__________________
btrayfors is offline   Reply With Quote
Old 18-01-2011, 09:45   #6
Registered User

Join Date: Oct 2007
Location: San Francisco
Boat: N/M 45
Posts: 236
Quote:
Originally Posted by sneuman View Post
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
__________________
beetle is offline   Reply With Quote
Old 18-01-2011, 09:56   #7
Moderator Emeritus
 
David M's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: San Francisco Bay
Boat: research vessel
Posts: 10,153
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.
__________________
David

Life begins where land ends.
David M is offline   Reply With Quote
Old 18-01-2011, 10:01   #8
Senior Cruiser
 
sneuman's Avatar

Cruisers Forum Supporter

Join Date: May 2003
Location: Jamaica
Boat: Tayana 37 Cutter
Posts: 3,167
Images: 37
Quote:
Originally Posted by beetle View Post
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.
__________________
Voyage of Symbiosis: http://svsymbiosis.blogspot.com/
sneuman is offline   Reply With Quote
Old 18-01-2011, 17:11   #9
Registered User

Join Date: Sep 2009
Location: Baltimore, USA
Boat: Irwin Citation 39 'Chesagansett'
Posts: 159
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.
__________________
steve_hendry is offline   Reply With Quote
Old 19-01-2011, 05:33   #10
Senior Cruiser
 
sneuman's Avatar

Cruisers Forum Supporter

Join Date: May 2003
Location: Jamaica
Boat: Tayana 37 Cutter
Posts: 3,167
Images: 37
Quote:
Originally Posted by steve_hendry View Post
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.
__________________

__________________
Voyage of Symbiosis: http://svsymbiosis.blogspot.com/
sneuman is offline   Reply With Quote
Reply

Tags
sextant

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Overvoltage Error Eddie Electrical: Batteries, Generators & Solar 11 22-09-2010 10:01
Prosine 2.0 Error Message saildad_2000 Electrical: Batteries, Generators & Solar 2 13-06-2010 11:48
Bluechart error? cal40john Navigation 8 24-02-2009 23:08
404 Error Sunspot Baby Forum Tech Support & Site Help 47 14-11-2007 15:34



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 14:12.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.