Manufacture of mirrors

Manufacture of mirrors for astronomy

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.

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.

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.

Curve generating:

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.

Smooting:

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.

Polishing:

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.

Figuring:

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:

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 page.

Couting:

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:

The 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.