NavList:

   

Reply

A late acquaintance of mine who repaired sextants and other nautical
instruments for decades, told me that even the finest sextants can have a
bit of backlash. His suggestion to me was to always rotate the tangent screw
one way when making observations so as to preclude the possibility of
backlash errors. I have followed this practice for years even though I own a
C.Plath which, theoretically, should have no backlash whatsoever.

But back to removing backlash: my understanding is that this is not a
repairable problem; that backlash is a result of poor machining of the arc,
hence my earlier question about how the instrument shop removed this error.

Robert

----- Original Message -----
From: "Alexandre Eremenko" 
To: 
Sent: Tuesday, May 24, 2005 4:32 AM
Subject: Re: Visit to Freiberg


> On Mon, 23 May 2005, Bill wrote:
>
>> Interesting that your IC was zero, which matches the arc error at 0d.
>
> Before checking the arc they SET Index error to be ZERO,
> by adjusting the horizon mirror.
> Zero is used as a point of reference, so arc correction
> at zero is zero by DEFINITION.
>
>> I am
>> surprised the backlash problem did not show up in your IE checks.
>
> I am also somewhat surprised. One explanation is that I
> always tried to follow the Russian manual recommendation
> to always rotate the screw (when touching the bodies) in one direction:-)
> Including the index error checks.
> The recommendation is given exactly for the purpose of
> reducing the backlash effect.
>
> On the other hand, I did try to estimate backlash myself
> (with stars) and came to the conclusion that it was nonsubstantial.
> So the relatively large errors in my star to star distances (like
> 0.3'-0.6')
> remain unexplained. Fred also tried my sextant once,
> the result had a similar error to my ones: about 0.3' with
> art horizon.
>
> Alex.

   

Reply