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Copied below is a thread which started life by being sent by Alex to Navlist, 
and has been reflected back to Nav-l as a result of a
reply by Frank Jones. Otherwise Nav-l readers that do not subscribe to Navlist 
would not have seen it. And (presumably) anyone
subscribing to Navlist alone would not be aware of the reply. Such problems 
illustrate precisely the complications I foresaw in an
earlier thread "Two lists with a common topic".

Back to the substance of the matter.

Frank Jones' posting is interesting. Why should any defect due to flexure of 
the mirror become worse when the sextant is used at
large angles? I can think of one reason. Index mirrors are often made large 
(and often rectangular, with an increased vertical
dimension) to help cope with the apparent foreshortening that occurs at large 
angles of altitude, when the index mirror is tilted so
far that light falls on it at a near-grazing angle. Under those circumstances, 
the top and bottom parts of the index mirror come
into view, parts which are normally out of view of the telescope.

If there's any curvature of the index mirror, so that it's bent in the same 
way as would a bow (held vertically) be, then this will
give rise to a vertically smeared image, and the amount of the smear would 
increase as the upper and lower edges of the mirror came
into view, at large altitudes.

Glass is a flexible material, which responds to stresses by bending. Ideally, 
locating points for a mirror should be in an "L"
configuration, with a non-adjustable point at the junction of the two arms of 
the L, and two independent adjustment screws, for
horizontal and vertical alignment, at the tips of the arms. Although both 
mirrors require adjusting screws to allow them to be set
perpendicular to the plane of the frame, only one of them (either will do) 
needs an adjustment to tilt in the other direction to
allow index error to be preset to near-zero.

Each of the three points should ideally be opposed by some sort of flexible 
spring, exactly facing the screw. Designed in that way,
no bending forces are imposed on the glass. My cheap Ebbco fails in that 
respect, the side-adjustment screw is near the centre of
the horizon glass, its counteracting spring being right at the edge; a poor 
bit of design. Springs need to be no stiffer than
necessary to keep the mirror in firm contact with the locating-point if it 
gets shaken by the accelerations involved in handling (or
mishandling). Small stainless spiral springs, as fitted to the horizon mirror 
of my plastic Ebbco, seem ideal for that job. However,
in the past sextants have often used small L-shaped bits of thin brass (maybe 
phosphor-bronze, which is springier) for that purpose,
and depending on the extent to which these have been bent, they may bear right 
on the edge of the glass, or at some indeterminate
point on the glass back from the edge. With that arrangement, it can be 
difficult to be sure that no bending stresses ensue as the
mirror is adjusted. The return-spring for the index glass perpendicularity 
adjustment on my Ebbco follows that design, which I think
is a poor one.

Glass is an elastic material, in that when stresses are removed, it usually 
springs back exactly to its original shape, and doesn't
take up any permanent "set". It responds to overstress by breaking, not by 
perrmanently bending. So when Frank Jones' sextant failed
its test at large angles, and needed a  redesign, I doubt whether the glass 
had been permanently distorted by the long-term stresses
on it. Instead, the redesign would, I guess, be to reduce such bending stresses.

Care needs to be taken when adjusting a mirror, not to exceed the limits of 
normal adjustment, particularly when all three location
points are adjustable (as on the horizon mirror of my sextant, in a "delta" 
configuration). In that case, a "sextant-worrier" can,
after repeated rounds of adjustment, end up with a mirror that's been 
progressively overtightened, so all the play in a spring has
been taken up, and further tightening forces it to bear on the solid frame. 
If, then, the other screws are shifted, enormous bending
forces can result, without the user being aware, which could break the glass 
(or bend it). Equally, with three adjustment points,
it's just as easy  to misadjust a mirror the opposite way, backing the screws 
off so that it's no longer located by pressure from
the return springs, and can flap around in the breeze.

====================

The complaint that Alex makes, about a doubled star image, with a weaker 
displaced image or tail as seen in the mirrors, appears to
be a different matter. Could it be due to a lack of parallelism between the 
two surfaces of his horizon mirror? Not the index
mirror, which is front silvered. With a back-silvered mirror, a small fraction 
of the light is reflected off the front surface, most
from the back. Normally, if the glass is parallel and uniform, those 
reflections will exactly coincide for an object at infinity.
But then, Alex states that it gets rather worse at large angles, which it 
should not do if the problem lies in the horizon mirror. I
wonder if he can cobble a different mirror, perhaps front-silvered, closely in 
front of his horizon mirror (with sticky tape and
plasticine) just to see if the star images improve.

If it's due to some bending of the index mirror towards its edges, however, he 
may try "stopping it down", obscuring any reflection
from the outer parts of the index mirror with bits of black paper, to see if 
that makes a difference. It might make an interesting
investigation, the sort of challenge that Alex appears to enjoy.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

=====================================================================

----- Original Message -----
From: "FJones" 
To: 
Sent: Friday, May 05, 2006 2:57 AM
Subject: Re: [NAV-L] [NavList 120] Mirror problem


|I can provide the following brief account regarding distorted mirrors.  Some years
| ago I sent my metal sextant to the Original Equipment Manufacturer (OEM) for
| certification.  The OEM later informed me the mirrors were distorted due to overly
| strong springs they originally installed years earlier.  I recall the OEM stating the
| sextant was okay for angles less than 70-80 degrees but the pinpoint star images
| would deteriorate (blur)  quickly for the higher angles making back-sights impossible.
| For a fee of course, new mirrors and new design springs were installed, the sextant
| was then remounted on the collimator and passed certification.  The mirrors are
| rather thick optical plates and I was very surprised that little springs could cause such
| distortions.
|
| I am always impressed with the high precision measurement a good sextant is
| capable of making.  Since your instrument passed certification by not one, but two,
| recognized experts I doubt your mirrors are the problem.  When a star is viewed
| without the telescope (mirrors are in the optical path of course) is the distortion
| perceptible to the naked eye?  Well maybe not naked, eyeglasses are okay too.
|
| Frank J.
| Rochester, NY
|
| Date sent:              Thu, 4 May 2006 11:22:34 -0400 (EDT)
| From:                   Alexandre E Eremenko 
| To:                     NavList@fer3.com
| Subject:                [NavList 120] Mirror problem
| Send reply to:          NavList@fer3.com
|
| [ Double-click this line for list subscription options ]
|
|
|
|
| Can anyone give an explanation/advise
| on the following problem I encountered.
|
| 1. When I focus my telescope on a star,
| a weak star really looks like a point,
| but a very bright star looks like a little
| spot, approximately round. This is OK.
|
| 2. But the image of the star reflected in the
| two mirrors does not look like this.
| It looks sometimes as a little dash, sometimes
| a like a comma, or a little spot of irregular shape,
| or a little disk with a little tail sticking from it.
| My estimate of the size of this irregularity is about
| 0'.5. So when taking star distances I have to aim
| at the "middle of the star":-)
| At the same time, the direct image of the star is fine.
|
| The problem is evidently in the mirrors.
| But what is this exactly? Some microscopic scratches?
| Not exactly flat surface? Double reflection from
| the horizon mirror?
| The image becomes somewhat worse at large angles,
| but it is not very good even at small angles.
|
| My index mirror is front-silvered, and the horizon mirror
| is back silvered, as on most of modern sextants.
|
| I've read that the mirror's flat surface can be a bit bent
| because of the pressure of the adjusting screw...
|
| Alex
|
|
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