Glass For Mirrors
There are very many glasses that can be used for making mirrors and we often get asked about the glass we use and which glass is really the best? This page may not give you a perfect answer, but we hope it helps you understand the question.
What may be "best" for one person may not be the right answer for another. The answer will depend to some extent on how much money is available and what environment the mirror is to be used in.
The first thing to note is that a glass with good optical transmission properties is not necessary, - light will not be travelling through the glass, - it will be reflecting off the surface.
(Perhaps Cassegrain secondary's excluded? - See the testing page for an explanation.)
So all that is needed is a glass with good physical properties, - but that still leaves plenty of different glasses to choose from.
The second thing is cost and manufacture related. Most glasses used for astronomical purposes are a "spin off" from another major market. Plate glass has thousands of uses with windows being the most obvious. Suprax is used for high temperature lamps and laboratory glass-ware and Pyrex is used for millions of household and cooking utensils. Both are trade names for what is really low expansion Borosilicate glass. They are well known by their trade names and the point is that it is large volume production in another market that drives down the cost of these glasses and makes them available for the astronomical community at reasonable prices.
Glass, Temperature & Distortion
The main physical issue with glass for telescope mirrors is the distortion suffered when temperature changes. It is a fact of life that all materials change their size to some extent when they change temperature and glass is no exception.
The amount of size change is always small, but can still be significant when compared with the wavelength of light. It is measured by the "Coefficient of Thermal Expansion" and with glass it is generally measured in "Parts Per Million Per Degree Centigrade" or "PPM/Deg C" for short.
As an example: Plate glass can be made of various constituents and has a range of thermal expansion values around the generally published value of 8.6 PPM/Deg C.
So assuming 8.6PPM/Deg C: This means that if a sheet of Plate glass 1" thick has its temperature raised by 10 Deg C, it will expand and end up as 1.000086 thick.
Since the wavelength of light is about 0.000022", the mirror surface would have moved nearly four wavelengths from its original position.
That's not too bad by itself if you can allow the mirror time to stabilise after the temperature change so that every part of the mirror has expanded and reached the new temperature.
Where a glass mirror experiences a sudden change in temperature, - such as moving your telescope from the house to the garden, - the glass close to the surface will adapt fairly quickly, but the glass deep inside the mirror will take longer to adapt. This will mean the mirror distorts and the parabolic curve on the mirror surface "ripples" out of shape until the temperature of the glass stabilises.
(And you can forget about obtaining good images from your nominally 1/4 Wave mirror until it does!)
Opposite is an example of what happens to the mirror surface. Glass nearer the surface cools quicker than inside. The corners cool faster than anywhere else. This example is an exaggeration as the real movements are microscopic, - but it will be enough to disturb the images seen through the telescope until the mirror temperature stabilises.
The thicker the mirror, - the longer it takes to adapt. Since larger mirrors have to be thicker for strength, larger mirrors take longer to stabilise.
Different glasses have different Coefficients of Thermal expansion, so it is a good idea to use a "low expansion glass" with a very small coefficient if you can - then there is very little expansion to worry about?
If a mirror is made of a glass that does suffer significant expansion, then the time the mirror will take to stabilise after a temperature change is affected by two other factors, the "Thermal Conductivity" and the "Specific Heat"
Thermal Conductivity means how fast heat travels through the glass, - the faster it can travel, the faster the glass stabilises. Metals like copper have very high values. Unfortunately any type of glass is very poor in comparison with metals.
Specific heat means how much heat energy is needed to be put into the glass to raise the temperature. This is a bit harder to explain? It is the capacity of the material to absorb heat. A high specific heat means the glass takes longer to stabilise.
An analogy is with batteries. There are many different types and sizes of 9V batteries. Each will keep equipment like a transistor radio working for a different length of time depending on its capacity. The higher the capacity, the longer the radio will keep running. The capacity of the battery is analogous to specific heat in glass. The higher the specific heat, the longer the glass takes to stabilise after a temperature change.
So in a glass for a mirror, we are ideally looking for a glass with a low "Coefficient of Thermal expansion" to limit the expansion suffered with temperature. As subsidiary factors we may take into account high "Thermal Conductivity" and low "Specific Heat" to reduce the time it takes for the glass to stabilise. Finally - cost has to be a consideration - the mirror should be as cheap as possible.
Unfortunately Oldham Optical does not use thousands of tons of glass a year and is therefore not a priority customer for any of the glass makers. We only use relatively small amounts of glass and have to wait our turn for deliveries.
A lot of Pyrex was made in Sunderland until recently. Sunderland is only about 80 miles from our workshop, - so you would expect it was very easy for us to obtain supplies of this glass? Unfortunately the vast majority of Sunderland's output went abroad and there was a very poor distribution network in this country for the type of disks and sheet we needed. It was often easier to obtain supplies of the similar glass - Suprax - from Germany, - but even then, delivery dates are not guaranteed.
