JELT

Optical system

One strong candidate for an optical design is a Gregorian optics in which a secondary mirror can be placed in a position conjugate with the lower atmospheric turbulent layer. However, in this configuration, a telescope tube would become long and a dome would become large. As an alternative, a new optical system involving three aspherical surfaces is being considered.

1. Three aspherical surface optical system

Figure 1 shows a layout of an optical system in which three aspherical surfaces are used to produce a flat focal plane without aberrations across a field of view spanning 10 arcmin. Element mirrors comprising this optical system are listed in Table 2. In contrast to a three aspherical surface optical system based on the Korsch system used in Japan Astrometry Satellite Mission for Infrared Exploration (JASMINE), a magnification of the third aspherical mirror in the present design is close to 1, ensuring that a wide field of view is maintained.
The primary mirror is ellipsoidal with a diameter of 30m and focal ratio of F/1.5, and is composed of 1080 hexagonal mirror segments. A 4m hyperboloidal secondary mirror, a 4m flat folding tertiary mirror, and a fourth mirror produce an ordinary Nasmyth focus and a virtual image plane in the vertical direction. This virtual plane becomes the final image plane after the light is refocused by a fifth 4m ellipsoidal mirror.
As the first virtual image plane and the final image plane produced by a two-mirror system are effectively in the same position, bent light rays are superimposed. However, since the final image is an inverted mirror image of the virtual image, superposition of light rays can be avoided by using only half of the circular field of view. If a fourth mirror is added and the opposite side of the Nasmyth platform is used, two semicircular fields of view can be produced at one Nasmyth focus, making it possible to sample a full 20 arcmin field of view.

Figure. 1: Three aspherical surfaceoptical system

2. Imaging performance

A field of view occupies an effective area of approximately 1m in radius, allowing several instruments to be placed at each focus. A new focus can be established on the opposite Nasmyth platform, providing suitable locations for additional instruments.
As the beam is bifurcated at the center of the optical axis, a maximum 50% of the total light is cut off by vignetting. However, as this vignetting occurs only 1mm or less from the center of the field of view, which is beyond the diffraction limit, this optical system in practice can be regarded to be free of vignetting.
A spot diagram is shown in Fig. 2. Within a radius of 8 arcmin, the size of a spot is within the diffraction limit of visible light.

Figure. 2: Spot diagram of JELT three aspherical mirror optical system.
Note that spots out to a radius of 8 arcmin fall within a diffraction limit of 30m telescope.

3. Peripheral optical system

By introducing a lightweight truss structure, the telescope structure can be constructed as a natural extension of present 8m-class telescopes. However, as a diffraction size of a 30m telescope is 1/3.75 that of an 8m telescope, and with advanced adaptive optics providing ultra-high compensation, ultra-high resolution observations are expected to be possible. It should be noted, however, that requirements for atmospheric dispersion compensation in a 30m-class telescope will be an order of magnitude more demanding than those for the present 8m-class telescopes. Such requirements are being examined in parallel with other ongoing investigations.

Telescope Structure

1. Basic investigation of overall structure

A change in the direction of gravity for any part of the mechanical structure will result in structural deformation. To eliminate this deformation, it is necessary to increase stiffness of structural elements. However, increasing the stiffness often results in increased weight, which exacerbates deformation. Furthermore, as a telescope requires a precise drive system for tracking of celestial objects, limiting an increase in weight is critical to the design of the drive system.
Such deformation is generally dealt with by employing a Serrurier truss structure, which ensures that deflection of the primary mirror section and the secondary mirror section are matched under any orientation and degree of deformation. Serrurier truss is a structural arrangement of triangles that support the top ring and bottom mirror cell with respect to the center section of the telescope. A correction optics for primary focus or the secondary mirror are supported by tension applied by a spider (plate or rod-shaped structure) suspended from the top ring. The plane of the center of gravity of the top ring, including the secondary mirror, is connected to the center section by the Serrurier truss, which is coupled by hinged joints to ensure that external force does not exert a bending moment on incident beams. Thus, when the telescope is tilted, the top ring undergoes parallel shift in a lateral direction while retaining its perpendicular orientation with respect to the axis. The primary mirror is also connected to the center section by the Serrurier truss structure and undergoes parallel shift when tilted. The system is designed such that this shift matches that of the top ring.
The Serrurier truss structure effectively compensates for deformation that occurs when the telescope is tilted. However, to ensure that the weight of the top ring balances the weight of the entire structure, including the section that supports the primary mirror, the top ring must be made very heavy. Large telescopes built using the Serrurier truss system will therefore be very heavy, limiting an aperture size to around 10m.

