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Optical Systems.

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I'll try to cover some of the concepts of optics as I understand them. My understanding comes from years of research for astronomy reasons. There are functional differences between astronomy and sports optics but the optical concepts are identical. I'll also try to lay this blog out in some logical manner but there's no guarantee that my logic is the same as yours .


In this case we are talking refractor optics as compared to reflector optics. For those who are unaware of the differences refractors are a series of lenses in a line straight to the focal point. Reflectors are a "folded" light path in which the light is reflected off of a mirror, back up the tube to the focal point. There are a few different types of reflectors but as they are not part of sports optics as this blog is directed I won't cover them.



Since this is about optics first, I'll start with the glass.

There are several "types" of glass used for optical lenses but most are based on silica (silicon dioxide), basically sand but in a pure form. In most optical systems you will find one of two types, borosilicate glass (Pyrex) or soda-lime glass. You will also find combinations in an overall optical system.

Borosilicate glass is simply the addition of boron oxide to the base silica. This creates the lowest dispersion and thermally stable glass making it excellent for optics.


Soda-lime glass used for optics is often referred to as water white glass. Essentially the only difference between this and the borosilicate glass is the additive compound. This glass is the most common in that it's what you find your glass containers and windows made of. For optical purposes it is produced in very pure forms and the additives are chosen for their optical qualities including dispersion and thermal stability.



Enough of the chemistry lesson. Let's move on to making glass into lenses.

The grind (polishing) of optical lenses is done different ways but always at least starts mechanically. Lenses generally start out cast, essentially in their finished shape, and are then polished to tighter tolerances and finished dimensions. It is just as it sounds, they are "sanded" into shape but using extremely fine polishing compounds. As with any figuring and polishing technique, this is accomplished with progressively finer grits of polishing compound. Other polishing techniques such as ion milling can be used and are very expensive. Ion milling, or ion beam figuring, is done in vacuum and an ion beam of argon is used to remove glass at the molecular level. As you can see, this is a very accurate polishing technique and is done after the final mechanical polishing.

Once the final polishing is complete they are finished with various coatings according to optically oriented details. I'll add a little detail about this later. Often lens edges will be painted in flat black to reduce unwanted reflections.

There are several aberrations taken into account when building an optical system. I'll note the normal aberrations although other "unique" properties of optics and light are inevitably taken into consideration.

Spherical aberration has simply to do with the tolerances of the polishing. Imperfections in the shaping of the lens will focus light from different areas of the lens onto a slightly different focal plane. This focus can be behind its intended plane, in front of it as well as up, down, left and right. Though we are talking tolerances in fractions of the length of a light wave for quality optics, this is an aberration that is unavoidable but can be mitigated greatly by the quality of the grind. Generally if you see a polishing quality stated it will be in an "x/y wave" value. Quality optical companies making this statement will usually give this figure as an edge to edge "flatness", i.e. 1/4 wave edge to edge. If you see it stated simply as 1/4 wave they are playing a word game as this would be measured peak to valley and the "flatness" could be many times that. Generally the "wave" is roughly wave length of the center of the visible light spectrum of roughly 550 to 600 nanometers. Some perspective of these tolerances is that a human hair is roughly 50 to 100 microns or .000050 to .000100 meters. A micron is one millionth of a meter. A nanometer is one billionth of a meter or a thousand times smaller and we're dealing with a quarter of that in 1/4 wave lenses.

Chromatic aberration is physics plain and simple. Light of different wavelengths will focus on a different plane. This is where achromatic doublets come into play. Essentially this is done by using two stacked lenses of differing properties to focus basically the red and blue wavelengths onto the same plane. The apochromatic triplet is roughly the same concept except three lenses are used to focus the red, green and blue wavelengths onto the same plane. You can see that it costs more to build apochromatic systems.

Coma is an aberration that makes off axis objects appear stretched or "comet like". This can be controlled or even eliminated in a refractive system by lens surface shape.

