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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Sextant mirrors

   
Author Topic: Sextant mirrors
SteveH


 - posted February 16, 2006 12:50 AM      Profile for SteveH           Edit/Delete Post 
Should these be optically ground flat? Or can any mirror be used? Thanks!
From: Gibraltar
Capt Steve Miller


 - posted February 16, 2006 07:00 AM      Profile for Capt Steve Miller           Edit/Delete Post 
It is my belief that the mirrors should be flat mirrors. If they were ground with a shape there would be a specific focal length involved that probably could not be compensated for with the current adjustments on the sextant.
The mirrors purpose is to reflect the light in a plane through the filters (shades) to your eyes. The position of the mirrors relative (angle bewteen them) to each other allows us to measure the objects' altitude. We are not trying to magnify the image with the primary mirrors, magnification is accomplished with the 'scope used.

From: Starpath
SteveH


 - posted February 16, 2006 07:20 AM      Profile for SteveH           Edit/Delete Post 
I realise the mirrors should not be concave or convex but are they actually ground to be completely flat, or optically flat? Thanks
From: Gibraltar
Capt Steve Miller


 - posted February 16, 2006 08:13 AM      Profile for Capt Steve Miller           Edit/Delete Post 
I am fairly certain that they have to be 'flat', as to the flatness specifications I do not know if there are any industry specs that are followed. I can check into that for you and let you know later.
From: Starpath
David Burch


 - posted February 16, 2006 08:25 AM      Profile for David Burch           Edit/Delete Post 
I believe a key issue here is not just the mirror part of the two "mirrors" (horizon and index) but also the clear glass part beside the mirror on the "horizon mirror," especially if the mirror coating is on the outside of the glass... though there are arguments for having it on the bottom side of the glass as well.

When the light passes through a sheet of glass that is not flat—and here i mean one side strictly parallel to the other side—then the mirror is effectively a thin prism and we know prisms bend light rays. And since we are measuring the angle of light rays this could be a problem.

On the other hand, i belive that any standard high quality "plate glass" is adequately "flat" for t his purpose.

At one time i lived in a house from 1897 with still some very old glass windows in it. And you could easily see the distortion when viewing the outside though some parts of the glass.

To me it seems the main question would be can a glass worker cut the plate glass mirrors to the right size at a reasonable cost. I will ask Bill Cook at Captains for his thoughts on this. he is an expert on all aspects of the instrument.

This is a good question these days since the price of re-silvering is very high. several of the companies that used to do this no longer do and it is now some $60 or more to do one mirror..

From: Starpath, Seattle, WA
David Burch


 - posted February 16, 2006 08:37 AM      Profile for David Burch           Edit/Delete Post 
It might help if can find some definitions of "plate glass" and "optically flat".

From a practical point of view, we know what we want: No bending of light more than say 0.1'.

With that as a tentative criterion, we might be able to make some conclusions from basic principles. More later.

From: Starpath, Seattle, WA
David Burch


 - posted February 16, 2006 08:58 AM      Profile for David Burch           Edit/Delete Post 
Getting started....

What is the difference between 1/4 wave, 1/10 wave, and 1/20 wave mirrors?
These values refer to the surface accuracy specification for the polished substrate of a mirror. Surface Accuracy describes the maximum allowable deviation of an optical surface from a perfect surface. If the mirror were flat, the value would give a reference as to "how flat". Since the test method used for inspecting surface accuracy uses a specific wavelength, the value is defined in terms of this wavelength. All catalog values refer to a maximum peak-to-valley value at 632.8nm. For high accuracy parts, the amount of deviation is so small that the value is defined as a fraction of a wavelength of light. For example, a ¼-wave mirror has a surface accuracy of 158.2nm (0.25 x 632.8nm), which is equivalent to 6.2 micro inches. The lower the value of the fraction, the higher the accuracy. Typically only values less than ¼-wave are considered as precision and values less than 1/10-wave as high precision quality. As a comment on notation, the following values are equivalent: ¼-wave, ¼l, and l/4, where l is the value of the test wavelength.

What is the difference between a first-surface and a second-surface mirror?
A typical mirror is a flat glass substrate with a metallic reflective coating applied to one side. If the coating is applied to the top surface, it is called a first (or front) surface mirror. The other surface may be clear (during fabrication of the glass or by polishing) or ground and the mirror is oriented so that the coating faces the source. If the coating is applied to the bottom surface and overcoated with black paint, then it is called a second (or back) surface mirror. The other surface in this case must be clear and the mirror is oriented so that the glass is facing the source in order for the light to pass through the glass before reflecting off the coating. The black paint (not always used) is used to protect the coating from the other direction and prevent any minimal transmission. An example of a second surface mirror is a common bathroom mirror. A second surface mirror is usually not preferred over a first surface mirror in most applications due to many inherent characteristics. A second surface mirror suffers from lower reflectivity due to absorption by the glass (especially in the UV and IR). In addition, there are often ghost images due to two reflections (one from the glass, one from the coating) and an increased optical path length since light passes through the glass twice (once to reach the coating and once to reflect back). A second surface mirror does have the advantage of increased protection of the coating. If the coating is very delicate or the environment is harsh, a second surface mirror may be selected (typically with a thin glass substrate).