Figure. 3: Telescope structure of JELT

Therefore, an alternative to the Serrurier truss structure is being investigated. The 45m radio telescope at Nobeyama Radio Observatory is a good example, in which the primary mirror is supported by a structure that resembles a truss structure, but the telescope itself is supported not by the center section but by an R-shaped rail structure attached to the bottom of the primary mirror. Therefore, the lateral displacement of the primary mirror that occurs when the telescope is tilted is practically negligible in this configuration. However, whereas a top ring of radio telescopes can be made very light, allowing lateral displacement to be neglected, the top ring including the secondary mirror in an optical/infrared telescope is very heavy, and lateral displacement must be considered. For JELT, it is proposed that the entire top ring be made heavier and that the secondary mirror itself be moved in the lateral direction. Using this system it is expected that the telescope will perform well without relying on the Serrurier truss structure. A conceptual design of a telescope with this support structure is shown in Fig. 3.

2. Eigenfrequencies

As a telescope becomes larger, its weight increases, shifting its eigenfrequency to lower values. A relationship between tube length and eigenfrequency is shown in Fig. 4.

Figure. 4: Relationship between telescope tube length and eigenfrequency

3. Elevation bearing section and horizontal rotation section

Selection of Bearing: Use of a roller bearing makes it possible to drive rotational motion of a large load smoothly with high accuracy. Roller bearings are therefore commonly employed for altitude and azimuthal axes of small telescopes. However, the largest roller bearings in use today are 7m in diameter, for use in tunnel excavation. It is practically infeasible to fabricate larger roller bearings, and bearings of such size will be very heavy. Hydrostatic bearings are therefore used for larger telescopes, such as Subaru telescope. For JELT project, a drive method based on hydrostatic bearings, similar to that used for Subaru telescope, is being considered.
Figure 6 shows the concept of the hydrostatic bearing [4]. An amount of gap (h) of support provided by a hydrostatic bearing is given by

where Q is an oil flow rate, eta is dynamic viscosity of oil, sigma p is a loss of pressure in the gap, and b is a gap width ( =2(a1+a2) ). In the OWL project, a 100m telescope proposed by European Southern Observatory (ESO), a bogie car has been proposed instead of a hydrostatic bearing. Advantages of a bogie car are twofold: (1) the length of the bogie car can be changed dynamically to compensate for any lack of flatness of the base rail, and (2) the bogie car itself provides the drive power, whereas a separate motor is required to drive conventional hydrostatic bearings. However, as the bogie car system requires very complex technology, and has never been implemented in practice, it is not being considered for the JELT concept.

Figure. 5: Conceptual diagram of a hydrostatic bearing

4. Overall structure of JELT

Required specifications and design parameters for JELT structure are listed in Table 3. Examples of the structure design for JELT are detailed below.

Table 3: Required specifications and design parameters for JELT structure

Item	Required value	Design value
Effective aperture	20 m or more (nominal 30 m)	30 m
Segment diameter	-	diagonal 1 m (hexagon) or fan-shaped
Optical system	Gregorian (or Cassegrain)	Three aspherical mirrors, 2 Nasmyth foci
Instruments	Camera, Spectrograph (1 each optical and infrared)	two Nasmyth foci
Field of view (non-block field of view)	5 arcmin or more	5 arcmin
Field of view (adaptive optics field of view)	1 arcmin or more at 2 μm	1 arcmin at 2 μm
Wavelength coverage	0.39 - 5 μm	0.39 - 5 μm
Drive range	Altitude: 20 - 80 deg	Altitude: 20 - 80 deg (range within which performance is guaranteed)

Elevation bearing section: Highly rigid horseshoes are arranged on both sides, suppressing deformation of the mirror cells by gravity while maintaining high rigidity around the elevation axis. Two points on left and right sides of the horseshoe cylindrical surface (four points total) are supported in the radial direction by hydrostatic bearing pads. The thrust direction is constrained by clamping one of the horseshoes using two hydrostatic bearing pads. This kind of support system constrains 5 degrees of freedom, providing support without excessive constraint. A weight is to be reduced by changing a weight distribution of the horseshoe and mirror cell.

Primary mirror cell: The most important issue to be resolved in designing the structure of the primary mirror cell is a number of mirror support points. Three support points are required for each 1m-diameter hexagonal mirror, giving a total of 3000 support points for 1000 mirrors. One conceivable way to reduce the number of support points is to combine a number of mirrors into a cluster, as has been proposed for TMT project. If a cluster structure is employed, assuming 19 mirrors per cluster, a total of 50 clusters will be still necessary; corresponding to 150 support points given 3 points per cluster. A question then becomes how to support these 150 points. It is difficult to obtain high rigidity using a hexagonal support pattern proposed for TMT. It is therefore proposed in the present case that the primary mirror be placed in square layout, which is expected to provide high rigidity with a smaller number of cells. One of issues that remains to be investigated is an optimum structure to connect the square structural members of the grid to the horseshoe.

