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    Re: problems with a mirror ah compared to a Davis Artificial Horizon
    From: Brad Morris
    Date: 2012 Sep 14, 18:47 -0400

    Hi Alan

    1) Are you turning the level end for end?  That is, when you measure the in the same line, with the level turned 180 degrees, do you measure the same result?  This would be an error in the instrument.
    2) Is the weight of the leveling instrument deflecting the surface of the mirror? 
    3) After leveling front to back, then left to right, are you checking the front to back again?  Both have to be simultaneously correct.  That's non-trivial and a procedural error if not faithfully followed.
    4) Is the instrument accurate enough to measure what you want.  A machinist level may get you the precision but may be too heavy (see #2)
    5) Is the adjusting mechanism capable of the adjustment?  A fiddly mechanism will cause a short circuit in adjustment due to frustration.
    6) Is your mirror flat to start with?  A front surface mirror of any quality will tell you how flat it is to within one wavelength of light.  A cosmetic mirror MAY be flat, but you will never know how flat.
    7) Is the reference surface for the level a parallel plane to the surface of your mirror?  The reference surface can be the surface of the mirror, but if its not a front surface mirror, then the parallelism of the glass surfaces come in to play.

    All of these features are inherent in a quality AH.  Two sensitive bubbles, at right angles to each other and of light weight, are referenced to the front surface mirror with a quality adjustment mechanism. 

    If any other ideas occur to me, I drop you another note

    Best Regards
    Brad

    On Sep 14, 2012 5:44 PM, "Alan S" <alan202@verizon.net> wrote:

    I have come to the realization that most, possibly all of the difficulties I've had trying to use a Mirror Artificial Horizon revolve around inaccuracies in leveling the assembly. The bubble level I have used is simply not sufficiently accurate for this class of leveling (10-20 seconds of arc). Using a Davis AH, I seldom have any problem with AM and PM sun shots, usually deriving a calculated fix that is well inside 5 nm of a known position (GPS coordinates).

    Given that water seeks it's own level, it is "self leveling", even here in western PA I believe, I tried floating the mirror, figuring that this would do away with the problems of mechanical leveling. Didn't work, though I got a painfully brilliant "mirror sun", I could not find the "sextant sun".

    As a check, this afternoon at about 1620, I set the two ah's up immediately adjacent to each other. With the Davis ah, I readily got a "lower limb" sun shot, sextant sun above reflected sun, Hs reading 65 degrees 20 minutes.

    I then, without changing anything on the sextant, looked at the mirror sun. I could not find the sextant sun in the floating mirror ah. Both ah's were properly oriented, which leaves me curious as to what the problem might be, the possibility of operator error is always there.

    I would like to try ah shots of the moon and perhaps stars too, which I do not be believe can be done with the Davis ah, ergo my messing about with a mirror unit, about which there have been a number of posts.

    Does anything come to the mind of readers? If so, any assistance would be much appreciated.

    Alan
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