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Brad,

> Its no secret that to appropriately calibrate to 0.1 arc minutes, the
> sensitivity of measurement should be an order of magnitude better.
> So as
> long as your sextant measurements are good to 0.6 arc seconds, you can
> calibrate your arc.

You cannot even read the sextant scale with this accuracy, so there is
no question about such measurements.

> In review of Comdr Bauer's book, he indicates that his table of
> interstellar distances is for sextant practice and for calibrating.  I

Same does one Russian book on nautical astronomy. They give distances
from the Polar star to 10 other stars.
When I bought my sextant, first I was enthusiastic about doing this.
Until I computed refraction correction to this table:-)

As the book does not explain how to make refraction corrections to this
table I conclude that the respected authors just did not think well
before giving this recommendation.

Unless we are talking of making an arc correction table rounded to whole
minutes, which is useless for most modern sextants which have smaller
arc errors. Or maybe they thought of emergency use, after having dropped
the sextant on the floor, or something like this.

Having computed the corrections, I made many observations, and found
that I am unable to measure star-to-star distance reliably to the
required 0'2 or 0'3 accuracy.
I can do better with bright objects like Sun-Lunar distances, but even then,
one needs averaging long series of repeated measurements to obtain
reliable results.

According to Freiberger-made certificate of my sextant, the arc error
is never more than 10", with one exception of 14" at 120 degrees.
So I could never verify these numbers with y own observations.
(I also had my sextant certified by Cassens-Plath the same summer,
and they just wrote their usual "This instrument is free of error
for practical purposes" whatever this means:-)

Alex.




> lots of respect for his knowledge and advice.  He, too, must have been
> aware the in-exact nature of the calibration, since he treats refraction
> in
> such a cavalier fashion.  Its as if he expects to only calibrate to within
> a few minutes of arc.
>
> Brad
> On Dec 17, 2013 6:07 PM, "Alexandre Eremenko" 
> wrote:
>
>> ------------------------------
>>
>>
>> Brad,
>>
>> > Has anyone ever matched the results published on their certificate
>> from a
>> > calibration facility while using star to star distances?  Open call:
>> > Anyone?
>>
>> Exactly the same challenge was posted on this list more than 10 years
>> ago
>> (by Frank, if I remember correctly).
>> To my knowledge nobody on this list ever claimed that s/he could do
>> this.
>>
>> Alex.
>>
>>
>>
>> > Brad
>> > On Dec 17, 2013 2:44 PM, "G Becker"  wrote:
>> >
>> >> ------------------------------
>> >>
>> >> I agree. When shooting Polaris in the daylight (not with a sextant)
>> it
>> >> reduces to a distinct tiny white pinpoint, it disappears behind the
>> >> crosshairs.
>> >>
>> >>
>> >>
>> >> -----Original Message-----
>> >> *From:* NavList@fer3.com [mailto:NavList@fer3.com] *On Behalf Of
>> *Peter
>> >> Monta
>> >> *Sent:* Tuesday, December 17, 2013 2:10 PM
>> >> *To:* george{at}gwbeckerpls.com
>> >> *Subject:* [NavList] Re: Sextant calibration
>> >>
>> >>
>> >>  ------------------------------
>> >>
>> >> Hi Brad,
>> >>
>> >> Rather than distinct points, stars are fuzzy blobs.
>> >>
>> >>
>> >>
>> >> It might help to illuminate the entire field with some background
>> >> light---that way the overall contrast of the scene would be reduced,
>> so
>> >> that the eye would see just the bright cores of the stars against a
>> grey
>> >> background.  Start with no light, acquire the blobs in the field of
>> >> view,
>> >> then gradually turn up the light source until the stars are maximally
>> >> pointlike, but not so much that they're not easily trackable by eye
>> as
>> >> the
>> >> sextant is swung and adjusted.
>> >>
>> >> The few times I've looked at Venus during the day with binoculars,
>> I've
>> >> been struck with how pointlike it seems against the blue sky.
>> >>
>> >>
>> >>
>> >> I have some camera images of star-star sights---they don't show the
>> >> "fuzzy
>> >> blobs" problem provided the exposure doesn't saturate the stars.  At
>> >> some
>> >> point I hope to finish the job of estimating my sextant's arc error
>> >> using
>> >> these images.
>> >>
>> >> Cheers,
>> >> Peter
>> >>
>> >> : http://fer3.com/arc/m2.aspx?i=125795
>> >>
>> >> : http://fer3.com/arc/m2.aspx?i=125796
>> >>
>> >
>> >
>> > : http://fer3.com/arc/m2.aspx?i=125798
>> >
>> >
>> >
>> >
>>
>>
>> : http://fer3.com/arc/m2.aspx?i=125800
>>
>
>
> : http://fer3.com/arc/m2.aspx?i=125801
>
>
>
>

   

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