Transmission optics

The following general requirements can be identified for mounts in the case of transmission optics:

Kinematic mount

The position of an optical element can be defined uniquely in terms of six independent coordinates, degrees of freedom (DOF): three translations (Tx, Ty, Tz) and three rotations (Rx, Ry, Rz). A mount is called “kinematic” when each DOF of the element is at most only once constrained. Kinematic couplings can be used to constrain all 6 (accurate positioning) or any (accurate movement) of these DOF without distortion and with excellent repeatability.

Non-kinematic mount

In practice it is not always possible and/or necessary to create full kinematic mountings. Sometimes support points are needed in order to avoid too much deformation of the optical element under its own weight (sagging), or there is not enough space available around the optical elements to provide room for the necessary structures/compensation mechanisms.

In special cases the choice is even made to overconstrain one or more of the DOF. By accomplishing high manufacturing accuracies e.g. super flat and accurate mounting surfaces the optical element can remain unstressed when mounted ‘non-kinematically’.

Radial mount principle

There are several ways for radial mounting of the lens.

Kinematic springs

Most commonly a set of three leaf spring/pivot hinge constructions (located symmetrically at 3x120° around the Centre of Gravity (CoG) of the optical element) is used. The leaf springs are very effective to release or constrain specific DOF, free of friction, wear and hysteresis. When possible this type of mounting is integrated within the optical element itself, but for transmission optics this needs to be provided within the support structures or in a dedicated substructure.

v-block

The v-block solution will cause decentre errors between ambient and cryo conditions. The necessary spring loading would make it susceptible to errors at changing gravity directions. High spring loading would be necessary to survive launch loads, causing additional (to high) stresses and thus Wave Front Errors (WFE).

Compensation

In special CTE compensation mounts the shrinkage differences between the support and the optical element are compensated by special kinematic couplings. These special pads have higher CTE than the structural or optical material and are dimensioned for each optical element individually. The pads are dimensioned such that they shrink sufficiently to compensate for the CTE differences between support and optics. The CTE compensation mount is successfully used within several ground based high accurate cryogenic instruments, SPIFFI (2K camera) and X-Shooter (NIR arm).

Bonding lens and holder

When fully detachable optical elements are required, the combination of maximum allowed stress on the element and the relatively large CTE differences makes the design of a traditional kinematic mount with leaf springs/pivot hinges virtually impossible. At all times only part of the springs can bear the lens load. So the spring rate of each spring should be sufficiently high to keep the optical element from sagging, but also low enough to avoid overstressing (resulting in deformation, birefringence or even breakage) of the optical element. It is very difficult to find a spring rate that fits both requirements at ambient and cryogenic temperatures, especially for larger optics because the absolute shrinkages differences increase with size.

Using a kinematic spring mount the lens requires the lens to be bonded to the springs. This allows all the spring at the circumference of the lens to bear the load of the lens. Particular issues arise when bonding to the typical lens material. The CTE of cryogenically suited glues are generally very different in comparison to the lens materials. This means that extreme care should be taken to avoid stresses due to the glue spots.

In addition the adhesion of the glue to the lens is so high (which is normally preferred for glue constructions) that, instead of simply delaminating, the glue pulls small chips of the brittle material from the actual lens, causing severe damage and high risk of cracking. This effect could be predicted [1], but requires detailed knowledge on the glue properties; adhesion/cohesion/viscosity/CTE etc. of the glue at the full temperature range. Also precise dimensioning of the glue spot [2]; size/thickness etc. is needed. Generally rather small glue spots [3] are required and special tooling is needed to apply the glue with sufficient accuracy.

Axial position of radial support

The axial position of the radial pads is another important aspect to reduce the deformation effects on the lens. An axial position at the full material of the lens is important or (unnecessary) additional deformation is the result. This effect can be illustrated with a meniscus shaped lens. When mounted at the lens centre plane:

This way additional deformation is created. By mounting at the thickest part in this case more at the edge:

The additional deformation is limited due to the guiding of the stresses through the main bulk of the material.

