SALT-3197AE0001 Cryostat Document
Southern African Large Telescope
Prime Focus Imaging Spectrograph
SAAO Detector Subsystem
SALT-3197AE0001: Cryostat Document
SAAO PFIS Detector Subsystem Team:
James O’Connor
Etienne Bauermeister
Dave Carter
Geoff Evans
Willie Koorts
Darragh O’Donoghue
Faranah Osman
Stan van der Merwe
Issue 2.4
28 February 2003
1
SALT-3197AE0001 Cryostat Document
2
Issue History
Number And File Name
SALT-3197AE0001 Cryostat
Issue 2.0.doc
SALT-3197AE0001 Cryostat
Issue 2.4.doc
Person
Issue
2.0
Date
08 Nov 2002
Change History
First pre-PFIS CDR draft
2.4
28 Feb 2003
Truly the final CDR draft!
Table of Contents
1
Scope............................................................................................................................................3
2
Overview......................................................................................................................................3
3
Requirements ...............................................................................................................................3
4
3.1
Mosaicing ............................................................................................................................3
3.2
Thermal ................................................................................................................................4
3.3
Vacuum ................................................................................................................................4
Final Design .................................................................................................................................4
4.1
Structure ...............................................................................................................................4
4.2
Field Lens ............................................................................................................................9
4.3
Thermal Control System ....................................................................................................10
4.4
Detector Assembly.............................................................................................................14
4.5
Detector Interface ..............................................................................................................16
4.6
Vacuum System .................................................................................................................16
4.7
Electrical Connections .......................................................................................................17
5
Mass Properties ..........................................................................................................................17
6
Fabrication .................................................................................................................................18
7
6.1
Components .......................................................................................................................18
6.2
COTS items .......................................................................................................................18
6.3
Cryostat body .....................................................................................................................18
6.4
Surface finishes ..................................................................................................................18
Risks ..........................................................................................................................................18
7.1
Thermal ..............................................................................................................................18
7.2
Vacuum ..............................................................................................................................18
7.3
Mechanical damage ...........................................................................................................18
7.4
Risk Management ..............................................................................................................18
SALT-3197AE0001 Cryostat Document
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3
Scope
This document describes the mechanical design rationale of the Final Design for the Cryostat as
required for the Prime Focus Imaging Spectrograph (PFIS) Instrument.
NOTE: This is the final design configuration and the only entities that might change are the
mounting arrangement of the field lens to the vacuum wall and the internals of the plug box
containing the connectors and pre-amplifier boards.
2
Overview
In this instrument the Cryostat has its function of housing the detector modules and the carrier for
the field lens (which also forms the window of the Cryostat).
3
Requirements
3.1 Mosaicing
3.1.1
Requirements
The individual CCD’s have the following specifications on the focal plane:
Surface undulation 10m Peak to valley.
The Optical Alignment requirements are as follows:
Tip/tilt of average focal plane: 20 m of one long edge compared to the other and
10 μm on the short edges.
Deviation in focal plane orientation between CCD’s: 1 arcmin.
Alignment of pixel columns and rows: 6 pixels in 4096 (5 arcmin)
3.1.2
Options
It has been decided to do the mosaicing at SAAO for the following reasons:
a.
SAAO will have to have an ability to maintain the detectors for SALT and as such needs to
develop a mosaicing facility.
b.
Experience gained on SALTICAM will minimize the risks on PFIS mosaicing. (The only
reasons for having the SALTICAM mosaicing done at E2V were time and manpower
constraints.)
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3.2 Thermal
The temperature requirements of the CCD’s are as follows:
Control temperature:
160 K
Peak to valley fluctuation:
less than 0.5 K
Cool down time:
less than 4.5 hours
3.3 Vacuum
The vacuum in the cryostat will have to be maintained to less than 10-4 Pa.
4
Final Design
4.1 Structure
The structure of the cryostat consists of 3 main parts, i.e. the front plate, the main body and the
interface plate for mounting the cryostat to the camera flange.
