1. MECHANICAL DESIGN DESCRIPTION
NGS2-NGS2-SDN-0004
Revision: 1.0
Document Approval
Name Date
Prepared by: R Gardhouse 17 Mar 2015
Checked by: F Rigault 17 Mar 2015
Approved by: N Paulin 17 Mar 2015
Revision Record
Rev. Author Description Approval & Date
1.0 R Gardhouse Initial release 17 Mar 2015
Research School of Astronomy and Astrophysics
ANU College of Physical and Mathematical Sciences
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1 Acronyms and definitions
2 Purpose
The purpose of this document is to describe the mechanical layout of the proposed new tip/tilt sensor for
the Canopus instrument.
3 Applicable Documents
1. ICD-G0013 revision B Gemini Environmental Requirements
2. 20141111 Optical data sheet NGS2
3. GeMS Mech Calc + COGs
4. Precision Mounting of Optical Components, Yoder
5. Introduction to Opto-Mechanical Design Vukabratovich
6. Principles of Exact Constraint Mechanical Design, Blanding
4 Description
4.1 Layout
The NGS2 tip/tilt sensor sits in a prescribed volume slightly larger than the original sensor (Figure 1).
The extra volume was achieved by moving the ADC closer to the Rugate filter.
The major components of the sensor system (Figure 1) are:
1. Entrance fold mirror pairs
2. Tube lens assembly
3. 3rd
fold mirror
4. Objective lens/detector assembly
Although the system is still in design, major design strategies are understood.
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Figure 1 Part Layout
The optical tolerances were calculated at 20ºC as plus/minus values. As the operational range is 20ºC to -
5ºC, the lens sag and spacing were biased so that the system is at nominal position at the median
operating temp of 9ºC.
Inter lens spacing tolerances are at the most ±100μm. To maintain their nominal position, the second fold
mirror, the tube lens and the third fold mirror are pinned to the base under an appropriate optical surface.
Not only does this prevent inadvertent Z shifts of the optics, it also allows the optic assemblies to be
removed and replaced with only residual alignment required.
As a general philosophy, the design is physics driven and based on the alignment method. As such, all
clamping and nesting forces are calculated to provide gravity invariance and compressive loads of 7MPa
±1MPa on the lenses. These calculations were done for the storage temperature range; the optics should
remain aligned and undamaged.
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The coordinate description in this document of the optics is the standard ISO right hand system with the
beam moving +Z and +Y is up from the table. This may or may not be equivalent to the official Canopus
mechanical and/or prescription coordinates.
Current mass of the system is 30kg of which 10kg are the optics, detector and stge. Although this is above
the desired weight of 25kg, it is expected that once the design is finished, weight reduction will bring the
mass within specification.
4.2 Fold Mirror Pair
The fold mirror pair are mechanically identical to each other. Each mirror (Figure 2) is first surface
mounted to a pair of steel balls on its lower surface and a ball end screw at its top. A wavey washer
provides the force to hold the mirror against these points under any gravity vector. This design allows for
possible replacement of the mirror in situ with minor realignment.
This assembly is free to rotate around a dowel in the base with center of the dowel is located along the
mirrors first surface. The same ball end screw as used for the top is used for this adjustment. To prevent
creep from the screw after the adjustment is locked, the adjuster system is designed to be easily removed.
The commercial 254 tpi screw provides a per degree screw rotation movement of 0.4asec for the top and
0.8 asec for the bottom. The total range of movement is ±0.5º for the top and ±04º for the bottom.
Figure 2 Fold Mirror
Although both fold mirrors are identical assemblies and have a dowel hole to pin the assembly to the base
plate along the mirror’s first surface, only the second fold mirror will be constrained to the base. The first
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fold mirror has nominal translation movement to help position the f16 focus at its nominal position in
front of the second fold mirror
4.3 Tube Assembly
The pair of lenses that make up the tube assembly (Figure 3) each have an appropriate flat for mounting:
surface 1 on the first tube lens and surface 2 on the second tube lens. Each lens is centered in the tube by
a wavey spring acting on a tangential contact spacer located within the tube by a location transition fit.
The lens diameters are then undersized preventing radial compressive stress from the tube during
temperature changes.
As the lenses are retained against the spring force by a bolted clamp ring, it is possible to replace either
lens in situ as long as the lens is made to the same tolerances.
The tube assembly is then held in its V mount by a spring loaded clamp. The tube is pinned to its base by
a dowel whose center line lies along the sag of first lens’s second surface. The V mount is located to the
base by a standard pin and slot where the pin is matched to the pin for the tube. Mounting in this manner
maintains the nominal inter surface distances repeatable during temperature changes as well as allowing
the tube to be removed and replaced with residual alignment required.
