Concave Primary Mirrors
Manufacture of Mirrors
This is our main business. The process is basically the same for all sizes and types of concave mirror. This includes parabolic mirrors used in Newtonian and similar telescopes or spherical mirrors destined for use in Schmidt cameras or similar.
Plate glass is available from several reputable suppliers in Britain and we generally use it for elliptical flats. It can also be the basis for mirrors say up to about 300mm diameter for amateur telescope makers.
Professional makers may use it in bigger diameters but plate glass is only available up to 25mm thick and for mirrors over 300mm, this may mean a more complicated suspension cell is required. This is not a problem for a professional telescope builder but for an amateur we would honestly recommend using thicker glass which in practice means going to a low expansion glass readily available in thicker sizes.
The thickness of the mirror will increase with the diameter. Our guide for an amateur is to use a thickness of 19mm for 150mm; 25mm up to 300mm and 35mm for 500mm diameter mirrors. Manufacturing costs of a thick mirror are not much more than for a thin one.
There is also an advantage in using Low Expansion Glass for smaller sizes if the telescope is going to stored in a warm house and brought outside on a cold evening to use.
Like most materials, glass expands as its temperature increases and the coefficient of thermal expansion value is a measure of how much. What this means in practice is that if a telescope is taken out of a warm house into a cold garden, the temperature change temporarily distorts the shape of the mirror until its temperature stabilizes.
The telescope will not give a clear view until that happens. You will simply have to wait longer for a mirror made of plate glass to stabilise than a mirror made of low expansion glass.
See our seperate page on glass for a lot more details.
At present, the difference in cost between using plate glass and low expansion glass for sizes up to 300mm diameter mirror is less than 10%. See the price list for further comparisons. The common low expansion glasses we use are "Suprax" from Schott and "Pyrex" from Corning.
Low expansion glass used to be considerably more expensive than plate glass, but the price difference has been narrowing over the last ten years. Where previously we sold lots of plate glass mirrors, we are now finding most of our customers opting for low expansion glass - even in the small sizes.
Most mirrors are manufactured from glass sheets. At one time the glass sheets were cut up into squares and then individually shaped into disks. The edges were ground and smoothed to the final diameter. The more modern technique is to directly cut the disks from the glass sheet with a water jet cutter.
The photograph adjacent is a water wet cutter seen cutting out sample discs from a small sheet of glass.
More details about water jet cutting are on the Water Jet page.
Moulded disks are sometimes used for the larger sizes. They are cast directly into a circular mould. They are bought in at standard diameters and the edge is ground down to remove the chamfer from the mould.
The first operation is to roughly contour the mirror to the approximate (concave) curve. This is done on a fully enclosed machine with a diamond cutter.
It has to be enclosed to capture the glass particles generated and contain the splash from the water used to cool the cutter. This stage removes most of the glass not required. The process leaves a relatively smooth concave surface to proceed to the next stage.
The shape is further refined in a different machine that can work to more accurate tolerances and has a finer cut. The cutting arrangements are set to approximate the curve required - Parabolic, Spherical or some other curve. A drip feed of abrasive material is fed to the cutter to assist in the process. After cutting the mirror is examined for surface defects before progressing to the next stage. The surface of the glass is opaque at this stage and useless as a mirror.
The polishing machine removes very small amounts of glass and transforms the opaque surface into a clear translucent finish. This is done with a pitch lap with a fine polishing abrasive. It is the abrasive material that does the cutting - the pitch lap serves only to hold the abrasive material to the glass during the process.
The final adjustment to the curve of the mirror is known as “figuring”. It is slower machine polishing with smaller laps. Occasionally the final figure is done by hand. Figuring is interspersed with frequent testing as the final curve is approached.
Testing takes place at intervals during the figuring process as the final figure is approached. We use several test methods depending on the size of the mirror and the accuracy required. Testing is covered in more detail on the testing section at the top of the page.
Following the final test all our mirrors are coated with enhanced aluminium with a silica overcoat. This is done in a coating chamber. After coating, the mirror is finished and ready for dispatch.
The Finished Result
Our standard product is manufactured to PV 1/4λ wavefront accuracy with yellow light, equivalent to PV 1/8λ surface accuracy. We can offer better accuracy such as 1/10λ wavefront (diffraction limited), if required.
See the Optical Standards page for a lot more details as to what this standard means.
Since the wavelength of light is about 0.00055mm or 0.000022". This is an incredibly small quantity and needs to be maintained even as the diameter of the mirror increases.