Axial mount principle

The axial positional accuracy is provided by the hard mounting against the holder/mount on a preselected number of support points. For positional accuracy (at operational conditions) the lens should remain pressed against this reference at all times. A simple spring can be used:

Specifically when facing high loads (e.g. during launch) gapping should be avoided thus the dimensioning of this spring can be problematic. A similar CTE compensation as for the radial support can be used for the axial support eliminating this gapping problem.

The axial mounting is less critical than the radial mounting due to the location and direction of the forces. When the spring or CTE location is opposite the support points the lens will only experience local compression forces, but no additional surface deformations will occur. Birefringence effects due to the compression stresses around the mounting points should be checked, however the affected zone will be limited.

Number of mounting points

As previously described a proper kinematic mount requires no more than three mounting points between the lens and the holder.

Especially larger rather thin lenses may require more support points. Not just the total surface deformation is but also the actual surface shape should be considered, which has a direct impact on the wave front error. Some typical shapes can be more easily compensated than others; e.g. a defocus shape error can be compensated by a simple translation of the component while trefoil and derivative forms are very difficult to compensate for.

Figure 2 shows the effect of either three or six radial support points on a lens. The 6 points allow for a smaller overall deformation together with a more favourable and easier compensated surface form.

Figure 1:

The theoretical representation of the classical “cone, groove and flat” kinematic mount (figure: courtesy Newport Corp.). The three balls are part of the element. With this type of mounting extra precautions are necessary to ensure correct positioning of all parts e.g. under varying gravity orientation. That makes this type of mounting less suitable for many astronomical applications (instruments in cassegrain foci, space instruments).

Figure 2:

Deformation of a lens (SPIFFI L1) with 3 and 6 pads. The total force is identical for both situations but divided over respectively 3 or 6 points, resulting in a surface deformation of L1 of respectively 3.83 nm and 2.04 nm. The 6 mounting pads show less deformation 2.04 nm, compared to 3.83 nm for three pads, and in addition a much favourable surface shape

By means of special Whiffletree constructions the number of support points can be increased without compromising the kinematic quality. However these constructions are rather complex and space consuming:

So the design of a proper kinematic mount should remain the key driver, however as mentioned before sometimes it is necessary and more practical to have a semi/non-kinematic mount. This decision cannot be taken mildly, and proper risk assessment should be provided to prove necessity and to analyse the solution.

Isostatic point of optics

Every component inside the instrument will have its individual isostatic point; this is the point that remains stable and unchanged in either warm or cold circumstances. This point is a result of component shape and mounting principle. Within the special CTE compensated mount this isostatic point can be (re)located at a more favorable location which has a positive influences the warm-cold conversion through less additional component and thus beam shifts. This is illustrated in Figure 3.

Figure 3:

Isostatic points for various mounting methods of lenses and prisms. The circle indicates the location of the isostatic point The situation at operational temperatures is shown with the red dotted lines.

Figure 4:

Kinematic mount with 3 double ended clamped leaf springs at 120 degrees each at the circumference, for accurate a-thermal alignment. The three centrally located dowel pins are key for the alignment accuracy of this kinematic mount

Figure 5:

Thermo elastic analysis of the SPIFFI L1 lens. Left the maximum von Mises stresses of 8.2 N/mm2 due to the temperature gradient of 18 K (right) caused by a cool down rate of 24 W/h. cooling input at three spots at the lens circumference.

Figure 6:

X-shooter corrector lens with additional thermal leads; the copper strips at the lens circumference

Figure 7:

Additional expected lens deformation due to CTE uncertainties within SPIFFI; L1 1.4 nm (lower left), L2 1.9 nm (upper left) and L3 (right)8.3 nm at an operational temperature of 77K.

Figure 8:

Stress distribution within the total assembly (left) and the lens (right).

Figure 8:

2K camera assembly; at the front the triplet assembly, the doublet is located inside the camera structure (at the right hand side) and thus not visible in the photo.