A brief mention of astigmatism. The easiest way to put this is that two focal planes come into play which makes the entire viewing object look out of focus. Lens figuring and alignment are the main culprits here and in any quality system this should not come into play. There are types of reflector systems where astigmatism is inherent and is correctable. In a refractor system it is completely avoidable.

Now for some details on coatings. I am adding this after the aberrations as some reasoning behind which coatings are used will make more sense. Generally speaking coatings will be applied almost like "staining" the glass as you don't want to introduce any of the aberrations which were removed by the quality of the polishing. The normal coatings you will find in optics systems are for chromatic aberration, anti reflectivity and light transmission. These may also be laid down my ion depositing, which much like ion milling, is at the molecular level and is expensive.


So much for the glass. Now I'll go into some the basics of a refractor optical system. Initially it will be more of an overview as the details can be quite technical and boring. I won't go into the reflector type here in order to stay in line with the objective of this blog.

Essentially the concept of the overall system is to bring the object of viewing to a clear, consistent focus at the individualís eye, whether magnified or not. The objective lens (or lens system) is the only part of the optical system that gathers light. Every other part of the system is designed to then focus that light for viewing.

The objective lens is designed to converge light waves entering the system to be focused via the ocular lens at the individualís eye. The larger the objective lens, the greater the light gathering capability of an optical system and the better the resolution. A good example here is a quality 10 inch telescope will have a resolving power of .48 arc seconds. This translates to roughly 1/250 of an inch at 100 yards. The light gathering capability of this same system is roughly 5 fold better then a system with a 5 inch objective.

The ocular is the lens system that ensures the light waves are focused on a single plane. The design of this lens will determine the field of view as well as the eye relief.

That pretty well covers what is actually a fairly simple system but as you've seen it is a fairly complex set of steps and procedures to get there.


Now for my thoughts as to how this works in our rifle scopes.

I suspect when I hear the statement that "our oversized" erector allows more light into the scope is simple BS. You could in fact have one that is too small and will restrict the light. Remember, the objective lens will converge light waves towards the ocular. That light path will require an erector diameter of "x" so anything larger is just overkill. Obviously the erector has the job of ensuring a "correct image", in other words, making sure up is up and left is left. Here is where my knowledge of rifle scopes falls short. I don't know if the reticle is part of the erector or the ocular. In either case, the reticle must also focus on the same plane as the light from the objective lens. Rifle scopes are certainly more complex in that way.

The ocular in a variable power scope also handles the magnification, and because mechanically moving parts will "shift", so will the point of aim. Some are better then others and some are worse. As I understand it for instance, the Leupold VXIII scope's design requirement was for no more the 1/4 minute of shift. While this sounds like quite a bit, compare that to Redfield and the old Bushnell scopes which are reported to be as much as 2 minutes. The new Elites for instance are some of the best. I suspect the higher end scopes, and obviously those used in long distance shooting, are also quite good to almost perfect. In the long run, it's a moving mechanical piece and there will be some shift, even if it's as little as 1/100 minute.

Other coatings come into play with rifle scopes and other sporting optics for things such as moisture on the objective, contrast enhancement and scratch resistance. Moisture and scratch resistance coatings are self explanatory whereas contrast enhancement will have to do with lighting conditions as well as the individual eyesight. These will usually be color tints, though some may not be obvious, which bring out detail in other colors that the hunter might need under differing conditions. As stated earlier, for the cost of ion depositing and ion milling, I doubt there are many, if any, rifle scope manufacturers that will use these techniques.

At this point I'll call it good and will update as I find new or differing information.

Ken....

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Updated 04-02-2008 at 15:30 by Heat

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Comments

  1. JB257460's Avatar
    Very, very interesting read!! Thanks, Ken! I look forward to the next installment
  2. KingCobraTX's Avatar
    Thanks Ken for the intel! Very good information!
  3. 's Avatar
    Very good info I learned a lot. Thanks
  4. skipper's Avatar
    nice articel
    the blog are used, yes
  5. canuck's Avatar
    Excellent Ken....thanks.
    Shane