Over the past several years there has been much made of the virtues of ultra low expansion substrate materials – ULEs, as they are known. Simultaneously, there have been increasing attacks on Pyrex suggesting that it is a fundamentally substandard material suitable only for those who cannot afford better. Let me say at the outset that there is absolutely nothing substandard about Pyrex as a substrate material for the very finest of astronomical optics. Added to that, ULE substrates are completely unnecessary for telescope mirrors of moderate sizes. This is something I have commented upon in prior writing, but, I think, needs additional explanation.
Occasionally, amateur astronomers desiring the very finest optics request ULE substrates, feeling that something other than Pyrex is in order when contemplating the very finest system. Almost inevitably these individuals are literally stunned when they find out the true cost of these ultra low expansion substrates. The myth of the panaseic ULE is one of those unfortunate and baseless "optic legends" take on a life of their own, and, in this case, lead to needless frustration on the part of people who feel that they will never be able to afford the very best. And, I should add, not just to pick on the amateur world, professional users also often request ULE substrates when they are really unnecessary. The request for a ULE substrate almost becomes a knee-jerk reaction. When I sometimes question the need for an expensive substrate I hear, "It’s the industry standard," as if I should have known better. The problem is that while the optical work alone for a particular item or system might cost, as it did in one recent case, $2,100, the ULE substrate cost would have been an additional $6,600, instead of $650 in Pyrex.
Common ULE materials include: Zerodur, Astro-Sitall, Cervit and fused quartz. Zerodur is a product of Schott Glass Technologies, Cervit was manufactured by Corning Glass (the developers of Pyrex), fused quartz was originally a product of General Electric Corporation. Astro-Sitall is a product of the former Soviet Union. Of these products, Zerodur and fused quartz are readily available and of excellent quality. Cervit is apparently no longer being produced and Astro-Sitall has an uneven history regarding both availability and quality. ULE materials share in common one factor, their coefficients of expansion are either extremely small or designed to be virtually zero. In the case of fused quartz, the material is extremely hard and free from imperfections of any kind, allowing for the creation of a particularly high-quality optical surface. Such is not the case with some of the other ULE materials which have surface characteristics that have resulted in difficulty in polishing and final surfaces that offer optical characteristics significantly inferior to Pyrex and other ordinary optical and plate glasses. Of all the ULE products available, the two most highly regarded are Zerodur and fused quartz.
As I have stated in other writings, the low expansion characteristics of exotic substrate materials are really immaterial to astronomical mirrors of moderate aperture sizes up to and including 16 inches. Far more detrimental to superlative performance are problems connected with thermal unloading from insufficiently acclimated mirrors. No matter what a mirror is made of, it will not perform optimally until it reaches full thermal stabilization. I have conducted numerous experiments in this area and have found them to be conclusive. A Zerodur mirror that is not thermally stabilized will not perform as well as a Pyrex mirror that is fully stabilized. A Zerodur mirror this thermally stabilized will perform exactly the same as a Pyrex mirror that is thermally stabilized. The only difference between the two mirrors is that one is likely to cost 10 to 12 times more than the other.
I suppose this raises the very legitimate question as to why manufacturers of high-quality astronomical telescopes use ULE materials in their construction. Aside from the fact that it has simply become appealing and indicative of high quality, ULE substrates in some cases benefit from the advantage of being harder than Pyrex, and thereby offer the potential of a smoother more scratch-free surface under production polishing conditions. In the case of fused quartz this is particularly true and fused quartz is routinely employed for mirrors and lenses used in solar telescope systems where scattered light as well as uneven expansion and contraction are particular problems. In the case of astronomical telescopes, the additional scattered light from Pyrex or even plate glass or optical glass is inconsequential and completely undetectable. It should be noted that the scattered light problem even in a solar telescope is of significant consequence only when observing things of extremely low contrast. The use of quartz optics in solar telescopes was pioneered by Lyot in his famous chronograph. Lyot’s attempts at white light coronal observations were made in extreme high altitude locations and were only marginally successful, but nonetheless demonstrated the value of exotic materials requiring very special conditions very much different than nighttime astronomy. Also, fused quartz is particularly transmissive in the ultraviolet range; another specific requirement not directly related to common astronomical use. Zerodur was developed for the space program and is a ceramic glass product designed for extreme applications. Schott has a very informative web page at http://www.schottglasstech.com/products.htm where you will see some of these applications and get a full and complete explanation of the chemical, thermal and optical properties of this most amazing material.
What I am trying to point out here is that ULEs have been produced for very special applications which may offer no distinct advantages over Pyrex for ordinary astronomical use. I should also mention that Pyrex, though not developed specifically for common astronomical use, was brought into use as an astronomical substrate because of its (at that time in history) special and unique qualities. Pyrex was considered to be a distinct advantage over plate glass for one overriding reason, a lower coefficient of expansion. During the early 1900s the first very large and high-quality telescope mirrors were being produced and found to suffer greatly from the effects of uneven expansion of the plate glass substrate. Unless very special precautions were taken in their use, and the astronomers fortunate in their prediction of evening temperature, the effects of thermal expansion and contraction so corrupted the images as to make them useless. Entire nights of potentially good observing were wasted. And scientists were contemplating an even larger 200 inch telescope. Something better was definitely needed. The Corning Glass Works had been developing a new low expansion glass product for general commercial use. It just happen to turn out that this was the best product available for the practical production of large telescope blanks. This new product offered only advantages and no disadvantages over plate glass. Even the cost was only slightly greater than plate glass.