Assembly build up

The holder consists of a dedicated mounting ring with a cover. Within the mounting ring 6 slots are provided for the 6 combined axial and radial CTE compensation pads. The pads can slide within their slots in axial direction. The CTE fixation screws are used to accurately manoeuvre and fixate the CTE pads in radial direction. Axial fixation is provided by the cover with 6 integrated but elevated pads.

The CTE compensation pads are made of high CTE material, higher than the used holder and lens materials. Accurate dimensioning of the CTE pads will ensure that the dimensional differences between the (faster shrinking) holder and the lens (not/limited shrinking) is compensated for. Resulting in accurate positioning together with no/low internal stresses.

Material choice

The main building material used within cryogenic instrumentation is (still) aluminium, due to the favourable material properties from (relative) low density, to ease of manufacturing. Unfortunately aluminium is amongst the metals with the highest CTE. The material for the CTE compensation needs higher CTE than either the lens or holder material, also the higher the CTE the smaller the overall packaging.

Thermo elastic plastics like POM and PTFE have suitable high CTE together with some less desirable properties like; high thermal resistance, sensitive to creep, non-linear CTE (and different progression than metals). However by proper dimensioning these issues can be overcome.

Alignment

Axial

The axial positioning of the lens and holder assembly can be provided by means of the cover. This way the same reference is used for the lens position in the holder as well as for the final positioning of the lens assembly in the instrument/barrel. Accuracy of this reference plane can be improved by precision lapping of the surface.

Radial

The radial positioning of the lens within the holder is accomplished by means of several accurate references. For the radial alignment the holder is provided with accurate flat surfaces at the circumference of the holder. These surfaces can be accurately positioned with respect to the interface features of the holder with respect to the instrument/barrel. The CTE pads and thus the lens will be referenced to these flat surfaces, thus no referencing with respect to the inner geometry of the holder is used/required.

The positioning of the lens assembly with respect to the instrument/barrel can be done in several ways either by an accurate fit between the holder outside periphery and the instrument/barrel inside periphery:

Or by means of two small dowel pins:

Due to the decrease in manufacturing accuracy of the large diameters the accurate fit solution turns out to be less accurate. The table shows final positional accuracy with a precise but normal manufacturing strategy.

	Tolerance	Displacement
		2 pins	1 fit
Pos. Tol. holes mount	ø 140 ± 0.02	0.01	0.01
Pos. Tol. holes barrel	ø 140 ± 0.02	0.01	0.01
Play 2 pins - holder	5h6/H7	0.0042
Play 2 pins - barrel	5h6/H7	0.0042
Play barrel - mount	140h6/H7		0.029
RSS		0.0153	0.032

Another disadvantage of the accurate fit is the difficult assembly and integration of such a construction. The lens assembly must be assembled accurately perpendicular with the centre axis or there is a high risk for getting stuck before the final mounting position is achieved.

Isostatic barrel mounting

When necessary, if for instance a different material for the instrument/barrel compared to the holder additional isostatic mounting features can be easily added to either the holder cover or (partly) within the instrument/barrel geometry.

5.

ANALYSES

CTE uncertainty

One remaining risk at operational temperatures is set by the uncertainty regarding the actual CTE values of the various materials and the manufacturing accuracy of the CTE pads. This results in deviations regarding the final dimensions of the various components at operational temperature which subsequently may result in additional stresses in the lens or play between the lens and holder.

Analyses on three specific lenses show that the effect of these CTE uncertainties is rather small.

This minor effect can be explained by the intrinsic flexibility of the holder and the CTE pads. Effectively the lens is mounted within a very stiff spring (the mount) which provides some flexibility to cope with the dimensional differences, though very small, caused by the CTE deviations.

Because the relative stiff nature of the holder the alignment problems and stresses, as described previously for kinematic spring mounts, are avoided and the lens remains accurately located.

Launch

During launch it is important that the optical element survives the extreme loads without alignment or other implications. However the stringent positional and surface form accuracies do not apply during launch.

At the ambient conditions during launch the lens is held accurately and play free by the CTE compensation mount. Finite Element Analysis show, as can be expected, a relative high Eigen frequency and low stresses.

