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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Sextant sights: with and without telescope

   
Author Topic: Sextant sights: with and without telescope
Dale Alexander


 - posted March 11, 2005 05:33 PM      Profile for Dale  Alexander           Edit/Delete Post 
I am currently working on the Starpath Coastal Nav course and at the same time teaching myself Cel Nav with a Davis Mark 25 sextant. I cannot find any information on the use of the telescope vs. the basic sight piece. What is the recommended procedure for using the telescope? Does it depend of the body being shot? I seem to have more accurate sights without the telescope no matter which body I,m shooting. Any info would be appreciated.
Dale Alexander

David Burch


 - posted March 12, 2005 12:29 PM      Profile for David Burch           Edit/Delete Post 
Let us separate for the moment the use of sextant for coastal navigation (your first ap) and for celestial navigation, the topic at hand. The answers are different.

For cel nav sights of any body you should do better with the telescope in essentially all conditions, which is not what you have observed. So we need to look into why that might be the case.

(1) there could be an issue of personal eye sight not matching the telescope. that is, one has an
Rx for glasses and this cannot be replaced by use of a telescope. This is certainly possible, but not very common, since even those with large corrections in their glasses usually can see the bodies OK with the telescope... or they can keep their glasses on and do the sights.

With the Davis sextant and with the Mark 25 in particular we have other more likely issues.

(2) For the Mk 15 or Mk 25, there is a black plastic eye piece cover that is intended (i think) to make collisions with eye glasses less abrupt.

These cups are slightly too long in that -- without knowledge of this effect -- can prevent you from focusing the scope properly. It is most crucial that the telescope be in focus for the distant objects. If that cup keeps you from pulling the lens out or pushinhg it in far enough, you might not have the body in proper focus. Try just removing it, or investigate this effect in some other way.

If this is an issue, you may want to trim a couple mm off the lip of it so it does not interfere.

(3) Another possibility, that is unique to the Mark 25 is the full view mirror they use. That glass must be clean or it does not work well and even when clean, you may experience difficulties in seeing faint objects reflected from it. I can imagine that perhaps these fainter objects might be more discernable without the telescope, especially if the lens on the telsescope are at all dirty.

In summary, with clean mirrors and clean lens and a properly focused telescope, the sights of celestial bodies should be improved with the scope. You see the horizon better for stars and it gathers in more light (even for the relatively small scope on the Davis units), and you see the alignment of the edge of sun or moon better with a scope.

With all that said, however, when using a sextant sideways for piloting, you may find that in some conditions you do better without a scope. It is not so much the optics, as without the scope you get a better overall view of what you are looking at and can pick out your target objects on the horizon more easily... Not always, but sometimes. Also in this ap as well, if you can find the targets OK with the scope in place, then chances are you will get better sights here as well as the alignments are easier... but this ap does not require tenth-minute precision, so it is not so curcial. More on this should go into the coastal nav forum.

From: Starpath, Seattle, WA


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