4.1.1
Front plate
The front plate (Fig. 1) has the functions of:
a.
Carrying and constraining the field lens in place.
b.
Serving as mounting base for the detector assembly.
c.
Attaching the Cryostat to the interface plate.
The window is carried in a stainless steel clamp. Silicon RTV is used to bond the lens to the clamp
in a similar fashion as lenses are bonded into cells. The clamp will pull the lens assembly down
onto an O-ring to form the vacuum seal (fig 2). Locating pins will determine the centering of the
lens to the detector optical center. Tip/tilt adjustments will be possible with shims between the lens
clamp and the front plate of the Cryostat.
The detector assembly bolts to inside of the front plate in such a manner that the detector center
will be on axis with the center of the field lens. The inside face of the plate will also serve as the
reference face to which the focal plane will be positioned. Fig 3 depicts the inside of the complete
front plate assembly.
The front plate assembly also serves as attachment points (Fig 4) for the Cryostat to the interface
flange (Fig 5). This design will enable removal and replacement of the Cryostat without upsetting
the optical alignment.
SALT-3197AE0001 Cryostat Document
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Heat Shield
Cold Braid Attachment
Tip/Tilt Adjuster
Guide Bracket
Front Plate
Lens Clamp
Figure 1: COMPLETE FRONT PLATE ASSEMBLY
Front Plate
O-Ring
Shim Gap
Lens Clamp
Field Lens
RTV
Figure 2: SCHEMATIC OF WINDOW AND CLAMP
SALT-3197AE0001 Cryostat Document
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Flexure
Front Plate
Cold Plate
Connection Board
Pedestal
Heat Shield
(Top Shield)
Flexure Clamp
Thermal Manifold
Pedestal
Pedestal
(Side Shield)
Figure 3: FRONT PLATE ASSEMBLY (INTERNAL)
Front Plate
Guide Bracket
Tip/Tilt Adjuster
Guide Shims
Hold Down Flange
Figure 4: ATTACHMENT POINTS
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Guide Pedestal
Interface Flange
Hold Down Bolts
Figure 5: INTERFACE FLANGE
4.1.2
Main body
The main body has the functions of:
a.
Providing space for the detector package.
b.
Carrying the cold end.
c.
Carrying the plug box containing the connectors and pre-amplifier.
d.
Mounting of the Ion pump and vacuum valve.
Fig. 6 shows an exploded view of the cryostat and Fig. 7 and Fig. 8 show cross sectional assembly
drawings.
SALT-3197AE0001 Cryostat Document
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Vacuum Pump Assembly
Vacuum Valve
Cold End Assembly
Main Body
Plugs
End Cap
Plug Box
Front Plate Assembly
Interface Assembly
Figure 6: EXPLODED VIEW
Figure 7: CROSS SECTION 1
SALT-3197AE0001 Cryostat Document
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Figure 8: CROSS SECTION 2
4.1.3
Interface Flange
Fig. 5 depicts the interface flange. This flange is permanently attached to the camera flange, but
has the facility for rotational adjustment during installation.
The cryostat is connected to the interface plate in such a manner that it can be adjusted for tip/tilt,
but no rotation will be possible. This arrangement will facilitate removal/replacement of the
cryostat without disturbing the optical alignment.
4.1.4
Structural Analysis
The FEA analysis for Salticam indicated that no undue flexures or stresses were present in the thin
area (5 mm wall thickness) of the lid. The lid on the PFIS Cryostat is substantially thicker (10 mm
minimum) with a wall/thickness of 17 mm where the field lens is located. The remainder of the
structure is a clone of the Salticam design. For these reasons it was decided not to do a FEA
analysis for PFIS.
4.2 Field Lens
The field lens is used as window to the cryostat as well. A flow of dry air is provided to prevent
the front surface form frosting up. Shims will facilitate limited tip/tilt adjustment, but decenter and
rotation will be locked by the mounting arrangement. Fig. 9 depicts the mounting arrangement.
