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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Mark 15 Sextant Index Correction

Author Topic: Mark 15 Sextant Index Correction

 - posted July 23, 2011 02:19 PM      Profile for Tedder           Edit/Delete Post 
I am learning my Davis Mark 15 sextant by means of a Davis Artificial Horizon. At first, I adjusted the sextant's mirrors by using distant objects, e.g. a neighbor's roof, a flag pole, the case of the artificial horizon. I was able to take sun sights, although they were not very accurate.

After getting Dave Burch's How To Use Plastic Sextants and making a Baadar solar filter, I superimposed the sun by tweaking the horizon mirror's adjusting screws.

After getting rid of all apparent error, however, I could no longer align the sextant sun and the sun in the artificial horizon. Furthermore, sighting with the sextant at zero degrees, the artificial horizon was several degrees out of alignment seen through the sextant, with the mirrored part far below the horizon part, i.e. way out of whack!

I believe my method is sound, but I wonder, what am I doing wrong here?

From: Apalachicola, FL
David Burch

 - posted July 23, 2011 09:04 PM      Profile for David Burch           Edit/Delete Post 
somewhere we have addressed this issue recently. As noted, using the artificial horizon for index correction is not a good idea. if you have a filter that is the best method.

not exactly clear what you mean with not aligning in the AH, but looking at anything that close (such as the edge of the AH box) will not line up.

best to do the index correction in normal manner using solar filter, then do a normal sun sight in the AH, overlapping the two images, then reduce as if it were a star. This is covered in our cel nav text book.

also as stressed in the plastic sextant book, you must treat the instruments gently. do not squeeze any part or push it or put any force on it at all.

From: Starpath, Seattle, WA

 - posted September 11, 2011 10:27 AM      Profile for Tedder           Edit/Delete Post 
To be clear, with my sextant horizon mirror carefully adjusted using the Badar filter, when I try to take a sight with the AH, I cannot find the AH in the horizon mirror and keep the sun in the index mirror. I can see the sun in the AH out of my left eye and it appears to align horizontally, i.e. the vertical adjustment seems close, but the reflected sun is to left of the AH. So far, nothing I do has brought the two images together so I can take a sight.

Also, with the sextant set at zero, I can see the AH sun in both mirrors.

One more thing, I use two half-inch nuts to align the index mirror; otherwise the same as the dice method.

Before I made the Badar filter, I adjusted the horizon mirror using a nearby roof line and telephone pole. My sights in the AH were not very accurate, however!

From: Apalachicola, FL
David Burch

 - posted September 12, 2011 09:46 AM      Profile for David Burch           Edit/Delete Post 
You make a statement that you cannot find the AH in the horizon mirror. It sounds like you are looking for some sort of line in your horizon mirror with which to measure the altitude of the subject body(Sun). This is not the way the AH works. As I said in my post to the AH question yesterday bringing the image in the AH (my blue disc) and the Horizon mirror image (my reddish disc) tangent (just touching) is a lower limb sight. The theoretical horizon would be at the tangent point of the two discs. OR when the two discs are superimposed as Instructor suggested the theoretical horizon would be at the center of the discs.
Look again at my post on AH from yesterday for the steps to use the AH and how to get the disc images to be visible.

Also hopefully the attached photo can help you realize how the AH works. The yellow lines are from the Sun. The line from the upper center is coming from the Sun to the horizon mirror and reflected through the sextant to your eye. the line from the lower left side bounces off the liquid in the AH also to the horizon mirror straight to your eye. The angle between these two rays of light is double the actual angle between the horizon (green horizontal line in photo) and the body.

From: Starpath, Seattle, WA

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