Precision Engineering behind European Astronomy Programmes
Shore P1 and Tonnellier X1
1

CUPE, Cranfield University Precision Engineering, Bedford, England

Abstract –The fields of astronomy science have presented significant precision engineering challenges. Numerous solutions for thesefields of science have achieved unprecedented levels of accuracy, sensitivity and sheer scale. Notwithstanding of their importance toscience understanding, many of these precision engineering developments have become key enabling technologies for wealthgeneration and other human well-being issues. This paper provides a brief historical overview of astronomy instruments. Later, detailsof critical precision engineering developments that supported the establishment of leading European astronomical instruments areillustrated. Finally, significant precision engineering demands to enable future sciences programmes are introduced.
Key Words: Telescopes, precision, machines

In this short paper, we provide a brief historical overview ofenhanced seeing performance. Using his telescope Galileo discovered

European astronomical developments. The paper subsequently

moons around Jupiter and consequently reinforced support for theprovides details of significant precision engineering demands toCopernician model. James Gregory and Isaac Newton’s reflecting

enable recent European telescope programmes. Thereafter criticaltelescope concepts were enabled in the 1660’s through the combined

precision manufacturing developments that will progress futuretechniques of grinding and polishing metal mirrors in speculum.

European astronomical projects are presented. The paper has been

These precision machining techniques enabled William Herschel toadapted from a comprehensive CIRP Keynote on Precisionbuild his telescope which discovered Uranus and numerous nebulae.

Engineering for Astronomy and Gravity Science [1].

Léon Foucault invented the metallised-glass reflecting telescope,
producing a 80 cm telescope in Marseille in 1864. By employing a

1. Introduction“knife edge” measurement test during the mirror manufacturing

It has long been the dream of humankind to understand its place(in-process metrology) a more controlled machining process waswithin the universe. This aspiration has led to detailed measurements

attained. The silvered glass reflecting telescope became the preferredwhich accurately described to movement and interactions of celestial

technology for large ground-based telescopes, particularly after thebodies. From the advent of astronomical instruments it has been clear

limits of refractors were reached. Following a suggestion of Newton

that these should have high effective resolving power, which isnearly 200 years earlier, Charles Piazzi Smyth in 1880 carried out

essentially a function of both; scale (eg. diameter of telescope) andobservations from the top of Mount Teide, Tenerife. Smyth’s

quality (eg. accuracy of mirror surfaces). From experience theseobservations demonstrated the benefits of observation made through asimultaneous demands are known to be difficult to reconcile. This

reduced atmospheric layer. From that time onwards major grounddual requirement is at the centre of “Precision Engineering” andbased telescopes were positioned on mountain tops; the first notable

fundamentally coupled to being able to measure with high accuracy

example was the Lick Observatory.

and with minimal levels of uncertainty. The maxim ascribed toDuring the first half of the 20th century the major development ofGalileo tells us to “measure what is measurable, make measurable

telescopes was that of size scale-up. Large reflecting telescopes bywhat is not”. Scientific progress made in the field of astronomy

George Ritchey and George Ellery Hale led to reflectors of up to 200

research has been achieved through significant precision engineering

inches (5 metres) in size. The 200 inch Hale telescope was based on

developments. Numerous engineering solutions have demanded

low expansion Pyrex mirrors, ship building structural engineering andunprecedented levels of accuracy, sensitivity and sheer scale.

passive flexure compensators. In 1929, using observations from theNotwithstanding of their importance to science understanding manyfirst of these large Californian telescopes, Edwin Hubble confirmedof these precision engineering developments have become key

the expansion of the Universe. For the majority of the latter half of theenabling technologies for wealth generation and other human

20th century telescope performance was most significantly advanced

well-being issues.

through implementation of electronic array detectors replacing
photographic plates in visible and IR. Through the greater sensitivity

2. Astronomical instrument development

and linearity of charge coupled devices (CCDs) and IR arrays, it was

From the beginning of astronomy as a science, discoveries havepossible to improve the performance of existing 2 - 4 m scale

been made following the introduction of new manufacturing

telescopes. Better detectors achieved an order of magnitude

technology. Considering Galileo’s telescope, built over 400 years ago

improvement of sensitivity during this time. Computer control of ain 1609, the key manufacturing technology was the newly developed

telescope was pioneered on the Anglo-Australian Observatory in 1975.

ability to fabricate glass lenses precisely, together with some critical

The implementation of computer control paved the way to actively

innovations from Holland such as “stopping down the aperture” forcontrolled telescopes and ultimately the 8-10 m telescope era of today.