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Figure 3 Tube Assembly
4.4 Third Fold Mirror
Currently the third fold mirror design is in flux; its tolerance stack up is not currently calculated if it can
be a simple fixed position mount or requires a more complicated adjustable solution. This is due to the
alignment tolerance between the tube lenses and the objective/detector assembly of 20μ decenter and tilt
of 20 asec over 268.66mm between the surfaces.
The third fold mirror bends the optic path between the tube lenses and the objective/detector assembly.
The tube lenses are fixed & pinned to the base while the objective/detector assembly requires translation
in X, Y and Z as well as rotation in X & Y to align it to the nominal optics path.
Once the design of these mounts are finalized and their tolerance stack up is calculated, it can then be
determined if the third fold mirror needs to be adjusted to aid in alignment of the two assemblies.
4.5 Objective/Detector Assembly
4.5.1 Overview
The Objective/detector assembly contains the objective lens assembly and the detector (Figure 4). The
assembly is aligned to itself and then aligned to the optic path exiting the tube assembly.
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Figure 4 Objectiv/Detector Assembly
The required assembly to allow adjustment in translation in X, Y, and Z as well as rotation in X and Y has
yet to be designed.
4.5.2 Detector
The detector is a commercial unit with a standard C mount interface (Figure 5). The detector is attached
to its mount using 3 of its 4 mounting holes allowing it to be removed in situ. As the C mount does not
have any alignment features, the top fastener is countersunk to reduce the number of degrees of freedom
during installation. Because of this, the camera will require alignment to some unknown level every time
it is installed.
The detector produces 35W of heat from the chip heat sink and a further 35W from the chip controller
PCB. To reduce the conduction of heat from the camera to the rest of the instrument, a 0.3mm thick G10
sheet lies between the detector and its mount provides isolation.
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Figure 5 Detector
4.5.3 Objective Assembly
The objective assembly comprises the objective tube and lenses, a 2 part alignment base and a precision
stage (Figure 6). A spring clamp holds the objective tube to the upper half of its base.
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Figure 6 Objective Assembly
Of the 6 lenses in the objective assembly 4 are CaF2 including the CaF2 lens in the doublet (Figure 7).
To prevent these lenses from mechanical damage, the spacers between the lenses have a tangential
interface which provides the lowest contact stress. The 2 exceptions to this is surface 1 of objective 2
and surface 2 of objective 6. As with the tube assembly, the tangential spacers are located within the
tube by a location transition fit.
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Figure 7 Objective Interior
The wavey spring loading the complete lens stack is in tension to provide 7MPa ±1MPa. As glass fails in
a brittle fashion, this figure is generally accepted as the value to prevent failure. The line contact of the
spacers either side of a particular lens are aligned to reduce any bending moment the lens may experience.
A clamp ring is bolted to the front of the tube to hold the complete assembly together.
To correct for residual coma and spherical aberrations specific lenses are adjustable. Objective 4 is moved
in X & Y by a pair of M4 ball end screws against a spring plunger acting on the lens radius at equidistant
spacing.
Because the optic beam is being reduced from f16 to f1.1, objective 6 is 2.6mm from the C mount face.
As the stage has ±10mm of motion, there is a possibility of damaging objective 6 by running it into the
detector C mount face. The prevention of this is 3 fold:
1. A plastic bump stop will be bolted to the objective/detector mount to prevent
the objective tube from coming closer that 1.5mm to the C mount face.
2. The tube for holding the lenses is 1mm longer than objective 6 so that the
tube will hit the C mount before the lens
3. A pair of adjustable limit switches indicate when the stage has overrun its
normal limits.
A precision stage moves the optic assembly by 1μm/motor step providing finer resolution than the 5μm
specification. As this movement is in full step increments, the fine pitch of the lead screw prevents
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unpowered motion of the stage. There is no motor heat to manage nor the additional complexity of a
brake.
4.5.4 Objective Tube Alignment to Stage
Between the lens tube and the stage is a 2 part mount. Both parts of the stage and the lens tube are co-
located by a dowel pin whose center line intersects the sag of surface 1 of objective 6. A spring loaded
clamp holds the lens tube into the upper mount’s V groove. The lens tube can then be removed and
replace with residual alignment required.
Misalignment of the lens tube to the stage results in a translation of the optics as the stage moves reducing
the assembly’s available alignment budget. To correct this misalignment the upper stage/lens tube can be
rotated in X and Y. Reducing run out in Y is achieved by a shim stack in the front of the mount which
will allow adjustment to within 15μm of parallel over the total run of the stage. This becomes a Y
translation of no more than 0.4μm over the expected focus range.
