WFIRST-PLAN-06635.pdf

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National Aeronautics and Space Administration

Goddard Space Flight Center

Greenbelt, Maryland

WFIRST-PLAN-06635, Revision -

Wide Field Infrared Survey Telescope (WFIRST), Code 448

WFIRST Contamination Control Plan

GSFC WFIRST CMO

August 10, 2017

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WFIRST Contamination Control Plan

Review/Signature/Approval Page

Prepared by:

David Hughes

Approved by:

Kenneth Anderson

Electronic Approval available on-line at: https://gddms.gsfc.nasa.gov/Windchill/app/

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Preface

This document is a Wide Field Infrared Survey Telescope (WFIRST)

Configuration Management (CM) - controlled document. Changes to this document require prior approval of the applicable Configuration Control Board (CCB) Chairperson or designee.

Proposed changes shall be submitted to the WFIRST CM Office (CMO), along with supportive material justifying the proposed change.

In this document, a requirement is identified by “shall,” a good practice by “should,” permission by “may” or “can,” expectation by “will,” and descriptive material by “is.”

Questions or comments concerning this document should be addressed to:

WFIRST Configuration Management Office

Mail Stop 448

Goddard Space Flight Center

Greenbelt, Maryland 20771

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Change History Log

Revision Effective Date Description of Changes

(Reference the CIR/CCR & CCB/ERB Approval Date)

Revision - August 1, 2017 Initial Release of document per WFIRST-CIR-02458

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Table of TBDs/TBRs/TBSs

Item No. Location Summary Individual/

Organization

Actionee

Due Date

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Table of Contents

1 INTRODUCTION

1.1 Purpose

1.2 Scope

1.3 Related Documentation

1.3.1 Applicable Documents and Forms

1.3.2 Reference Documents

2 GENERAL

2.1 Contamination Control Organization

2.1.1 WFIRST Contamination Engineering Manager

2.1.2 Instrument Contamination Control Engineer

2.1.3 OTA Contamination Control Liaison

2.1.4 OTA Contamination Control Engineer

2.1.5 Spacecraft and IC Contamination Engineer

2.2 Contamination Control Board

2.3 Definitions

2.4 Sensitive Hardware

2.5 Implementation Plan

3 CLEANLINESS REQUIREMENTS

3.1 General

3.2 Derivation of Surface Cleanliness Requirements

3.2.1 Optical Losses From Molecular Films

3.2.2 Optical losses and scattering from particles

3.2.3 Thermal effects of particles

3.3 Derivation of Contamination Budget

3.3.1 Particle accumulation

3.3.2 Molecular Accumulation

3.4 Contamination Transport Analysis

3.5 Requirements for Non-Sensitive Surfaces

3.6 Communication Between Organizations

4 CONTAMINATION CONTROLS

4.1 Materials Selection

4.2 Design Features

4.3 Cleaning and Processing

4.4 Bakeouts

4.5 Facilities

4.6 Verification Methods

4.7 Discrepancies

4.8 Protection and Packaging

5 ENVIRONMENTAL CONTROLS

5.1 Facility Design

5.2 Facility Certification

5.3 Facility Monitoring

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5.4 Facility Maintenance

5.5 Facility Corrective Actions

6 PERSONNEL CONTROLS

6.1 Garment Requirements

6.2 Personnel Practices

6.3 Training

6.4 Prohibited Activities and Materials

7 OPERATIONAL CONTROLS

APPENDIX A SHALL STATEMENTS

APPENDIX B ABBREVIATIONS AND ACRONYMS

List of Figures

Figure 1. WFIRST Project Organization Figure 2. WFIRST Contamination Control Organization Figure 3: Absorption Coefficients

Figure 4: Calculated BRDF Curves Figure 5: Correlation of fallout rate with ceiling height

Figure 6: Acoustics Chamber Vibration Table Figure 7: EMI Room Figure 8: Thermal Vacuum Chambers

Figure 9: SSDIF Cleanroom Figure 10: Portable HEPA Filter Wall (back side)