The Russian 6 m BTA telescope was one of the first to rely solely on
computer control.
In 1923, Hermann Oberth proposed the benefits of space based
astronomy. Lyman Spitzer in 1946 presented a seminal paper thatdefined the wider attributes and capabilities of space based telescopes.
The launch of Sputnik in 1957 “changed everything”. It led to thebirth of NASA, the race to the moon and the consequent availabilityof space technology that led to the great NASA Observatories such as
the Chandra X-ray telescope and the 2.4 m Hubble optical/near IR
telescope. These NASA space telescopes, together with ESA’s ISO
and XMM-Newton telescopes, have produced a broad range of
discoveries and provided beautiful images that have captured wide
public interest.

Fig. 1: European Extremely Large Telescope (42 m segmented
primary mirror)

By the late 1980s it was generally recognised that the scope for
improved sensitivity of ground based telescopes was again only to be

2. Precision manufacturing technologies

afforded through an increase in size. This situation was brought about

Ultra precision manufacturing technology for producing advanced

through spectacular improvement of detector sensitivity during theoptical surfaces has been, and continues to be, a fundamental

1970s and 1980s together with improved positioning and motion“technology enabler” for telescope development. In this section we

control. The late 1980s saw astronomy entering the era of the 8 – 10

present European developments considered to have made a significant

m telescopes. Numerous enabling technologies that led to the very

contribution.

large telescope era (VLT, 8-10 m scale telescopes) will be covered in
more detail later in this paper. Those considered having historical

2.1. Mirror (lens) fabrication

significance include: Segmented primary mirror technology, applied

The ESO VLT and Gemini telescopes employed Zerodur based

to the US Keck and European Grantecan telescopes, as proposed bymeniscus substrates made by Schott. They were subsequently ground,

Jerry Nelson [2]; and 8 m scale monolithic honey comb spun castpolished and measured in-situ. An 8 m scale capacity mirror

mirror moulding technique developed by Roger Angel [3] and applied

processing facility has been established in France by REOSC [8].

to the Magellan Telescope and the Large Binocular telescope. In
parallel, an 8 m mirror capability was also established in France
during the 1980s. It is based around a meniscus mirror fabrication
capability and use of active optics technology. This technology,
developed by REOSC, was first employed by ESO on the 3.6 m New
Technology Telescope and subsequently the four 8 m Very Large
Telescopes [4]. Similar technology was subsequently employed for
the Japanese Subaru telescope [5]. During the late 1990s and the first
decade of the 21st century the 8-10 m scale ground based telescope
technology matured. Adaptive Optics, discussed later on, significantly
advanced the “seeing” performance of Keck, Gemini and the VLTs
producing revolutionary high resolution images of gas giant planetarysystems (much like Jupiter) and provided evidence of a massive blackhole near the centre of our galaxy.
However, as was seen during the first half of the 20th century theFig. 2:VLT Mirror (8 m) undergoing optical measurement using

in-situ measuring tower [9]main demand of 21st century astronomers is increase in telescope sizefor ground and space based systems. The detection of so called

REOSC processes 175 mm thick so-called “meniscus” mirror

earth-like planets, presently a prime objective, is a central justification

blanks. The meniscus blanks are held on active supports during

for the proposed US/Japanese Thirty Metre Telescope and thepolishing as employed within the telescope itself. At REOSC a large

European -ELT 42m telescope projects [6-7].

