A freeform optical surface is typically defined as any surface that does not have an axis of rotational symmetry. These surfaces provide additional degrees of freedom that can lead to improved performance compared to systems that make use solely of conventional optics.
Applying freeform optics in high-end optical systems can improve system performance while decreasing the system mass, size and number of required components. Their widespread application is however held back by the lack of a suitable metrology method. TNO, TU/e and NMi VSL have therefore developed the NANOMEFOS measurement machine , capable of universal non-contact and fast measurement of aspherical and freeform optics ranging from convex to concave and from flat to freeform, up to ⊘500 mm, with an uncertainty below 30 nm (2σ).
Three mirror anastigmat (TMA) telescope designs  had been implemented in different projects ranging from the narrow Field-Of-View large instruments as Quickbird (2° FOV)  to smaller telescopes as JSS 12° FOV developed for RapidEye mission .
This telescope configuration had been also selected for the PROBA-V payload, the successor of Vegetation, a multispectral imager flown on Spot-4 and subsequently on Spot-5 French satellites for Earth Observation and defence. PROBA-V, small PROBA-type satellite, will continue acquisition of vegetation data after the lifetime of Spot-5 expires in 2012.
The PROBA-V TMA optical design achieves a 34° FOV across track and makes use of highly aspherical mirrors. Such a telescope had become feasible due to the recently developed Single Point Diamond Turning fabrication technology. The telescope mirrors and structure are fabricated in aluminium and form an athermal optical system.
This paper presents the development of the compact wide FOV TMA, its implementation in PROBA-V multispectral imager and reviews optics fabrication technology that made this development possible. Furthermore, this TMA is being used in combination with a linear variable filter in a breadboard of a compact hyperspectral imager. Moreover, current technology allows miniaturization of TMA, so it is possible to use a TMA-based hyperspectral imager on a cubesat platform.
Melt spinning is a rapid quenching process that makes it possible to create materials with a very fine microstructure. Due to this very fine microstructure the melt spinning process is an enabler for diamond turning optics and moulds without the need of post-polishing. Using diamond turning of melt spun aluminium one can achieve ≤2 nm Rq surface roughness. Application areas are imaging and projection optics, mirrors, moulds for contact lenses and spectacles. One of the alloys that RSP produces is RSA-905. This alloy has a solid track record as a better and cheaper concept in the application of moulds for optical components such as contact lenses. The RSA-905 is a dispersion hardened amorphous-like alloy that keeps its properties when exposed to elevated temperatures (up to 380°C). This gives the material unique features for optics moulding applications. RSA-905 moulds are cheaper and better than traditional mould concepts such as copper or brass with or without NiP plating. In addition logistics can be simplified significantly: from typical weeks-months into days-week. Lifetime is typically in the range of 100.000 – 200.000 shots. For high volume production typically ranging from several 100.000 – several 1.000.000 shots, NiP plated steel moulds are typically used. By using an appropriate optical coating concept RSA-905 can be upgraded to a competitive alternative to steel in terms of price, performance and logistics. This paper presents some recent developments for improved mould performance of such concept. Hardness, wear resistance and adhesion are topics of interest and they can be applied by special coatings such as diamond-like carbon (DLC) and chromium nitride (CrN). These coatings make the aluminium alloy suitable for moulding mass production of small as well as larger optics, such as spectacle lenses.
Aluminum mirrors and telescopes can be built to perform well if the material is processed correctly and can be relatively low cost and short schedule. However, the difficulty of making high quality aluminum telescopes increases as the size increases, starting with uniform heat treatment through the thickness of large mirror substrates. A risk reduction effort was started to build and test a ½ meter diameter super polished aluminum mirror. Material selection, the heat treatment process and stabilization are the first critical steps to building a successful mirror. In this study, large aluminum blanks of both conventional AA-6061 per AMS-A-22771 and RSA AA-6061 were built, heat treated and stress relieved. Both blanks were destructively tested with a cut through the thickness. Hardness measurements and tensile tests were completed. We present our results in this paper and make suggestions for modification of procedures and future work.
For years now conventional aluminium 6061 T6 has widely been used for mirrors in astronomical instruments, being
diamond turned or since a few years also being optically polished. This allows the development of optical systems that
can be tested and operated at any temperature, without being affected by CTE effects. Using traditional aluminium the
manufacturing methods could in some cases not deliver the required surface shape, accuracy and roughness due to the
increasing demands from optical systems. Over the last few years RSP Technology developed a new series of aluminium
alloys for several applications, produced with a Rapid Solidification Process. Both on a macroscopic and microscopic
level these new aluminium alloys have different material characteristics compared to the traditional aluminium alloys.
TNO and NOVA-ASTRON have performed diamond turning and polishing tests on these new aluminium alloys. This
paper presents results of several diamond turning and polishing tests obtained over the last year and show the potential of
these new alloys with surface roughness values of 1 nm on RSA 6061 and RSA 708 acquired with both diamond turning
For the ESO Very Large Telescope, TNO is making four Optical Tube Assemblies for the Four Laser Guide Star
Facility. Each OTA is a large 20x Galilean beam expander, which expands a Ø15 mm, 25W CW 589 nm input laser
beam to a steerable Ø300 mm output. The L2 lens is a Ø380 conical convex lens with a radius of curvature of 637 mm
and conic constant k = -0.4776.
This paper describes the flexible manufacturing technique that TNO applies to make these kind of optics. With the
combination of deterministic polishing and the NANOMEFOS measurement machine, these optics can be manufactured
quickly and efficiently, without the need for a dedicated test setup. Final performance testing of the OTA validates all the
intermediate steps in the flexible process value chain.
The breakthrough of freeform optics is limited by manufacturing and metrology technology. However, today's
manufacturing machines like polishing robots and diamond turning machines are accurate enough to produce good
surface quality, so the question is how accurate can a freeform be produced. To investigate how accurate freeform optics
can be diamond turned, measurable freeforms (e.g. an "off-axis" sphere) were diamond turned and they were compared
to there on-axis equivalents. The results of this study are described in this paper. Furthermore, an overview of the
accuracies of freeform optics that TNO diamond turned are presented. An indication of freeform accuracy for diamond
turned optics is derived from this, which can be used for optical designers as a guideline in their design work.
This paper present the results of a diamond turning study of a rapidly solidified aluminium 6061 alloy grade, known as
RSA6061. It is shown that this small grain material can be diamond turned to smaller roughness values than standard
AA6061 aluminium grades. Also, the results are nearly as good as nickel plated surfaces, but the RSA6061 has the
advantage that no additional production steps are needed and that no bi-metallic bending or delamination can occur in a
thermally changing environment, e.g. when cooling to cryogenic temperature. Therefore, RSA6061 is a good material for
optical applications in the visual spectrum.
This paper presents the results of an experimental study on the influence of aging on the cutting mechanics of glassy polymers. Polystyrene (PS), a glassy polymer, typically behaves brittle when subjected to a stress, it can be made ductile by rejuvenation. It was expected that PS would show a different cutting behaviour when it would be aged or rejuvenated. To investigate this two different molecular weight PS grades were used. Both aged and (mechanically) rejuvenated samples were made from each grade and cut. Cutting forces, chip morphology and surface quality were investigated. Although the chips showed no differences in brittleness and ductility, the measured cutting forces indicated that there is a difference between aged and rejuvenated PS. Also an interesting difference in cutting forces between the two PS grades was found. Investigation of the surface quality of the PS samples showed that the aged samples have smoother surfaces than the rejuvenated samples.
It can be concluded that aging does have effect on the cutting mechanics and the obtained surface roughness.