SALT-3197AE0001 Cryostat Document
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Front Plate
Field Lens
Lens Bracket
Locating Pin
Cap Screw
Figure 9: FIELD LENS MOUNTING
4.3 Thermal Control System
A great deal of time and effort was devoted to the design of SALTICAM to minimizing the heat
load as well as ensuring a good temperature distribution across the detector surface. This
experience and knowledge was applied to the design of the PFIS Cryostat
The cold path will consist of the following elements (Fig. 10):
a.
Heater elements.
b.
Temperature sensors.
c.
Thermal manifold.
d.
Cold braid.
e.
Junction.
f.
Cold end.
g.
Trap.
h.
Compressor/heat exchanger.
SALT-3197AE0001 Cryostat Document
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Figure 10: SCHEMATIC OF THERMAL SYSTEM
The thermal manifold is designed such that the most even temperature distribution through the
CCD Invar package is ensured. This is achieved through contacting on the cold plate and with cold
blocks directly onto the underside of the package in appropriate places. (Fig. 11) Heater elements
and temperature sensors are attached to the manifold such that a temperature fluctuation of less than
0.5 K can be achieved. A cold braid will lead from the manifold to the junction that is integral to
the cryopump end (Fig. 12).
The final adjustment to reach the required stabilization temperature of 160K on the focal plane will
be made by reducing or increasing the conductivity of this cold braid. The arrangement is such that
it will facilitate easy connection/disconnection upon assembly/disassembly of the cryostat.
SALT-3197AE0001 Cryostat Document
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Cold Plate
Top Clamp Block
Thermal Spider
Bottom Clamp Block
CCD
Figure 11: COLD BLOCKS ONTO CCD
SALT-3197AE0001 Cryostat Document
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Cryotiger
Heater Recesses
Cold Braid Attachment
Thermal Manifold
Cold Braid Attachment
Figure 12: COLD END ASSEMBLY
4.3.1
Heat Load
By polishing the inside of the cryostat walls and installing a gold plated heat shield assembly the
thermal loading was substantially reduced.
Heat load:
Radiative load on surface other than focal plate
90 mW
Heat generated by amplifiers on chips
150 mW
Heat generated by pixels on chips (during readout)
750 mW
Radiative heat on detector surface
2400 mW
Conductance through mechanical connectors
150 mW
Conductance through electrical wiring
120 mW
Total heat load
3660 mW
 3,7 W
SALT-3197AE0001 Cryostat Document
4.3.2
14
Temperature Distribution
Following the Salticam analysis it can be said the temperature distribution in the manifolds and
surface of the chips are well within acceptable limits.
4.4 Detector Assembly
The assembly consists of the following:
3 x CCD’s
1 x cold plate
4 x G10 flexures
4 x aluminum pedestals
1 x heat shield assembly
Fasteners and clamp plates
Pedestal
Flexure
Flexure Clamp
Cold Plate
Figure 13: FLEXURE MOUNTING OF COLD PLATE TO PEDESTALS
SALT-3197AE0001 Cryostat Document
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Four G10 flexures carry the CCD/cold plate assembly (Fig. 13). This material has good flex as
well as thermal properties. Aluminium pedestals are used to mount the flexures to the front plate.
A set of thermal shields gold coated for minimum radiation encapsulate the assembly except for the
focal plane and where the cold plate attachment points protrude through (Fig. 14).
Top Shield
Side Shield
Pedestal
Figure 14: THERMAL SHIELDS
4.4.1
Mounting Method
a.
The CCD’s are mounted on an Invar cold plate 6 mm thick. The standard Marconi
method of attachment and alignment is used.
b.
The focal planes of the CCD’s will be aligned to a dedicated bolt on bracket to give the
zero reference for the planes. This bracket is then utilized to mount the detector
assembly to the front plate of the cryostat. The face of the front plate serves as the
reference for the offset of the focal planes to the back plane of the lid (Fig. 15). This
method removes any dimensional criticality from the cold plate (except for flatness).