Removing X run out is achieved by rotating the upper assembly around the rear dowel pin using gauge
blocks for reference allowing adjustment to the 5μm level becoming an X translation of no more than
0.1μm over the expected focus range
4.5.5 Objective/Detector Alignment to Tube Assembly
The objective/detector assembly is aligned to the optic path by translations in X, Y and Z as well as
rotations along the X and Y axis. The as yet to be designed base will sit on two shim stacks.
Rotations in the X axis are accomplished by changing the shim stack in the front of the mount allowing
adjustment to the 15μm level. To achieve the required 0.3amin alignment will require an arm of 175mm
which will fit in the current space envelope.
Rotations in the Y axis will be achieved by rotating the complete assembly. Using gauge blocks for
reference allows adjustment to the 5μm level. Based on the above arm length of 175mm this provides
alignment to 0.1armin.
Translations in the X and Z will be accomplished by moving the mount with respect to the base plate
using gauge blocks for reference allowing adjustment to the 5μm level.
Translations in the Y axis will be accomplished by changing both the front and rear shim stacks between
the mount and the base plate providing height adjustment to within 15μm.
4.6 Thermal Management
As stated previously, the detector produces 35W of heat from the chip and an addition 35w of heat from
the controller PCB for a total of 70W. To reduce thermal bloom, the concept is to enclose the detector in a
box (Figure 8) lined with a thin high performance insulation such as Thremaflect heat shield cloth from
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Heatshield Products. Although this strategy protects the instrument form thermal bloom it poses several
challenges:
1. Coolant needs to be supplied to the chip’s heat sink
2. Cooling air needs to be supplied to the controller PCB fans
3. Hot exhaust air needs to be routed into a controlled environment and not the
telescope enclosure
4. The box requires a pass through for the camera services
5. The box needs to be removable to allow for camera replacement in situ
Fortunately, the RSAA has a 3D printer that allows as much flexibility in the box’s design as required to
solve these design constraints.
Figure 8 Proposed Detector Enclosure
The heat form the chip is removed by a liquid cooled heat sink. The heat sink is plumbed with MC1502
from Colder Product/Wainbee which are neither dripless nor the Gemini standard Swagelok B-QC6-D1-
600. To solve this, it is proposed a pair of right angle connectors within the box are connected by an
appropriate length of hose terminating in Gemini standard connectors which then connect to the coolant
system. To prevent spillage the system should be designed so that the hose stays with the detector when
the box is removed.
The box needs to have a fan to blow cool instrument air into the detector. Discussions are underway to
determine size, type, suitability and location of the fan. It is anticipated that the fan will be mounted to the
box with a vibration isolating system such as Richco’s rubber screws. Baffling inside the box can be
printed to optimize air flow to the controller PCB.
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To both exhaust the hot air and provide detector services routing, the box has an opening facing the hinge
end of the electronic enclosure (Figure 9). Flexible hose running from the outlet of the box to a flange
mounted on the rear of the instrument light baffling provide route for both the camera services and the hot
exhaust air.
Figure 9 Detector Services Access
As it is unacceptable to exhaust 35W of hot air into the telescope enclosure it is proposed that a flexible
hose connects the flange on the instrument baffling with the electronic enclosure. This would provide a
protected path for the camera services as well as ducting the exhaust air into the cooled electronics
enclosure.
4.7 Gravity Flexure and Vibration
The lack of correlation between actual and FEA predicted flexure and vibration response for kinematic
designs is well known. This is especially true for the significantly reduce functionality of FEA embedded
in modeling software.
In that light, instead of presenting misleading FEA results, the plan is to use best practices to mitigate
flexure and vibration in the design then physically test both regimes to ensure the instrument meets its
requirements. The RSAA has both a rotary stand and a vibration table to ensure actual on telescope
performance of the instrument. In the unlikely event that some portion of the instrument does not perform
as expected, a remedy can be implemented before shipment to Gemini.
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Should Gemini have a 3 axis vibration profile where the instrument will be placed on an operational
Canopus, the instrument can tested in this regime to further guarantee as delivered performance.
4.8 Installation
The best strategy for successfully installing the sensor is to align the optics to the nominal f16 focus. then
insert the complete assembling onto the bench then align the sensor f16 focus to the actual Canopus f16
focus. As can be seen in Figure 10 this may not be possible do to the structures supporting both the
optical bench and the electronic enclosure.
Figure 10 Installation Area
To determine how much of the sensor would be disassembled to place it on the optical bench, a full sized
card board mockup of the space limiting element of the instrument as well as the sensor were created
(Figure 11).
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Figure 11 Cardboard Mockup
From this mockup, it is currently believed that the objective/detector assembly needs to be removed to
install the remaining pre-aligned components. The mounting features between this assembly and the base
can be designed to accommodate removal and reinstallation of this component with residual realignment
required.