60G static load

A 60 G gravity loading analysis is performed in the three main directions. Highest stresses occur with the gravity z loading where the holder shows the maximum stresses, specifically in the (weaker) cover, though within the safety limits of aluminium 6061.

The lens itself shows smaller overall stresses, although on first glance outside safe limits. Due to the simplifications within the analysis, surfaces touching each other are assumed rigidly connected, locally high stress points are found. These singularities can be ignored.

Modal Analysis

The analysis show a first frequency of 3105 Hz. Due to the FEA simplifications, rigid connections between components, this number will be lower in practise.

SPIFFI 2K Camera

The first cryogenic lens mounts with CTE compensation mount were developed for the SPectrograph for Infrared Faint Field Imaging (SPIFFI) 2K camera. SPIFFI is part of the SINFONI instrument on the ESO Very Large Telescope (VLT) in Chili. The SPIFFI instrument is located at the cassegrain focus and thus feels a varying gravity. Due to the wavelength range λ: l.1μm to 2.45μm the instrument operating temperature is 77K.

A new camera was designed to allow the use of an extended 2Kx2K pixel detector. The camera consists of 5 different lenses of various materials and sizes, see Figure 9. The camera is folded by means of a folding flat in order to fit the camera within the available space envelope.

Figure 9:

Optical design of the SPIFFI 2K camera consisting of 5 lenses and one folding flat. The lens triplet consists of the first three lenses while the last two lenses are joined in the doublet.

The lenses are combined in two separate groups, the triplet and the doublet. The triplet assembly consists of the first three lenses L1, L2 and L3 (see Figure 10), the triplet and doublet are preassembled and then located accurately within the 2K camera structure. The triplet assembly consists of the L1/L2 assembly onto which the L3 assembly can be mounted. The L1/L2 assembly holds, as the name already suggests, the L1 and L2 lenses. Both have their individual radial and axial supports but share one axial spring. The L3 assembly consists of the radial/axial support an individual axial spring.

Figure 10:

The main housings of the triplet are accurately lapped to the required dimensions. The axial springs push the lenses (transparent) to the axial supports (darkest colored rings) that are referenced to these outside (lapped) surfaces. The aluminium contact surfaces with the lenses are treated by Teflon-filled hard anodization to reduce internal friction. Within the housing the six radial supports are located (small light grey circles).

Figure 11:

X-shooter camera

Figure 12:

X-shooter corrector lens

Figure 13:

The X-shooter test prism base support plate. Clearly visible are the special thermal contact pads and their elaborate spring connections that ensure thermal contact with the prism at all times. On the pads the small circles of indium foil to improve thermal conductivity. The Kapton layer between prism and support pads reduces friction for maximum compensation during cool-down/warmup. CTE compensation pads are located on either side of the prism area.

Figure 14:

The X-shooter prism bench. The fully integrated prism with bench with the spring loaded top supports. The special CTE compensation pads (white) are seen at the side of the prism.

Figure 15:

X-shooter near IR Spectrograph

The triplet was the most stringent assembly as the aaccuracies are determined by the most demanding requirements, in this case the image position

X-shooter

7.

CONCLUSIONS

The ever increasing ambition in observational astronomy is a driving force behind the technical research and development for infrared astronomical instrumentation. The development of astronomical IR instruments is evolving strongly and every new instrument tends to be more sophisticated.

The pursuit of a-thermal and adjustments-free instrument designs is important because it provides the new technology for the next generation of infrared instrumentation. High accuracy, stress-free mounts for cryogenic optics is the key enabling technology for these state-of-the-art IR instruments.

In this paper we discussed mount designs for cryogenic transmission optics. These mounts have been applied successfully in ground based infrared instruments like Sinfoni-SPIFFI and X-shooter. It has been shown that with simple adaptations these high accuracy, stress free mounts are well suited to supply a safe launch environment for transmission optics. This opens the possibility to use adjustment-free, cryogenic infrared instrument designs in space.