One big source of disks for astronomy over the last few years came from the "left over" glass in the kiln after a melt had finished. The kiln would typically contain more glass than was needed to fill the waiting molds. The remaining glass was often cast into simple plain 600mm disks. This emptied the kiln so fresh materials could be used for the next melt. These disks were ideal for astronomical mirrors but it meant the glass makers had to overestimate the amount of glass they required to fulfil their orders before any of these disks would be produced. Delivery dates were unreliable but they were cheap in comparison to placing a special order for the same size disk.
If you needed anything bigger than 600mm, - it always meant a special order.
There are still supplies of these disks to be had, but with better computer control, there is never as much excess glass remaining after a melt to cast into disks. We are able to get sufficient supplies of Pyrex and Suprax for our normal needs, but there is always uncertainty over delivery dates. Sometimes our orders are supplied 3-4 months late. This means we have difficulty offering firm dates for delivery of finished mirrors when an order is placed.
An alternate glass produced from 2009 is Supremax. This is produced by Schott in sheets of 1.2m x 1.5m up to 66mm thick. These sheets are then water jet cut into disks as required. This glass now comprises a fair amount of our general output. However delivery dates are still long.
We can usually firm up delivery dates once the glass has arrived. Please don't be afraid to pick up the phone and check the progress of your order.
If Pyrex is requested, we usually import direct from the USA,
For Suprax and Supremax, we usually obtain it from Germany through distributors,
We have also used some large BVC glass disks from Canada.
Plate glass is obtained locally through distributers.
The Best Glass?
So what Glass is best for primary mirrors? - There is no one answer.
If the mirror is to be always kept and used in a temperature controlled environment 24 hours a day, - or at least if the mirror can be allowed a fairly long time to settle down and equalise its temperature after being moved, then you can't beat the cheapest glass available, - Plate Glass. There would be no point in paying extra for anything else.
If however you are building a space telescope, or are a Professional Astronomer affiliated with a large company or institute, or perhaps a very rich amateur? - Money is no object to you and the mirror will typically be subject to large temperature changes but is needed to be available for use as soon as possible? - You may be considering Schott Zerodur, Corning ULE, Fused Silica or a similar glass with an extremely low thermal expansion.
If you are the "average amateur" who keeps his Telescope in the house and then either transports it to site in the back of a car, or perhaps just sets it up in the back garden, - then you may be looking for something between those two extremes, - say a low expansion glass that is not that much more expensive than plate glass?
(And don't worry! - You are in very good company, - the 200" Hale Telescope Mirror is Pyrex.)
In practice in the astronomical market, there is very little demand today for plate glass primary mirrors. This is even in the small sizes below 12" diameter where their thermal expansion does not cause any practical problems. While we can produce plate glass mirrors if requested, we just offer low expansion glass primary mirrors as standard.
Below is a table listing salient facts and figures of the main types of glass available and used by Oldham Optical for mirrors.
|Glass Type||Thermal Coefficient of Expansion (PPM/ Deg C)||Thermal Conductivity (W/M/Deg C)||Specific heat (J/Kg/Deg C)||Relative Material Cost (Plate Glass=1)||Comments|
|Plate Glass||>8.6||0.75||730||1||Produced by Everybody. Suitable for Elliptical flats. Quite suitable for mirrors up to say 12" as these smaller thinner mirrors stabilise quickly? but most customers are now going for a lower expansion Glass even for these smaller sizes|
|Suprax 8488||4.3||1.2||?||1.1||Produced by Schott. Our main "Low Expansion Glass" up to mid 2004 after which the kiln was shut down. A supply became available again in 2006. It is popular because its price is close to that of Plate Glass|
|Pyrex 7740||3.25||1.13||726||1.3||Produced by Corning. Thermal Coefficient marginally better than Suprax, but a bit more expensive than Suprax. Very expensive thicker than 25mm|
|BK7||7.1||1.11||858||1.1||Produced by Everybody. Our main use is Cassegrain secondary's where we need the good transmission characteristics for testing|
|BVC||2.4-2.8||?||?||1.2||Produced by ASM products of Canada. Definitely a serious option for very large mirrors, but not as popular at the moment for smaller mirrors due to its "looks" (It's Black!) Started getting more use from mid 2004. Readily available up to at least 30"|
|Supremax 33||3.25||1.2||830||2.5||Produced by Schott. Same materials as Borofloat but produced by a rolling process. Potential for mirrors up to 1m diameter up to 65mm thick.|
|Zerodur||>0.02||1.64||821||>10||Produced by Schott. This has extremely low thermal expansion, - but you have to be able to afford it!|
|ULE 7971||>0.05||1.31||776||#1||Produced by Corning as an alternate to Zerodur.|
|Borofloat||3.25||1.11||830||#1||Produced by Schott as an alternative to Pyrex.|
|E6||2.8||1.1||730||#1||Produced by Ohara as an alternative to Pyrex.|
#1 We simply don't use enough to offer a good price comparison, but assume the price is similar to the other glasses with similar characteristics. You can also assume that since we use less of these - delivery times are extended.