Now, here we might find an amusing corollary to the present infatuation with ULEs. In the 1930s and ‘40s Pyrex was suddenly considered to be the ideal material for all reflecting telescope mirrors, no matter how small. All amateur telescope makers simply had to make a mirror out of Pyrex. Ordinary plate glass was now considered to be substandard and inferior. Corning obligingly began turning out small to moderate size mirrors (3" to 12.5") much in the way they turned out Pyrex pie plates and baking dishes. Because of its low coefficient of expansion Pyrex was legitimately useful not only as baking equipment but laboratory "glass" tubings and containers for liquids. Pyrex tubing could be subjected to common glass blowing techniques without fear of cracking and test tubes and flasks made of Pyrex could be subjected to extreme heat and quenching without shattering - wonderful. But, of all of the new uses for Pyrex perhaps the most illegitimate and trivial was small telescope mirrors. Six and eight inch telescope mirrors made of plate glass work excellently. I have made them and used them and their performance is virtually indistinguishable from Pyrex – believe it, or not. However, when making telescope mirrors of ten or twelve inches and above the advantages of Pyrex become manifest, particularly during manufacturing where even the very minor heat generated during figuring will greatly distort the final figure and require extended settling periods. Even with Pyrex, these mirror sizes require settling, but not nearly to the degree of plate glass.
The one advantage that can be credited to Pyrex for the use of small as well as large telescope mirrors is the fact that it is substantially harder than plate glass or optical glass and is therefore more resistant to scratching and very minor surface imperfections that can cause scattering. The process of refraction suffers far less from surface scattering than the process of reflection. Reflective optical surfaces do need a better polish if they are to perform as well as refractors. But, I hasten to add, we are dealing in the range of the virtually undetectable. Pyrex produces an extremely high-quality optical surface not inferior to any of the ULEs substrates. I have all too frequently heard the epithet "garbage" referred to Pyrex in comparison with Zerodur. This is most unfortunate. In fact, nothing could be further from the truth. What is frequently the case with Pyrex mirrors is the "garbage" optical work I have seen applied to them. The unjust degradation of an absolutely wonderful optical material, a material that has advanced astronomy in the last 60 years perhaps more than any other single material, is indeed disturbing. There’s an old saying that familiarity breeds contempt, and I think that the ready availability of Pyrex and its inexpensive cost have made people take it somewhat for granted. The logical assumption appears to be that it can’t possibly be as good as something costing 10 or 12 times more by weight. I have very little doubt that mirrors made of Zerodur are often optically finished to a higher degree of excellence than Pyrex mirrors for no other reason than the people who have purchased high-cost Zerodur substrates have also engaged private opticians to do superior work.
Deficits connected with Pyrex are mostly restricted to the occasional bubble, stone or inclusion which can find its way close to the surface. In most cases, these defects are so tiny as to be completely inconsequential. In the vast majority of cases Pyrex blanks are entirely free from these defects. Another problem that can exist with Pyrex arises from the fact that Pyrex is not manufactured as having properties suitable for optical quality transmission. It is merely a substrate material, and, as such, suffers from striations. Striations are smaller areas within the larger body of the melt of differing refractive index resulting from imperfect mixing of the melt. These areas are often harder or softer than the surrounding glass and can result in local surface unevenness if a striation comes through the surface. Once again, this rarely happens to any extent considered consequential. I have seen it very rarely happen but have successfully figured mirrors with surface minor striation. When it appears, the striation is observed as a very slight discontinuity in surface smoothness over a very tiny area, usually as a narrow streak. Careful figuring can even mitigate most of this.
I have had the experience of working a number of interesting substrate materials into both transmissive as well as reflective optics. When one works these various materials it becomes evident under the critical eye of the test that certain materials polish better than others and result in superior optical surfaces. Other materials that take excellent polishes and make very good optical surfaces include: plate glass, BK7 and BaK 1 and related optical glass, and Schott ZKN7, a semi low expansion material, not as low expansion as Pyrex but having particularly good transmissive properties. Materials that do not take quite as good a polish are flint optical glasses such as F4 and F2 and related glasses. (Once again, the refractive use mitigates this problem.)
Materials that can be quite difficult to polish to high optical quality include the recently much admired Astro-Sitall. A product of the crumbling former Soviet Union economy, stockpiles of this material, along with high-quality optical glass that would not be replaced once the initial supply was exhausted, were used to make the now famous Russian Maksutovs. The Russian Maksutovs made use of this ultra low expansion material for their primary mirrors and the often high quality of these instruments seemed to propagate notion that Astro-Sitall alone was in some way responsible. Initial batches of this material entering the U.S. formed a relatively inexpensive source of ULE substrates for amateur telescopes. Individuals were acquiring random blanks of Astro-Sitall at bargain basement prices, having them fashioned into Newtonian primary mirrors, and claiming superior performance due largely to this material. The problem has now arisen that more recent batches of Astro-Sitall are simply not up to the quality of the earlier batches. The former Soviet Union economy and society simply do not have the resources or internal controls to produce a dependable export product. But, amateur astronomers still repeatedly request this material as if it is an off-the-shelf item of predictable quality and availability – it is not. I have been informed by people who have recently worked this material that it cannot be depended upon to take a good polish.
To all those potential purchasers out there, Pyrex is just fine for telescope mirrors. Really. You don't have to spend a fortune for the very best image.