Telescope tube: Rather than using wires to apply tension to the upper telescope tube as is generally the case, a self-supporting structure will be employed. In the current proposal, the spider will have an elliptical cross-section, allowing vignetting to be minimized while maximizing rigidity.

Horizontal rotation section: The horizontal rotation section is supported by six thrust hydrostatic bearing pads, with a radial hydrostatic bearing pad in their center constraining the horizontal rotation section within the plane. The thrust hydrostatic bearing pads are designed to slide on a top surface of a rail attached to its base. Magnets to read encoders will be mounted on the outside or inside of the rail, with coil assemblies mounted on the telescope side. A tape encoder to detect rotation speed will be installed on the rail side. To compensate for changes in gap distance between an encoder head and the tape due to effects such as differences in thermal expansion, a gap-maintaining mechanism is being considered. Furthermore, arrangement of drive points near detection points (to achieve co-location) to achieve precise control is being considered.

Yoke Structure: The same yoke structure as used for Subaru telescope is also to be applied in JELT. The yoke structure, which supports the Nasmyth table and an elevation shaft, consists of two groups of tripod trusses set on hexatruss structure on the horizontal rotation rail. This structure limits effects on the elevation shaft thrust pressure by movement of the six nodal points following any swell in the horizontal rotation rail, which can result in undulation. It is also possible to suppress fluctuations in pointing of the telescope. The features of this type of structure are listed below.

1.	High rigidity: The load is transmitted directly from the point at which force is applied to the point where it acts. Forces that support the telescope structure are transmitted directly and linearly to the hydrostatic bearing pads. Rigidity of an oil film in a hydrostatic bearing is very high, and the structure makes effective use of this high rigidity.
2.	The mechanism consists of a minimum number of parts, facilitating assembly and improving cost performance.
3.	The effect of swell in the azimuth rotation rail (which may be present due to error in manufacturing the system and/or due to change in the system over time) is effectively compensated by this structure.

5. Investigation of eigenfrequencies and oscillation modes using a simplified model

Using an analysis model shown in Fig.6, eigenvalue and static solution analyses (for the case in which a static acceleration of 1G acts on) were performed to determine eigenfrequencies, oscillation modes and deformation due to the telescope's own weight as a check of appropriateness of the basic structure.

Figure. 6: Model for eigenfrequency analysis

(1). Deformation due to gravity

Deformation due to gravity, under a static acceleration of 1G, occurs at the Nasmyth table. The maximum load is applied to the yoke legs, under the weight of the Nasmyth table.

(2) Eigenfrequencies and oscillation modes

The oscillation modes of the principal members are as follows.

Oscillation in the direction parallel to the table.

Oscillation in the direction parallel to the tube.

Rotational oscillation around the azimuth(Az) axis.

Rotational oscillation around the elevation(EL) axis.
The primary oscillation mode is forward translation of the table, which occurs with an eigenfrequency of 2.3Hz. The lowest eigenfrequency of tube oscillation is 3.2Hz. The eigenfrequency of rotational oscillation around the azimuth axis is 4.4Hz, and that of rotational oscillation around the elevation axis 4.5Hz

6. Secondary mirror support mechanism

Figure. 7: Trade-offs in secondary mirror support mechanisms

Figure 7 illustrates trade-offs involved in possible secondary mirror support mechanisms. A support system shown on the left side of the figure is Schwesinger support system used in the primary mirror of VLT, consisting of a rear surface axial support and an outer circumference lateral support. This system requires a lateral support mechanism in the outer circumference of a secondary mirror, which introduces a source of heat radiation in infrared wavelength. A mechanism shown in the center of the figure is a support mechanism used for the primary mirror of Subaru telescope. In this system, holes in the mirror are used for lateral support at a center in thickness direction. This system requires holes to be made in the mirror, which affects a fabrication process and a cost of the system.
In a system shown on the right of the figure, two support mechanisms are adopted to provide support in a lateral direction, positioned such that an intersection of the support force lines lies at the center of the mirror in thickness direction. This system does not introduce unnecessary infrared heat radiation, and also does not require holes to be made in the mirror. This system is therefore the most desirable at present.

Figure. 8: Secondary mirror support mechanism

Figure 8 shows specific proposals for secondary mirror support mechanisms. The proposal involves 45 passive support points, 9 of which are for the tip-tilt drive, with 3 fitted with sensors to detect rigid positions of the secondary mirror. A mirror cell is positioned with 6 degrees of freedom by 6 jacks. Reaction force compensators are provided on top of the mirror cell, on the opposite side from the secondary mirror, to cancel the reaction force from the secondary mirror. The features of the secondary mirror support mechanism are listed below.