List of Tables

Table 3: Hardware Contamination Sensitivity Table 4: Absorption Coefficient Data

Table 5: BRDF vs PAC Table 6: Alpha Change Due toParticles

Table 7: Emissivity Change From Particles Table 8: Precision Cleaning Methods Table 9: Cleanliness Verification Methods Table 10: Contamination Sources Table 11: Cleanroom Design Standards

Table 12: Cleanroom Monitoring Frequency Table 13: Cleanroom Housekeeping Frequency

Table 14: Facility Corrective Action Sequence Table 15: Contamination From Personnel Table 16: Garment Requirements for Cleanrooms Table 17: Settling Time Recommendations file:///C:/Users/jmoeller/Documents/1.WFIRST/Documentation/CIRs_CCRs/WFIRST-PLAN-06635,%20Contamination%20Control%20Plan%20(CCP)/Rev%20-/WFIRST-PLAN-06635.docx%23_Toc490129460

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1 INTRODUCTION

1.1 Purpose

The purpose of this document is to describe the Contamination Control Plan (CCP) to meet the

Cleanliness Specifications for the Wide Field Infrared Survey Telescope (WFIRST) Project as documented in WFIRST-SPEC-06862. These specifications have been derived from the Design

Reference Mission science requirements. In addition, the observatory design will not preclude servicing. The CCP includes on-orbit controls for nominal operations and servicing operations.

1.2 Scope

This Contamination Control Plan (CCP) applies to the WFIRST observatory, and to all elements of the observatory from manufacturing through end of life. This plan defines approved approaches to requirements derivation, cleanliness verification, operations, facility and personnel controls.

While the specifications in this document are not requirements that will be tracked in a verification matrix, they are procedural details that must be followed to ensure mission success. Use of the word "shall" indicates a mandatory item and non-compliance requires approval from the WFIRST

Contamination Control Board (CCB). Use of the word "should" indicates a best practice;

deviations must be communicated to the WFIRST CCB.

The system integrator and suppliers of subsystems and components are required to prepare a

Contamination Control Implementation Plan (CCIP) in accordance with section 2.0; these plans may have more stringent requirements than those listed in this document. In the event of a conflict between this plan and a Contamination Control Implementation Plan, the more stringent requirement will be followed.

1.3 Related Documentation

The latest versions of all documents below should be used. WFIRST documents can be obtained from URL: https://gddms.gsfc.nasa.gov/Windchill/app/.

1.3.1 Applicable Documents and Forms

The following documents are referenced within this document and are directly applicable or contain policies or other directive matters that are binding for the contents of this document. In the event of conflict between an Applicable Document and the content of this document, the

WFIRST Project Configuration Change Board has the final authority for conflict resolution.

Document Number Title ASTM E-595 Methods of Test, Total Mass and Controlled Volatile Condensable

Materials from Outgassing in a Vacuum Environment

ASTM E-1216 Standard Practice for Sampling Surface Particulate Contamination by Tape Lift

ASTM E-1234 Standard Practice for Handling, Transporting, and Installing Nonvolatile Residue (NVR) Sample Plates Used in Environmentally Controlled Areas for Spacecraft

ASTM E-1559 Standard Test Method for Contamination Outgassing Characteristics of Spacecraft Materials

ASTM E-1560 Method for Gravimetric Determination of Non Volatile Residue from Cleanroom Wipers

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Document Number Title ASTM F-312 Standard Methods for Microscope Sizing and Counting Particles from Aerospace Fluids on Membrane Filters

ASTM F-318 Standard Practice for Sampling Airborne Particulate Contamination in Cleanrooms for Handling Aerospace Fluids

FED-STD-209E Federal Standard Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zones

GSFC-Form 18-59 Materials and Process List

GSFC-TLS-PR-7324-01 Contamination Control Procedures for the Tape Lift Sampling of Surfaces

GSFC-MLS-PR-7324-01 Contamination Control Procedure for the Molecular Wipe Sampling of Surfaces

