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    Re: SNO sextants
    From: Joel Jacobs
    Date: 2004 Nov 11, 15:36 +0000
    No Alex you missed the point. I am on record all over the internet recommending the Russian SNO-T sextant as an outstanding instrument, and comparing it favorably to the premium brands.
    But, if your mirror doesn't accept adjustment as you've stated,  it means that it is potentially in a constant state of change or movement, which should affect your sights. You need to test your clips for their tension.
    Joel Jacobs
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    -------------- Original message from Alexandre Eremenko <eremenko{at}MATH.PURDUE.EDU>: --------------

    > Dear Joel, Jared and Lee,
    > Thank you for your advises about SNO maintenance.
    > At this moment I probably have no need in disassembling anything,
    > but I will watch the behavior of the horizon mirror adjusting
    > screw carefully.
    > I was just asking in general, and for the future.
    > On Thu, 11 Nov 2004, Yourname Here wrote:
    > > I am surprised you get such accurate results. ;-)
    > You probably mean the results I posted earlier on this list?
    > Why are you surprised? Did you think that SNO are inferior
    > to the best sextants? (I thought so myself, but now I am
    > not sure; my experience in comparisons is ZERO: I never
    > handled any other sextant:-)
    &! gt; My results are not always so good.
    > For example, LONG lunar distances (more than 100 degrees)
    > are not good.
    > That's why I started to think of very fine adjustment
    > of the horizon mirror.
    > The "star test" for it always gives normal results
    > (the stars come together so that I cannot tell them apart).
    > But it seems that there is more precise test:
    > I overlap two images of the sun, using two filters of different
    > color (red and yellow).Superimposing the two discs gives
    > an orange disc. But sometimes one edge of this disc (say left)
    > is more yellow, while the other edge (right) is more red.
    > This is an evidence of non-perpendicularity.
    > My attempts to make a table of the "arc error" by measuring
    > star-to-star
    > distances fail so far (I get contradictive results
    > for long distances). I attribute it to
    > my unsufficient
    > profici! ency in getting a precise touch of two stars.
    > It also seems to depend on the star.
    > The very bright stars (like Vega) are especially difficult.
    > I started to try various light filters on these stars
    > and the results improved.
    > Another thing I found was that filters on my SNO can ditrort
    > the measurements.
    > I tried all possible combinations of filters on the
    > "index correction test" with the Sun.
    > And found experimentally that more filters I use larger
    > the "index error" is.
    > This means that some of my filters are
    > distorting.
    > For example: I put one red and one yellow filter
    > (these give the best results for the Sun) and determine
    > my index correction from the sun. I get the values of index
    > correction
    > between 0.0' and -0.2', the average of a long series is -0.15'.
    > And the control value of 4SD is within ! 0.2' of its true value.
    > Seems good.
    > But then I put ALL my filters (3+4) and obtain the index error
    > of -0.4' to -0.5'.
    > Alex.
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