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Speculum Metal Mirrors

Published on Oct 1, 2007 at 4:10 pm. 1 Comment.
Filed under history, telescopes.

In my office and at home are a lot of books. Some of these books are quite old. I have a few hand-me-down books that an old astronomy professor gave to me when he retired. At least one of those, his professor had given to him. I like my old books. People keep asking what good any of these out of date books are. Well, I like them because they are a link with the past. You can look through them and see how far we have come. You can see what astronomers used to think and compare that with what we think today. Sometimes it helps you to keep in perspective that we don’t know everything yet. It is also instructive to see how our knowledge has evolved over the years. On top of those philosophical reasons for keeping the old books, there is a practical one, too. I have published a number of things related to the history of astronomy, and it is very useful to have access to historical works right here in my office.

In a number of the books that date back to the beginning of the 20^th Century, there are references to speculum mirrors. I have read in some of these books about the advantages and disadvantages of reflecting telescopes and refracting telescopes. This is a hot topic even today among amateur astronomers. It was even more so over a hundred years ago.

To explain why, I need to say a bit about mirrors and lenses. First of all, consider how lenses focus light. As light passes through them, the lenses bend light to a focus. The distance that it takes parallel beams of light to focus is the focal length of the lens. However, as anyone playing with prisms can attest, different colors of light are bent by different amounts as they pass through glass. This is true with lenses, too. So, the blue light is bent to a focus at a different point than the red light. When you use an eyepiece to view a distant object, then you can only get one color in focus. The object will appear in focus in this color, but with the other colors of the spectrum forming a colored haze around it. This distortion of images is called chromatic aberration. It is an inherent flaw in refracting telescopes. However, it can be minimized by using very long focal length telescopes (the less bending of light, the less the difference in the focal distances). A more modern (and far more expensive) approach is to match two or three lenses so that the effects of one lens are partially corrected by the others. The first method makes for telescopes that are very long and unwieldy, and the second method makes for very heavy lens sets and very pricey systems.

Now, one way to get around some of the issues with lenses is to simply not use lenses to focus light. Mirrors can be used to focus light, too. All that is needed is to bring light to focus, so it doesn’t matter if it is a mirror or a lens that does that. What does matter is that the mirror must be just the right shape. The proper shape to focus light is a parabola. Since all colors of light reflect the same way, then the mirror does not suffer from chromatic aberration.

But, this is where we run into a problem. In order to focus light the mirror must be reflective (or else it isn’t a mirror!) and must be the right shape. Figuring a mirror to the right shape is difficult. The larger the mirror, the more difficult it is to make it the right shape. Of course, it is also difficult to make lenses the right shape if they are large, too. But, mirrors have other issues that need to be addressed that make mirrors used in astronomy particularly problematic.

One of the problems with mirrors is the degradation of the reflective material itself. When most people think of a mirror, they think of the mirror in their bathroom. That sort of mirror is a piece of plate glass with some reflective coating put on one side. Most shiny surfaces, though, tend to tarnish and become less reflective over time. So, a coating of some sort is often applied to the reflective surface to keep oxygen (the usual culprit) away from the reflective coating. Unfortunately, this coating also keeps light from the reflective coating. But, that is OK, because the reflective coating is applied to glass, which is transparent to light. So, the solution is that you just look through the glass to the reflective coating on the far side of the glass. This is your common mirror. It is a back-coated mirror. But, the problem is that light passing through the glass also reflects and refracts. So, you often get multiple images, and even distorted images. For normal uses (like shaving, putting on makeup, adjusting your clothing, etc) these slight image distortions are not a major deal, and you can ignore them. But, for astronomers, who are already working on the limit of what the instrument is capable, these distortions can doom observations. So, the solution is to put the reflective surface on the front surface of the mirror. Then, light does not pass through the glass. It reflects directly off of the reflective surface. Most serious optical work uses front surface mirrors for this reason.

So, astronomers use front surfaced mirrors. However, this introduces a whole new set of problems. First of all, as I said, most highly reflective surfaces tend to oxidize and tarnish. Gold doesn’t readily tarnish, but gold is expensive and does not reflect all wavelengths of light equally (that is why it appears yellowish).

But an even greater problem is the difficulty of applying a perfectly even coat of the reflective material. For rear surfaced mirrors, this is not such a problem. You make the glass smooth and the right shape and you slop on the coating. If is it not uniform thickness, that is no problem. As long as the glass is the right shape, then the mirror is the right shape. But, for front surface mirrors, the situation is much different. Even if the glass is the right shape, if the reflective surface is not uniform, then the surface of the mirror will not be the right shape. It was not until the mid Nineteenth Century that techniques were developed for depositing a thin and uniform layer of reflective material onto the surface of a glass mirror blank of any reasonable size. So, mirror makers had to come up with some other way to fabricate mirrors.

The solution was to use a shiny metal surface as a mirror. Today, this would probably be done using stainless steel or polished aluminum. But, stainless steel did not really exist until the early Twentieth Century and techniques for fabricating and polishing aluminum mirrors did not exist. So, instead, a very shiny metal alloy called speculum was developed. Speculum was a mixture of copper and tin (similar to bronze). Different mixtures of speculum ranged from a composition of 30% tin up to 45% tin. Frequently about 1% arsenic or antimony was added to make the material whiter and better as a mirror. About 33% tin was considered ideal for reflectivity, but such speculum tarnished easily. A composition of about 45% tin resisted tarnishing but had other problems (it was brittle and not as reflective). Speculum could be fabricated into large mirrors of the correct shape. However, speculum reflected only about 65% to 75% of the light that was incident upon it under ideal conditions. As it tarnished, the proportion of light reflected drastically declined. Some older observing guides suggested that speculum mirrors had to be 1/3 larger than corresponding lenses to give the same image brightness (naturally that depended upon the type of speculum and the degree of oxidation). A further problem with speculum was that it was very brittle. Speculum mirrors often cracked during fabrication. They tarnished readily, so they had to be polished on a regular basis, and they often broke during polishing.

As a result of all the difficulties with speculum mirrors, you can imagine that it was not a popular material, but it was all that was available for large mirrors for a long time. For telescopes of under 12 inches in diameter, the general feeling was that it was better and less expensive in the long run to go with lenses. But, for telescopes larger than that, there was quite a bit of debate.

Fortunately, we don’t have to deal with speculum mirrors anymore! Techniques were developed to grind glass mirror blanks to the correct shape and to apply a very thin uniform coating of silver or aluminum to the mirror. Eventually the surface would tarnish, but the very thin layer could (relatively) easily be removed and a new one applied. Speculum fell by the wayside, and now hardly anyone makes it. Those that do make it often do so in order to construct a historically accurate telescope replica, or to study the material properties of the metal.

So, if you are ever looking in a very old book about astronomy, or perhaps a book about the history of astronomy, and you see a reference to a speculum mirror, then you will now know what the book is talking about.

-Astroprof

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