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I remember discussing this very issue with the late Frank Janicek, who was
the proprietor and owner of Maryland Precision Nautical Instruments in
Baltimore, Maryland.  For those of you not familiar with Frank, he was the
guru of sextant repair on the US east coast for many years. He was also a
licensed Captain, so in addition to his instrument repair skills, he
possessed a vast wealth of practical knowledge on navigation and seamanship.

I learned more about the mechanical aspects of the sextant from this man
than anyone I've since encountered.

I used to get wound around the axle, for example, about side error. Frank
told me that a little side error was a good thing because when it comes to
determining index error using a faint star, it is far easier to determine
the point at which they line up side by side, than it is to determine the
exact point of superimposition.

On backlash, Frank told me that even the finest sextants have a little
backlash and that it is always best to adopt the practice of rotating the
drum in the same direction when fine-tuning your sight. I have followed this
advice and have never gone wrong.

When the clouds clear and I can get a good view of the sky again, I will try
this out and get my results back to Alexander.

Robert

P.S. I know this is not supposed to be a "political site", however, I must
offer a salute and my heartfelt thanks and appreciation to our brave
soldiers, sailors and airmen, past and present, on this day, November 11:
Remembrance Day (Veteran's Day for my American cousins).

God Save the Queen and God Bless America.

Robert


----- Original Message -----
From: "Bill" 
To: 
Sent: Friday, November 11, 2005 11:56 AM
Subject: Re: Backlash


> Alex wrote:
>
>> Could you make the following experiment for me:
>> determine the backlash of your sextant.
>> This is very simple.
>
> Alex
>
> As we both know, you hand is steadier and eye better than mine. For other
> readers, my instrument is a year-old Astra IIIB deluxe, 3.5X scope, with
> traditional split mirror.
>
> I may be guilty of the politician/lawyer trick of deciding if I don't want
> to answer the question at hand, answer a question of my own choosing.
> (And
> after the last 4 posts, why read on;-)
>
> I find my ability to align a faint star to have a rather large standard
> deviation (especially after the first few observations) unless a good bit
> of
> side error is present.  I would reference the article Frank pointed out
> (Sept. 05 Sky & Telescope) as a possible cause.
>
> Referring to posts about the eye being able to beat the theoretical limits
> of acuity when dealing with lines, I prefer to use small diameter power
> for
> the test you proposed.
>
> What I "think" I know from this is:
>
> 1.  My ability to detect touch and leave (wire appearing to be in
> alignment
> and wire appearing to leave alignment) is at best plus/minus 0.1' in
> either
> direction.  That I regard as *my* ability to "see" with my instrument.
> Note
> these tests were tripod mounted so the sextant was plumb.  Note the focus
> was not changed (a problem noted in off-list posts) during the tests, and
> these were before the slight lateral play in the front of the index arm
> became noticeable. Also note my impression (based on answers to my list
> questions) for side-error or backlash tests do not seem to be distance
> dependent--they align or they do not align.  (Alignment of a phone or
> power
> line may be less than zero, but so what if if they agree?)
>
> 2.  When testing for touch and leave with drum rotation in either
> direction,
> any difference possibly caused by backlash was below my threshold (0.1')
> of
> seeing.  I can pretty much confirm that in determining IE from touching
> the
> limbs of the Sun in both drum directions.
>
> I am sorry that is not what you asked for, but I tend to play the odds and
> go with methods that I can statistically prove give me repeatable results
> (with the lowest standard deviation).
>
> That being said, I could notice no significant change due to rotating the
> drum in either direction.  Of course, there must be backlash in any gear
> system, but it is below my ability to detect it--just noise from my
> vantage
> point.  This was born out in recent Sun IE checks where, in one case, I
> separated the images in both directions--using opposite drum rotation; and
> in the other case I brought the images to touch (using the method in a
> recent post) for one set of limb observations and separated the images
> using
> the same drum rotation for the other set.  As my 4XSD averages were less
> than 0.1' of each other with a standard deviation well below 0.1', I
> pretty
> much feel I have hit the limits of this man/machine combination under
> ideal
> conditions.
>
> I imagine as the gears wear and develop more backlash (despite spring
> tension), or if I were using a plastic unit, I might pay more attention.
> For
> the moment nothing significant to report for practical use. Lunars and
> star-to-star are limited more by my lack of ability and possible errors
> along the arc than by backlash.
>
> Bill

   

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