If money is absolutely no object then Zerodur, ULE or similar glasses are the better choices as they have extremely low thermal expansions,
The Thermal Coefficients for Zerodur and ULE are 0.02 and 0.05PPM respectively, - please do not compare them directly and automatically assume Zerodur is better. The values do change with temperature and at one temperature ULE is zero. Both are so low that for use as a telescope mirror, - any difference is of no consequence.
Even owners of mirrors made of Zerodur or ULE are not able to use their telescopes immediately after moving them out of a warm house into the garden. Although the mirrors would suffer no distortion throughout the cooling period, - there would still be a temperature change to deal with. Convection from the warm surfaces causes air currents that distort the viewing until the mirror has cooled.
The same applies to the metal or wood structure of the telescope itself, - that would need to cool down before air convection currents die away. Taking those factors into account - it might be a bit pointless paying extra for a Zerodur or ULE mirror? These glasses would not generally be considered good value for money by most informed amateurs?
In practice, for small mirrors up to say 12" Diameter, even a Plate Glass mirror can cool down sufficiently before convection currents from the rest of the telescope structure die away to allow good viewing, - but we now offer low expansion glass as standard.
There is really very little to chose between Pyrex, Suprax or Supremax. All three are very similar Borosilicate glass's from two different manufacturers using two different casting processes. Pyrex and Supremax have nominally lower thermal coefficient of expansion than Suprax, - so on face value look better, - but Suprax cools down marginally faster and is about 20% cheaper. Supremax costs more than Pyrex, but can be available with shorter delivery dates.
As a practical example of a low expansion glass cooling, - a 16" (400mm), Diameter mirror made of 40mm thick Pyrex and manufactured to 1/6λ, was allowed to stabilise at about 10 degrees above the workshop temperature. It was then brought into the workshop and tested over a period of time as it adapted to the new temperature. It rapidly exhibited about 1/2λ of error, and took about 30 minutes to return under 1/6λ. You may expect similar times following transport of a telescope to site in the back of a car.
Note that the main markets for Pyrex and Suprax are in household and laboratory glassware respectively. Blanks are readily available up to about 5-600mm diameter, but sizes bigger than this are rarer and more expensive as their main markets do not require them. Consequently there is a large step change in price at about this diameter. The thickness of a large blank may vary within a few millimetres depending on the supplier.
For mirrors at or over 400mm, Black Vitreous Ceramic (BVC) is a serious competitor. It is made by ASM, a small Canadian company. ASM are a much smaller firm than Corning or Schott and it should not come as too much of a surprise that there are slight variations between the batches of glass they produce. They claim a range of 2.4-2.8PPM/Deg C, for their BVC glass.
We have been passed test figures from the Canadian company showing a range of 2.56 -2.64PPM/Deg C, which supports their claims. Note even the worst of the figures quoted, - 2.8PPM/Deg C, - is still lower than Pyrex.
The only drawback of BVC is the cosmetic aspect that it is black, instead of the semi-transparent appearance of most other glasses. After adding transport costs from Canada, it is still marginally cheaper than Pyrex at about 500mm diameter and it becomes progressively cheaper as the diameter increases. It can be readily supplied up to 750mm Diameter and it has been supplied up to 1.2M Diameter previously. (However in 2009, supplies of this glass became more difficult to obtain.)
Cassegrain Secondary Mirrors
After reading values in the table, some of you considering a Cassegrain might be worried by the high Thermal Expansion value of BK7 glass that Oldham Optical use for Cassegrain secondary mirrors? Don't forget that even though the thermal expansion of BK7 is twice that of Pyrex/Supremax, - the secondary mirror is less than half as thick as the primary. Even with the higher coefficient it will have stabilised before the primary mirror.
For a similar reason, we consider Plate Glass is perfectly adequate for elliptical flats in Newtonians. The flats are only a fraction of the size and thickness of the main mirror and will always stabilise before the main mirror does. However if a customer wishes the flat to be of low expansion glass - we have no problems supplying, - just make sure we know what you want.
"Normal" Glass Used By Oldham Optical
Unless you have specified a particular glass on your order, you will be supplied with the following:-
Primary Mirrors for Newtonians and Cassegrains will be low expansion glass from either Pyrex, Suprax or Supremax, mostly depending on levels in stock or availability at the time the order is placed. If you have a preference for one particular glass, please discuss it with us or make it clear what you want in your order.
Cassegrain secondary mirrors are usually BK7.
Elliptical flats are usually plate glass.
Note that for primary mirrors above about 450mm diameter, the exact thickness supplied will vary slightly depending on the type of glass and the source it is obtained from. If you do require an exact or specific thickness, then please discuss this with us or make it clear on your order.
To finish off this page, - What might convince you about our statement that plate glass is still perfectly suitable in sizes of up to 12" for good observing????
We have already used illustrations of the Hubble and the Hale Mirrors as examples of Fused Silica and Pyrex. Illustrated here is the Oldham 10" reflector. It was photographed early one evening while preparing to do some observing in the back garden of the Oldham residence.
Its primary mirror, - as you may well have gathered by now, - is genuine 100% Plate Glass.
But if we were replacing it today, we would probably follow fashion and make it of low expansion glass!
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