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» Online Classroom   »   » Public Discussion of Cel Nav   » precision of sights

   
Author Topic: precision of sights
navi


 - posted May 18, 2017 02:38 PM      Profile for navi           Edit/Delete Post 
Hello,

I have several of your books and have done your home study course.

Recently I bought a sextant, it is a brass sextant with a micrometer in 1' graduation and has an index correction around 1/2' of the scale. When i measured Spica to Arcturus I got 32 50' and according to Bruce Bauers numbers in the sextamt handbook it should be 32 47.7'.

The sextant is a Roger Poulain, "Instruments de Precision" from Seine, France. It has 1.5" x 2 " index mirror and the horizon mirror is about 1.5" high and about 1" wide, that is one sees the horizon without glass to the left of the horizon mirror.

The scope has no markings but is 30mm on the big side, do not know the magnification.

What is to be expected in precision of the fix?

I work from the Chicago shores and use Lake MIchigan as the horizon. I am using 249 for reduction and UTC from a cellphone. Using Sumner- St Hilaire with 3 hours between sights as you recomend in your book. I use the work forms, assumed position is 87 53'N and 87 35' W.

Best regards
Nicklas B




From: Chi
David Burch


 - posted May 18, 2017 08:10 PM      Profile for David Burch           Edit/Delete Post 
not sure what is in the bauer book. there is potentially a refraction correction to make. I will look into this and get back to you. You can always compute the right distance with data from starpath.com/usno.

but again, there may be a correction depending on relative heights. I think we have something on this in our textbook...

From: Starpath, Seattle, WA
David Burch


 - posted May 18, 2017 08:12 PM      Profile for David Burch           Edit/Delete Post 
to answer your question, there are two answers. how close should this type of calibration comparison be, and i would hope that is ± a few tenths. as for a fix underway, the best to hope for is ± 0.5' and this mainly depends on correcting for the motion of the boat properly.
From: Starpath, Seattle, WA
David Burch


 - posted May 18, 2017 08:35 PM      Profile for David Burch           Edit/Delete Post 
ARCTURUS 21 45.2 N19 05.7
SPICA 34 20.3 S11 15.1

distance = 90 - 57º 12.95’ = 32º 47.0

you will have to make the refraction correction listed in chapter 11 of our text book.

From: Starpath, Seattle, WA
navi


 - posted May 19, 2017 12:23 PM      Profile for navi           Edit/Delete Post 
Thanks!

How do you calculate this? That is how did you calculate the diagonal distance?

ARCTURUS 21 45.2 N19 05.7
SPICA 34 20.3 S11 15.1

distance = 90 - 57º 12.95’ = 32º 47.0

I re-measured and I got 32 48 Arcturus to Spica. Close to Bauers 32 47.7. The sextant only has whole minuts anyhow.

From: Chi
David Burch


 - posted May 19, 2017 02:23 PM      Profile for David Burch           Edit/Delete Post 
The process is explained in the text book. You can then use the precise digital sight reduction at www.starpath.com/calc

There is also the refraction correction which depends on the two altitudes.

From: Starpath, Seattle, WA


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