— from http://www.edmundoptics.com/TechSupport/DisplayArticle.cfm?articleid=285

From: Starpath, Seattle, WA
Capt Steve Miller


 - posted February 16, 2006 04:51 PM      Profile for Capt Steve Miller           Edit/Delete Post 
The quality of mirror surface as indicated by the 1/10 wave, 1/8 wave 1/20 wave is unnecessary in the use that we are discussing. We are not trying to observe minute details of the object as is done in astronomy. You certainly could not expect to take a mirror out of a sextant and use it for astronomical observations in a telescope with any hope of resolving to the arc-sec level. The mirrors used in telescopes are much larger in size than can be obtained from a sextant and those mirrors are also 'curved'(ie circular or parabolic) to create a specific focal length and magnification when used with various eyepieces. As I indicated earlier we are not trying to magnify with these sextant mirrors.
The first surface mirror, while eliminating the light from going through the glass twice does have a practical problem in that the 'silvering' or ground surface is fully exposed to the elements and the cleaning methods used. While the second surface has possible distortions due to the light passing through the glass twice, has the coatings on the 'back' side and will have a greatly extended lifetime.
The quality of mirror manufacturing processes today compared to the mirrors used a couple of hundred years ago allows even 'standard' mirrors to be far superior to those first sextant mirrors.

From: Starpath
David Burch


 - posted February 16, 2006 06:54 PM      Profile for David Burch           Edit/Delete Post 
yes, i agree completely with Steve that these first specs above are not crucial for what we want as a measure of flatness. these are flattness for one surface, whereas we want parallelism. I think there must be specs for that exactly as we want, meaning how much would a light ray of a given frequency bend when exiting the surface.

I have been in touch with Bill and he will check in here shortly... looking ahead, he noted that we could indeed use a commercial mirror and we could cut it, but there is a lot more to add and i will wait for him to fill that in. He at one time made the mirrors for cassens and plath sextants.

Bill also mentioned what Steve did that the quality of glass these days is very good but we still have issues to resolve about the quality of the reflecting material, etc.

From: Starpath, Seattle, WA
Bill Cook


 - posted February 24, 2006 02:36 PM      Profile for Bill Cook           Edit/Delete Post 
I am checking in on this matter as David requested.

Stay away from wavefront discussions whenever possible. They are much more at home in conversations involving photography and astronomy. Even there, those who talk the most often know the least. The fact is that smoothness and wavefront errors can contribute to imperfections in the image. However, navigators don’t care if the star looks like a pinpoint or a cornflake; it’s the position that counts.

Yes, in a pinch one COULD use a piece of “bathroom” type mirror—if not too thick. Soda-lime plate glass in itself is not the kiss of death for sextant mirrors. Still, there are things involved in some company’s crude, non-chemical stripping and coating processes that leave too much to be desired.

Parallelism is important to be sure. Even so, some of today’s worst plate glass is going to be more than adequate for sextant mirrors. When I was making mirrors for C. Plath—not Cassens & Plath as David mentioned—I used Bk7 optical glass, exclusively. Of course, that is because I am a professional and nothing less should be expected. Still, it would be good to remember that the crown element in the Navy “Big Eyes” of WWII were often garden variety plate glass, as is the 48-inch corrector plate on the world’s largest astronomical camera.

Finally, in case there are any among us who fear they will run out of things to worry about: under enough magnification, the smoothest optical surface ever polished will look like a high-altitude view of the Grand Canyon.

I hope this has been of some help.

Kindest Regards,

Bill Cook

From: Captains Nautical Supply
David Burch


 - posted February 28, 2006 04:22 PM      Profile for David Burch           Edit/Delete Post 
thanks Bill. Could you remind us of the issues that might arise if we make one ourselves. I recall you said once that they cut better underwater and so on. In short, what would be your advice to someone who wanted to make their own sextant mirror. thanks. --david
From: Starpath, Seattle, WA
Bill Cook


 - posted February 28, 2006 05:14 PM      Profile for Bill Cook           Edit/Delete Post 
My first recommendation would be not to do it; there are too many “work-arounds.” Not the least of which is simply buying an inexpensive mirror, like those for the Astra IIIb, and cutting it down to size.

“But Bill,” you say, “my horizon mirror is ROUND.” Not a problem. Simply head for your local eyeglasses lab with your glass—about 4mm thick—in tow, an accurately measured diameter in mind, and hope the shop is not too busy.

Hand the class to a tech—square, rectangular, whatever—and tell him what diameter you want. He will place the piece on his Coburn edger, install a circular template, dial in the desired diameter, turn on the cutting fluid, turn on the edger and head off to deal with a paying customer while you stand with your face pressed against the lab window, watching a miracle performed by high-speed diamond encrusted wheels. Before you know it, you have a perfectly edged and beveled sextant mirror blank.

As for the square or rectangular pieces, you just need a rockhound friend with a thin lapidary saw. These blades a very flexible and are not at all forgiving. Cover both sides of the glass with a tape that will not readily dissolve in water. Use a straightedge and indelible marker to draw the exact dimensions and go to work. The blade should just be touching the water in the reservoir. Too much water and you have a blinding mess; too little and the glass may get too hot and fracture.

Finally, there is nothing wrong with using a good ole glass-cutter for cutting rectangular or square mirror blanks. If you keep the glass submerged while trying to break it, you will have much better results.

The following is from the Spectrum Glass Company of Woodenville, Washington:

“We knew you'd want to know: According to Scientific American, water causes glass to crack more easily because when a water molecule enters the crack, a reaction occurs in which a silicon-oxygen bond at the crack and an oxygen-hydrogen bond in the water are cleaved, creating two hydroxyl groups attached to silicon. As a result, the length of the crack grows by the size of one bond rupture. The water reaction reduces the energy necessary to break the silicon-oxygen bonds, thus the crack grows faster.”

That may sound to some like so much mumbo-jumbo. However, I have that SA article at home; it’s a “for real.”

Cheers,

Bill

From: Captains Nautical Supply


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