20 m high optical test tower is provided above the grinding/polishing

The so-called Extremely Large Telescopes (ELTs) are presently insystem itself. The form accuracy achieved by Sagem-REOSC in

the design and preliminary manufacturing assessment stages. These

producing the 4 VLT primary mirrors was 17.6 - 33 nm RMS [10].

telescopes are targeted for “first light” by 2017 – 2020, at which time

The form accuracy achieved on the 8 m scale mirrors by

the pioneering NASA led segmented mirror-based James Webb Space

Sagem-REOSC corresponds to an optical resolution (relating to image

Telescope (JWST) having a 6.5 m diameter will be operational.

sharpness) of 0.03 arcsecond in the visible spectrum. It provides aUsed in combination the proposed ground based ELT’s and the
JWST will yield a new era of astronomical observation.

theoretical capability of distinguishing two objects separated by only
150 mm at a distance of 1000 km.
Brinksmeier et al., has presented a detailed review of thedevelopment of Ultra Precision Grinding [11] which includes details
relevant to astronomy surfaces.
2.2 Segmented mirror manufacturing
The idea was originally conceived by the Italian Horn d’Arturo in
the 1950’s. This segment mirror technology is pivotal as beyond 10 m
scale monolithic, mirrors were becoming unrealistic; even considering
their transportation. More complex shape primary mirrors (i.e. deep
aspheric) would enable more sophisticated and elegant telescope
designs. By making the primary telescope mirror from a number ofsmaller interlocking mirror segments, a scalable means for producing

Fig. 4: Grantecan mirror segments being readied for optical testing
using the REOSC test tower [16]any size of telescope would be enabled [12]. Clearly, producing theoff-axis aspheric shaped segments would be critical manufacturing

2.3 Space mirror manufacturingissue. Significantly these complex shape mirror segments would need

The Hubble Space Telescope is perhaps the most iconic and famous

to demonstrate very limited so-called edge “roll-off”. Any shape error

telescope. It has undoubtedly been a major scientific success,at the edges of the segments would reduce the optical performance of

operating in the UV to the near infrared, ~200 – 2400 nm.

the telescope and generate highly problematic stray light features.

Unfortunately a measurement error made during the final polishing of

Nelson applied it successfully to the two 10.2 m primary mirrors

the primary mirror led to a highly complex servicing mission. The

Keck telescopes. It was therefore important to develop aflight mirror was polished by Perkin Elmer in 1979 using pre-CNC

manufacturing process chain able to realise the 36 mirror segments oflapping and polishing technologies. Interestingly, a second (spare)~ 2 m scale. The developed fabrication route included a grinding

Hubble mirror was produced by Kodak. Whilst it took Kodak 16

process followed by a novel full-aperture polishing technique called

months to produce it is claimed the actual polishing time itself was“stressed” mirror polishing [13]. In addition a 2.5 m off-axis grinding

limited to 74 hours [17]. It was also made using traditional pre-CNC

machine was built at CUPE (Fig. 3) for Eastman Kodak’s segment

controlled grinding and polishing machines.

mirror programme [14].

In regard to developing advanced manufacturing technology, it isperhaps the X-ray space mirrors have led to the widest range ofadvancements. X-ray telescopes are themselves a relatively new
development, the first being launched in 1977. A highly detailed
account of their development has recently been provided byAschebach [18]. X-ray space telescopes are most significantly basedon the application of Wolter type mirror designs employing grazing
incidence optics [19]. The primary mirror optics are tube-like in shape
typically having internal mirror surfaces of parabolic and hyperbolic
shape. A number of the Wolter type mirrors are nested: The greater thenumber of nested mirrors, the greater the X-ray light collecting
capacity. Consequently the manufacture of thinner section mirrors has
been a significant X-ray telescope technology driver. This has led to a

Fig. 3: Schematic layout of the laser interferometer controlled
CUPE/CPE Ltd OAGM made for Kodak

notable ultra precision manufacturing demand. As might be expected,
these shorter wavelength mirrors demand ultra smooth surfaces
typically 0.3-0.8 nm RMS, having form accuracy demands below the

The final figuring process, an ion beam figuring (IBF) technology
causes ionised argon atoms to bombard the mirror surface causing
atom to be “knocked out”.

30 nm RMS region.
Pioneering X-ray telescopes, such as Exosat launched in 1983, usedmirrors produced by a replication process. This replication process

The fabrication of Grantecan mirror segments at Sagem-REOSC

used solid glass mandrels that were ground and polished to the inverse

was achieved using sub-aperture polishing with industrial robots andshape and to the form accuracy of the demanded mirror. Mandrels

final ion beam figuring [15]. Importantly, the ability to polish/ion

were subsequently gold coated. Carrier mirror shells machined to an

beam figure the edges of the segments without inducing edge roll-off

accuracy of 0.5 mm were produced. The gold coated glass mandrels

was developed, ensuring low levels of consequent stray light in thewere placed inside the carriers and epoxy injected between them.

telescope.