SALT-3197AE0001 Cryostat Document
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Reference Plane
Focal Plane
Field Lens
Front Plate
Figure 15: OFFSET OF FOCAL PLANE TO REAR OF FRONT PLATE
4.5 Detector Interface
The interior face of the front plate will be used as the mounting and reference plane of the detector
package. The cold plate/CCD assembly is attached to the inner mounting pedestals with G10
flexures. The method of assembly will be as follows:
a.
The four pedestals will be bolted to the front plate.
b.
The cold plate assembly (with the gauging bracket in place) will be placed in position
over the inverted lid.
c.
Once it is established that the detector assembly is in the correct location, the G10
flexures will be attached and tightened.
d.
After checking the installation the gauge bracket will be removed.
e.
Following this the electrical connections will be made and the front plate screwed down
in position.
f.
Once the front plate is in position the cold path will be connected through an aperture in
the body of the cryostat.
4.6 Vacuum System
A Varian 2 l/s ion pump (with noble gas capability) is used to maintain vacuum over an extended
period. An activated charcoal cryopump (getter) is also installed on the cold end (Fig. 16). The
vacuum will be monitored via the integral gauge reading from the ion pump controller.
Viton o-rings will be used on all sealing surfaces, except for the bonding of the window into the lid.
It is not foreseen that metallic sealing rings will be required on any of the sealing surfaces.
In order to minimize molecule attachment to interior surfaces, most of the internal surfaces will be
electro-polished, including the surfaces that will have gold coatings applied to them.
SALT-3197AE0001 Cryostat Document
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All internal holes and jointing surfaces are designed such that the potential for molecules traps are
minimized (bleed holes and shaved threads).
Cryopump
Cryotiger
Figure 16: CRYOPUMP ON CRYOTIGER
4.7 Electrical Connections
There are three electrical connectors going through the vacuum wall for the CCD’s.
connectors are of the CANON type and well suited to HVAC environment.
These
The connectors for the CCD’s have an external plug box enclosing them, also containing the preamp boards, external connectors to the SDSU II controller and shorting plug arrangement.
5
Mass Properties
The following mass properties are calculated for the cryostat:
Front plate and window:
1.15 kg
Cryostat body:
1.60 kg
Detector assembly:
0.90 kg
Cold end
1.75 kg
Adjustable Interface assembly
1.35 kg (not required at PDR)
Vacuum system
0.85 kg (0.75 kg Vac Pump not foreseen at PDR)
Connectors, wiring and plug box
0.85 kg (0.5 kg Plug Box not foreseen at PDR)
Total Mass
8.50 kg
The above does not make provision for the wiring looms going to the cryostat, nor the cooler hoses.
SALT-3197AE0001 Cryostat Document
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18
Fabrication
6.1 Components
The majority of components that are not COTS will be machined from solid.
6.2 COTS items
These items will be compatible with a high vacuum cryogenic environment.
6.3 Cryostat body
The body will be machined from solid as the vacuum cast option proved problematic in quality as
well as availability.
6.4 Surface finishes
All vacuum related surfaces will be finished to a very low RA value.
7
Risks
7.1 Thermal
Presently the only risk is that the parameters used for calculations (as published in literature) are
not exactly correct as applied in our design.
7.2 Vacuum
Most risks pertain to virtual leaks and physical leaks:
a.
Unclean surfaces or surfaces with bad surface treatment that are inside the container.
b.
Sealing elements and sealing surfaces that are not up to standard
7.3 Mechanical damage
As most components inside the cryostat are small and fragile, the utmost care will need to be taken
to ensure that no patent or latent failure will occur due to undetected damage during the
manufacturing and assembly.
7.4 Risk Management
Pro-active steps have been taken to address these risks. In some instances SAAO has sound
experience and the correct procedures and checks will minimize these risks.