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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » artificial horizons and lasers

Author Topic: artificial horizons and lasers
Bill Brucato

 - posted March 15, 2005 08:08 AM      Profile for Bill Brucato           Edit/Delete Post 
I was struck with an idea last night in regard to using artificial horizons. It seems to me that the new auto-leveling laser plumbline tool that I got as a gift will come in quite handy for shooting sights without an available horizon (shoreside). If I set this unit up so that I may "bring the body" down to the laser level line, I'd have a true artificial horizon. As long as the line is at the same level of the sextant, it should be a zero dip situation, yes?
I live in a heavily wooded area with no sightline to the horizon, I believe this will allow for sights in any direction as long as I have a surface that will allow the line to be visible ( the neighbor's house, or fenceline). What do you think?

From: Celestial Navigation Home/Online
David Burch

 - posted March 15, 2005 09:32 AM      Profile for David Burch           Edit/Delete Post 
Yes, i think in principle you are right, and dip is not an issue. BUT, we must remember that a 1' angle subtends only 1 inch at 100 yards, and as i recall even laser beams spread out on that level once very far off. It might be tricky, but worth playing with. If you pursue it, please follow up with your experiences here. thanks.

other options are plate glass mirror and high quality carpenter level and business card shims, with the mirror mounted about mid chest high and in a position you can walk around it to view any direction...

or a large quadrant with a weighted plumb bob to read the angles. this can be made relatively easy to get precisions of 10' and then with multiple sights you get fairly good results.

From: Starpath, Seattle, WA
Bill Brucato

 - posted March 15, 2005 12:45 PM      Profile for Bill Brucato           Edit/Delete Post 
Is the 1' angle you refer to in regard to the height of the sextant? I believe I need the sextant to be at the same level as the laser when taking the sight.
I'll try this in the next couple of weeks and post my experience with the experiment.

From: Celestial Navigation Home/Online
Bill Brucato

 - posted March 15, 2005 12:48 PM      Profile for Bill Brucato           Edit/Delete Post 
Oh OI see now what you meant, the spread of the beam will create a less well defined horizon at a distance due to the spreading beam of the laser. I won't be working off anything excessively far away, less than 50 yds. Again, I update you all with my results.
From: Celestial Navigation Home/Online
David Burch

 - posted March 15, 2005 03:28 PM      Profile for David Burch           Edit/Delete Post 
Right. At 50 yards off, if the beam pattern's vertical width is 1 inch, the difference between aligning with the top edge of the pattern and aligning with the bottom edge will be 2' of sextant height.

Our problem here is we are dealing with very precise measurements.

And i see other problems. You really need a horizontal reference line like a roof top or fence, but the chances of that being inline with your eye level -- even if you could confirm a precise alignment with a laser -- is very small.

In any event, see what happends.

An alternative is this.... find a roof in the distance that you can use for horizon, take a few sights of bodies above the roof, then reduce them from a known position, and just figure what the dip correction has to be to make the intercepts zero. then in the future, you can use that correction... But if the roof is close, then you must stand in the same exact position for future sights.

If there is interst in this approach, we can compute some numerical limits on the process.

From: Starpath, Seattle, WA
Bill Brucato

 - posted March 26, 2005 12:55 PM      Profile for Bill Brucato           Edit/Delete Post 
I gave it a shot and found that I could not make consistent sights using the laserlevel. It seems that the error generated by not being on a plane with the laser itself would cause dramatic differences in the sights. It exaggerates the error no differently than if I did not have a clear shot at the horizon and used dip short calculations (without a clear idea of my true distance to the visible horizon).
I tried the laser level using 2 methods. The first method I employed was shining the beam on my neighbor's north wall at a distance of maybe 35'. I then attempted to keep the eyepiece even wqith the laser's origin port and took the sight. Allowing for the time between sight (just enough to note each one) I was wildly inconsistent. The next method I tried was using the reference line up close. I set the beam on a door and stood within 3' of the mark and took sun sights. This generated a 2 degree error! Not 2 degrees every time, but it wandered around the dial a bit. The way I confirmed what my sights should have been was using the Cartes Du ciel (http://www.stargazing.net/astropc/) program with my known position and zero h.e.
I wish I could say that I believe I can fine tune this, but I'm afraid it was a good idea that didn't pan out. I'll give iut a few more tries before I give up completely. I'm damn stubborn that way.

From: Celestial Navigation Home/Online
David Burch

 - posted March 26, 2005 02:10 PM      Profile for David Burch           Edit/Delete Post 
Yes, i am afraid i would have guessed these results.

HOWEVER.... with a 2-foot quadrant on a post with a plumb-bop line to measure the angles, you can do remarkably well with cel nav standing in the back yard.

And thank you for the reminder about the wonderful Cartes du Ciel program. I will add this to our list of star map programs, it is certainly one of the best.

From: Starpath, Seattle, WA

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