After curing, mandrels were removed, leaving behind the gold mirror

A notable difference to Keck project, in the production of thesurface on the carrier. The EXOSAT mirror carriers were made fromGrantecan segments, was that they were polished as hexagonal

beryllium. The ultra precision mandrels were said to be reusable up tosegments prior to IBF. The achieved form accuracy of the Grantecan

6 times.

segments was 12 – 30 nm RMS.

Subsequent to EXOSAT, the Rosat X-ray telescope launched in
1998, employed an eight mirror X-ray system. Four nested parabolic
mirrors and four nested hyperbolic mirrors (Fig. 5).

based on an air bearing supported contacting probe, referenced using
a HeNe laser interferometer to optical straight-edges mounted within
the machine envelope. Circularity and axial profile accuracies were
better than 1 µm.
Post diamond turning, the EUV mirrors were processed by Ferranti
Astron. Ferranti carried out a pre-polish prior to coating withelectro-less nickel (~60 µm thickness). This was followed by a final
super polishing employing alumina oxide abrasive and a conventional
pitch polishing sub aperture technique. Achieved roughness wasFig. 5: Cross section of Rosat X ray space telescope showing 4
nested Wolter type mirrors [20]claimed at 0.3-0.5 nm RMS.
In 2009 the Herschel Space Telescope was launched. It is a far
infrared region telescope operating in the 60 – 670 µm wave-length

The largest of the X-ray mirrors was 0.84 m. Co-aligned with the
X-ray system was an extreme ultra violet optical system called therange and it complements the Hubble and the soon to be launched
James Webb Space Telescope.

Wide Field Camera (not shown in Fig. 5) having a three nested mirror
arrangement, the largest being 0.6 m. Rosat’s X-ray wavelength
mirrors were made of Zerodur and manufactured in Germany by Zeiss[21]. These Zerodur mirrors were ground on a vertical grinding
machine employing air bearings. During grinding, the Rosat mirrors
were supported in aluminium fixtures having many supporting pins.
The numerous support pins ensured grinding forces did not induce
significant distortion. Subsequent to grinding, polishing was carriedout using a sub-aperture free abrasive polishing technique. Finalpolishing runs were undertaken with the mirror in a constrained freestate, as suggested by Young [22]. The Rosat X-ray mirrors had a 3 Å
RMS roughness. Critical geometrical errors of X-ray telescope
mirrors have been discussed by Zombeck in 1981 [23].
The two most critical errors were stated as; circularity error about
axial direction (akin to roundness/cylindricity) and profile error again
in an axial direction. Rosat X-ray mirrors typically had circularity
quality and axial direction profile accuracy at 0.5 µm and 0.2 µm

Fig. 7:

Herschel Space telescope 3.5 m silicon carbide based

primary mirror [25]respectively. Data to confirm this is presented by Aschenbach andmeasured using in-house constructed profilometers at Zeiss [24].

As might be expected, the form accuracy requirement for its

Rosat’s EUV mirrors were electro-less nickel coated aluminium

primary mirror is less demanding than for shorter wavelengthand manufactured by CUPE and Ferranti in the UK. The aluminium

telescope mirrors. Its 3.5 m mirror has form accuracy around 6 µm

substrates were diamond turned by CUPE in 1981 using an in-house

RMS with a 30 nm RMS roughness. Nonetheless the Herscheldeveloped large diamond turning machine (Fig. 6) [24].

telescope represents a significant precision engineering achievement.
It is structurally and optically fabricated from silicon carbide. Its
primary mirror has been constructed by brazing together 12 large
mirror segments produced by the French company Boostec [26].
The light-weighted mirror segments are moulded using an isostaticpressed sintered silicon carbide. The segments were accurately
machined and subsequently joined using a silicon braze technology
establishing the 3.5 m mirror substrate. Since the Herschel spacetelescope operates at cryogenic temperatures it is clearly critical thesilicon braze technology is employed so that a “stress-free” bondacross the substrate in achieved. The application of silicon carbide forthe primary and secondary mirrors and the strut supports of the
Herschel telescope ensure its total mass is less than 300 kg. The use of
hard silicon carbide requires adaptation to conventional polishing. The
polishing of the Herschel primary mirror was carried out by Opteon in