Multi-point drive: By arranging an actuator in anti-nodes of oscillation mode, an effect of the lower-order oscillation modes can be eliminated. This means that the eigenfrequency of the secondary mirror is effectively increased, allowing control bandwidth to be increased substantially.
System without a support structure around the secondary mirror: No thermal noise.
Large thrust with compact actuator: (same type as the tip-tilt drive actuator used in Subaru telescope)

Maximum thrust: 500N

Dimension: 90mm diameter x 60mm height

Table. 4: Comparison of requirements for tip-tilt drive actuators

A comparison of requirements for the tip-tilt drive actuators in JELT and Subaru telescope, and numbers of actuators required, are given in Table 4. If chopping frequency is set at around 1Hz, a total of 6 - 9 actuators are expected to be sufficient.

7. Drive mechanism

Features of the proposed drive system are as follows.

Direct-drive (DD) system.

- Lower cost than a gear-drive system.

Smooth drive.

- The drive system of Subaru telescope, which has already performed successfully, can be applied.

- High resolution: Encoder resolution is 0.0005".

Features of the DD system are as follows.

General features of the DD system

- Zero friction

- High rigidity

The DD features of Subaru telescope are inherited without alteration.

- Low heat production, high torque

A praseodymium magnet is used to obtain high magnetic flux.

Low torque ripple

- Optimized skew of the magnet.

- Coil layout for minimum torque ripple.

For reference, comparison of a DD system, a gear-drive system and a friction-drive system is given in Fig. 37. In the gear-drive system, it is necessary to continuously apply torque via two gears in order to prevent backlash. Thus, large fluctuations in friction occur when the drive system is operating at low speed, particularly upon start-up. Error in gear shape becomes a source of external disturbance. Similarly, in the friction-drive system, friction fluctuation is also generated due to pressure applied between rollers. In the case of JELT, the roller would be too large to be made in one piece, and therefore must be divided into several pieces. However, the resulting seams may become a source of external torque disturbances. The DD system is completely free from such external torque disturbances.
For gear- and friction-drive systems, rigidity of the drive system decreases by an amount depending on rigidity of the mechanism itself, whereas the DD system has no mechanism other than motors, resulting in absolutely no loss of rigidity.
Considering the features of these systems, the DD system allows high tracking accuracy to be achieved relatively easily. Furthermore, a cost of fabricating gears, as required for the gear-drive system, is much higher per unit length than a cost of magnets in the DD system, a cost of which is comparable to the friction-drive system. Accordingly, based on an overall evaluation, the DD system has been adopted for JELT. The specifications of Subaru telescope DD system
are listed in Table 6.
Performance required for the DD system is given below, in reference to Technical Report No. 4 of TMT project.

1.	Slewing requirement 360deg Azimuth, 65deg Elevation / 5min
2.	1" on sky 1s
3.	10" on sky 3s
4.	100" on sky 10s
5.	1000" on sky 30s
6.	Maximum speed and maximum acceleration : Maximum drive speed (using a rough precision encoder) : 1.2deg/s Maximum tracking speed (using a high precision encoder) : 0.2deg/s Maximum acceleration : 0.05deg/s²
7.	Maximum torque : Azimuth axis : 1x10⁵ Nm ( ˜10 ton m) Elevation axis : 2x10⁵ Nm ( ˜20 ton m)
8.	Inertia : Azimuth axis : 1x10⁸ kg m² Elevation axis : 2x10⁵ kg m²

Specifications of DD system proposed for JELT, based on the DD system in Subaru telescope, are compared with these required specifications in Table 5.

Table. 5: Specifications of DD system required for JELT

8. Tracking control system

The tracking control system developed for Subaru telescope will be used for JELT. The features of the control system are listed below.

1.	High-order filtering of control devices to obtain a large control bandwidth
2.	Compensation for structural resonance and anti-resonance
3.	Speed estimation unit inside a control unit : At normal speeds, pulses from encoders are counted to estimate the speed. At very low speed (very low frequency), the speed is estimated by an observer's speed estimation unit.
4.	Total system analysis to degenerate structural modal order
5.	Structural design based on frequency response function
6.	Incorporation of predictive control Parallel actions including converging acceleration during slewing, converging acceleration during guiding, slewing, ADC driving, ImR/InR driving and guide probe driving are made possible.

In addition, following additional functions must be incorporated :

1.	A high-order compensator to suppress external disturbances such as those due to wind (H1 control)
2.	Automatic compensation for directional error due to swell in the horizontal rotation rail

Figure. 9: Block diagram of tracking control system for JELT

Adaptive optics system

Enclosure

Control software and computer system