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Re: SNO sextants
From: Joel Jacobs
Date: 2004 Nov 11, 15:36 +0000
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@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.