Fig.6: Rosat EUV mirror being measured on CUPE LDTM prior to
NC error correct diamond turning
The so-called LDTM employed a post-process profilometer whichallowed form errors to be measured in-situ. The profilometer was

Finland who developed and employed a novel diamond based
polishing technology [27]. The polished mirror was subsequently
coated with an adhesive nickel chrome layer, a reflective aluminium
layer and finally a protective silicon based polymer.

The James Webb Space Telescope, scheduled to launch in 2014,

correction.

will be the first operationally segmented mirror telescope to be

An alternative approach has emerged, which is geared towards

deployed in space. Although the joint NASA/ESA telescope has its

higher volumes. This utilises CNC sub-aperture techniques at every

primary mirrors made in the US, astronomy mirror demands are notstage of surface generation, with an essentially unlimited freeform

limited to large mirrors. Complex shape multi-mirror arrays are also

capability [33]. Targeting a 20 hour cycle time for each stage ofdemanded, especially within telescope instrument systems [29]. A

surface generation for a 1.5 m class mirror, it specifically addresses

significant example is image “slicers” (Fig. 8) and “re-imagers” as

the requirement for worldwide supply ramp-up, where existingused within spectrometers.

techniques take at least an order of magnitude longer.

Fig. 8: Photograph of an image slicer made by Cranfield University

Figu

Precision Engineering for the JWST MIRI instrument [30]Fig. 9: Rapid production of large scale mirror segment using the –Development of multi axes diamond machining techniques for

BoX® Cranfield ultra precision grinding/measuring machine

producing these mirror arrays has been undertaken [31]. Shore
reported the diamond machining technique employed to produce such

The approach is based on a high speed, high accuracy ultra-low

optics for the James Webb Space Telescopes MIRI instrument [30].

damage freeform CNC grinding stage, followed by a relatively short

Morantzminiature

CNC figure corrective polishing stage. These technologies have been

Twyman-Green interferometer mounted onto a stability enhanced

reportedtheirmeasurement

using

a

demonstrated; grinding achieving cycle time and form accuracy

ultra precision CMM [32].

targets below 1 µm RMS [34]. An additional CNC sub-aperture
Reactive Atom Plasma figure correction technology is at an advanced

3. Future demands

stage of development [35]. Mass production of ~1 m scale mirrors has

3.1 Extremely Large Ground Based Telescopes (ELT’s)

become a commercial demand defined by future astronomy

Extremely Large Telescopes are classed as telescopes possessing anprogrammes providing a clear potential benefit for optic fabrication.

aperture diameter greater than 20 m for observation in the UV, visible
and near IR wavelengths. Monolithic mirrors are limited to around 10

3.2 International X- ray Observatory

m diameter for reasons of manufacturing, measurement and

The International X-ray Observatory (IXO), a large deployabletransportation difficulty. Larger effective apertures are possible

Wolter type X-ray mirror structure with 20 m focal length, is a joint

through the aperture synthesis achieved by interferometry ofeffort (NASA/ESA/JAXA) currently planned to be launched for 2021.

combined smaller aperture telescopes, but there are significant

The IXO demand represents a new ultra precision production

advantages in an optically-filled aperture, not least light-gatheringengineering challenge and a major commercial contract.

power. Consequently, segmented mirror technology, as applied to theKeck telescopes, has become the main basis for telescope scale up.

