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JKP@obec.com wrote:
Actually you should touch the top of one image to the bottom of the 
other image rather than superimposing them, you get a clearer match 
point. Depending which way you do this you will either get an upper limb 
or a lower limb observation. Since the reflected image in the artificial 
horizon is inverted, the top edge of that image is actually the lower 
limb. So touching the top of that image to the  bottom of the image in 
the sextant index mirror results in a lower limb observation.

You then have to apply sextant corrections and it is important in which 
order you apply them. First apply the index correction. Then divide by 
two since the measured altitude is twice the actual altitude.  If you do 
this in the wrong order you will get the wrong answer. An example should 
make this clear Let's say you measure 71 degrees between the top edge of 
the reflected image with the bottom edge of the sextant image, a lower 
limb observation, and the index correction  is minus one whole degree. 
Subtract the one degree leaving you 70 degrees which you divide by two 
to obtain the sextant altitude of 35 degrees. But if you divide by two 
first, 71 divided by two equals 35 � 30' then subtract the one degree of 
index correction you end up with the incorrect altitude of 34� 30'. Then 
apply refraction and then semi-diameter. Obviously you don't apply dip.

gl




> Brad,
> Tnhnaks for your advice.  I'll see if I can check the parallelism as you describe.
>
> GTeorge, Thanks for your advice as well.  I will either get a piece of black 
glass (and devise a means of leveling it as I have read in past posteings) or 
at least try some sort of non-reflective treatment for the pan bottom.
>
> I don't particularly suspect that I'm getting a double image from the oil 
pan, however.  When sighting the sun, I use a colored cover over the 
artificial horizon pan, and different colored filters between my eye and the 
sun, and I get two distinctly colored images, say green from the sky and 
yellow from the pan.  It is these that can't seem to be brought very near 
each other horizonatally.  That is, until for one luck second they slip right 
over one another, and if I'm quick with the micrometer I can get my shot 
beforfe they agaion wobble apart.
>
> By the way, I had also meant to ask:  Should I ignore altitude correction 
when using an artificial horizon?  Clearly no correction for semi-diameter 
should be needed, because I am superimposing full discs of the sun and its 
reflection, center-over-center, rather than touching the upper or lower limb 
to the horizon.  But I guess I can't grasp the principle behind altitude 
correction sufficiently to decide whether it counts in this situation.
>
> -John
>
>
> >
>
>   


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