So since we work to incredibly small fractions on the mirror surface, you might be surprised that the focal length of the finished mirror is not as accurate. For example; a 500mm F/4 should have a focal length of exactly 2000mm. In practice there is always some variance on this value of a few mm each side. Up to ±0.5% is a typical tolerance. The same tolerance applies for the focal length of Cassegrain systems.
This is a reality of commercial manufacture. The mirror maker is concentrating on the final figure and is aiming for the perfect curve. As a side effect the focal length is not being as accurately controlled. This is well known by telescope makers and normally the telescope construction or the mirror cell have adjustments to cope with the small differences between mirrors.
While mirrors could be produced with the same surface accuracy and an accurate focal length it would push the cost up. It's cheaper to build adjustments into the telescope itself.
If you require any better accuracy on the focal length, please discuss it with us first for a price and make that requirement clear on the order.
The process is really no different to producing a concave mirror. We produce spherical secondary mirrors for Dall-Kirkham Cassegrain telescopes or hyperbolic for others such as Ritchey-Chretien. The same machines used for concave mirrors are used but are biased to remove glass from the outer edges of the disk rather than the centre.
Secondary mirrors are smaller than primaries and we generally produce them in batches using BK7 glass. The testing of convex mirrors is more involved than concave and is covered on another page of this web site.
Convex Secondary Mirrors
Manufacture of Elliptical Flats
Elliptical flats are needed for Newtonian or similar telescopes to divert the image out of the telescope tube where it can be viewed. We usually make them in batches.
Plate glass is cut into rectangles slightly larger than the Major and Minor axis required. The rectangles are then roughly nibbled (shanked), to an approximation of the elliptical form. The rough blanks are fixed to a vacuum chuck set at 45 degrees and spun in a lathe. The edges are ground down to a perfect elliptical shape with a diamond cutter with water cooling.
The back of the flat is now ground flat and a number of flats are then fixed to a circular bedplate using an adhesive agent. The top surfaces are progressively ground, smoothed and polished to an optically smooth finish. Following the final test the flats are given the same enhanced aluminium and silica overcoat as the other mirrors.
Schmidt cameras and Schmidt Cassegrain telescopes require a special lens called a Schmidt Corrector Plate. This is usually in the range of 8-12mm thick and at first sight an ordinary piece of plate glass from the local glazier would do as a base.
Unfortunately plate glass has inconsistencies and the refractive index can vary across the plate making it unsuitable as a lens or corrector plate. It is necessary to use a plate made of good quality optical glass.
(Good quality optical glass is more expensive than something suitable for a coffee table!)
The curve to be put on the corrector is unusual, but fortunately there is a relatively easy way to produce it which is illustrated in the steps below:-
Manufacture of Schmidt Collector Plates
a) A disk of glass of known dimensions and Young's modulus is placed on the mouth of a vacuum chamber.
b) The pressure in the chamber is reduced to cause a certain deflection of the glass (say pressure down to typically 2/3 atmosphere).
c) While in this state, the top is ground and polished into a spherical curve.
d) The disk is removed from the vacuum chamber and relaxes into the shape desired.
The vacuum chamber is the only piece of special tooling used, but we do need one for every diameter of corrector plate. It is necessary for the lip of the chamber to be optically flat to achieve a good seal with the glass and maintain the vacuum. It is usually sufficient to grind the glass disk lightly against the lip of the chamber for a short time prior to establishing a vacuum to obtain a good seal.
The amount of vacuum is calculated for the appropriate disk diameter, thickness and type of glass, but typically about 1/3 atmosphere is needed (i.e. the pressure in the chamber is about 2/3 atmosphere). The vacuum once established usually remains fairly constant over 24 hours, but is checked at regular intervals to avoid problems. The temperature rise from working the glass also has to be taken into account as it changes the pressure.
The illustrations above suggest the corrector is single sided - in practice we can chose to do half the correction on each side. If so, once one side of the disk is done, we simply turn it over and repeat the operation to obtain a double sided corrector. This has the advantage that it avoids the need for one side to be optically flat, which can be more difficult to achieve than a curved surface.
Although the process seems a bit involved, Schmidt Correctors are surprisingly easy to make, once you have a large polishing machine, a vacuum chamber and a fair bit of experience!
A mirror needs an surface accuracy of better than 1/8λ, but a corrector is a refractive device and only needs to be within a few waves at any point on the surface to give a matching performance. Polishing machines designed for mirrors of 1/8λ have no problems obtaining this accuracy. The correctors are tested with the spherical primary mirror as part of a complete system.