It employs segmented mirror technology similar to that employedin JSWT and the proposed ELTs. IXO requires 6948 parabolic andFunded ELT programmes scheduled for first light around or before

6948 hyperbolic mirror segments. The surface area demanded for the2020 include E-ELT at 42 m apertures, requiring 984 segments for its

polished forming mandrels is 60 m² (721 mandrels). Total mirror

primary mirror. The mirror supply issue is non-trivial: all 20 of thesegment area over 800 m². The form quality requirements forworld’s operational very large (> 5 m diameter) telescope primary

individual mirror segments equates to 100 nm RMS with tighter shortmirrors, manufactured over a period of more than 50 years, have awave-length demands.

combined area of around 1,000 m2; just TMT and E-ELT between

Mirror manufacturing technologies for the IXO mirror shellthem will require in excess of twice that for their primaries,

segments include “slumped glass” and “silicon pore optic”. These

manufactured within a period of only 10 years. This represents aprocesses are being used commercially: glass slumping is used inrequirement for a rapid increase in worldwide supply capacity,liquid crystal flat panel display industry, “silicon pore” optics are

currently underway, perhaps approaching an order of magnitude.

standard semiconductor wafer technology processes [36]. The IXO

The stress-lap and stressed mirror polish techniques, employed forsome existing large mirrors and segments, utilise modified sphericalsurface generation techniques with subsequent sub-aperture figure

demand is creating a higher accuracy capability from thesemanufacturing technologies.
For example, ESA silicon pore optics are 12” silicon wafers that arediced, wedged and ribbed. Rectangular silicon plates produced having[14] W.J. Wills-Moren, T. Wilson: The Design and Manufacture of athin ribs on one side and thin membranes between the ribs. The

Large CNC Grinding Machine for Off-Axis Mirror Segments. CIRP

sandwich construction is bent into the required shape and stacked

Annals 38(1):529-532, 1989.

automatically to obtain a stiff pore structure. The stacks are then[15] R. Geyl et al.: Large optics ion figuring. Proc. SPIE

mounted into "tandem" structures which are combined to form the3739:161-166, 1999.

X-ray mirror [37].

[16] E. Ruch: Presentation at Polissage Optique pour les Grands
Instruments de la Physique et de l ’Astronomie, Bordeaux, 2009.

4. Conclusions[17] ITT: http://www.ssd.itt.com/heritage/hubble.shtml, 2011.

This paper has introduced European precision engineering[18] B. Aschenbach: X-ray telescopes. Prog. Phys. 48:579-629, 1985.

achievements borne of the demands of large scale astronomy. These[19] H. Wolter: Mirror systems with glancing incidence as

have included:

image-producing optics for X-rays. Ann. Phys. 445:94-114, 1952.

Ultra Stable and Lightweight Materials, Ultra

Precision Surface Fabrication & Measurement (especially at large[20] ROSAT mission: http://www.mpe.mpg.de/xray/wave/rosat, 2011.

scale) and Ultra Precision Machine Tool Techniques.

[21] D. Reinhardt: Features of manufacturing and qualification tests

The paper has also identified key precision manufacturing demandsof a high-resolution Wolter I mirrors. Proc. SPIE 733:145-148, 1987.

of future astronomy ESA/NASA programmes and the even greater[22] P.S. Young: Fabrication of the high-resolution mirror assembly

demands that these future programmes place on Ultra Precision

for the HEAO-2 X-ray telescope. Proc. SPIE 184:131-138, 1979.

Technologies. The scale of future programmes, such as the Extra[23] M.V. Zombeck: Advanced X-ray Astrophysics Facility (AXAF).

Large Telescopes, requires greater attention to production engineering

Opt. Eng. 20:297-309, 1981.

capabilities in order for them to be realised in acceptable timescale[24] K. Beckstette, E. Heynacher: Contour line measurements onand cost.

ROSAT X-ray mirrors. Proc. SPIE 429:126-129, 1983.
[25] P.A. McKeown et al.: Experiences in the precision machining of

5 Acknowledgements
The authors would like to thank the following for theirgrazing incidence X-ray mirror substrates. Proc. SPIE 571:42-50,
1986.

contributions: Klaus-Friedrich Beckstette, Pat McKeown, David Allen,[25] Herschel: http://www.esa.int/esaMI/Herschel, 2011.

Bill Wills-Moren and Robert Hocken. Acknowledgement is given to[26] J. Breysse et al.: All-SiC telescope technology: recent progress

the EPSRC funded UPS2 IKC, ESA, ESO, NASA, STFC and theand achievements. Proc. ICSO: 659-671, 2004.

McKeown Foundation.

[27] T. Korhonen et al.: Polishing and testing of the 3.5 m SiC M1
mirror of the Herschel space observatory of ESA, Proc. SPIE 7210:

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