IES-RP-CC003.2 Garment System Considerations for Cleanrooms and Other Controlled Environments

IEST-CC-1246 Produce Cleanliness Levels - Applications, Requirements, and Determination

ISO-14644-1 Cleanrooms and associated controlled environments - Part 1:

Classification of air cleanliness

JSC SP-R-0022A Vacuum Stability Requirements of Polymeric Materials for Spacecraft Application – General Specification

JSC SN-C-0005 Contamination Control Requirements for the Space Shuttle Program

MIL-P-27401C Propellant Pressurizing Agent, Nitrogen

MIL-STD-889 Dissimilar Metals

MIL-STD-1246C Product Cleanliness Levels and Contamination Control Program

1.3.2 Reference Documents

The following documents are referenced herein and amplify or clarify the information presented in this document. These documents are not binding on the content of this document.

Document Number Title NASA-JSC-08962 Compilation of VCM Data of Nonmetallic Materials

NASA Ref. Pub. 1124 Outgassing Data for Selecting Spacecraft Materials. Online at http://outgassing.gsfc.nasa.gov/og

NASA MAPTIS Materials and Process Technical Information System data base center

Technical Reference A dynamic approach to monitoring particle fallout in a cleanroom environment, Radford Perry, SPIE Volume 7794: Optical System

Contamination: Effects, Measurements, and Control 2010

Technical Reference Optical Constants of Water in the 200-nm to 200-μm Wavelength Region, Hale and Querry, Applied Optics v. 12, No. 3, 1973 http://outgassing.gsfc.nasa.gov/og

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2 GENERAL

2.1 Contamination Control Organization

The NASA/GSFC Contamination and Coatings Engineering Branch, (Code 546), has oversight responsibility for GSFC projects’ contamination control activities. Code 546 assigns a

Contamination Control Engineer (CCE) to each project. For the WFIRST project, the CCE reports to the WFIRST Mission Systems Engineering. Figure 1 shows the WFIRST Project organization.

Figure 1. WFIRST Project Organization

The WFIRST CCE will direct and monitor the activities of this plan to assure conformance, identify the need for corrective or preventative actions, and implement such actions when necessary. Because of the size of the project, additional contamination support personnel will be required. Figure 2 shows the WFIRST Contamination Control organization.

CCE

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Figure 2. WFIRST Contamination Control Organization

In figure 2, boxes with bold outlines denote staffing by off-site contractors. These individuals report to their normal management structure, but are also considered part of the WFIRST contamination control team and report on an informal basis. Coordination of the contamination control implementation details, and resolution of discrepancies or non-conformances, is performed by a WFIRST Contamination Control Board. The members of this board are marked with an asterisk in the figure. WFIRST Safety and Mission Assurance (SMA) and Systems Engineering

(SE) representatives are also included in the board.

2.1.1 WFIRST Contamination Engineering Manager

The WFIRST Contamination Engineering Manager has prime responsibility for the overall cleanliness of WFIRST, its replaceable components, and the overall mission from initial delivery to GSFC through Launch. The WFIRST Contamination Engineering Manager is supported by the Payload Contamination Control Engineer, Facility Contamination Control Engineer, a Purge

Engineer, and a Coatings Engineer. The WFIRST Contamination Control Engineer is responsible for coordinating molecular and particulate contamination transport analyses to ensure that instruments and other observatory hardware systems do not cross-contaminate one another. The

WFIRST Contamination Control Engineer will also provide, upon request, analysis support to instruments and observatory systems to ensure that they will not self-contaminate.

2.1.2 Instrument Contamination Control Engineer

Both WFI and CGI will have an instrument CCE. The ultimate responsibility to control the effect of self-contamination on the on-orbit performance of the instrument lies with the designated

Instrument Contamination Control Engineer. Each instrument vendor shall designate a

Contamination Control Engineer who is responsible for the cleanliness of the delivered instrument to GSFC.

Each Instrument Contamination Control Engineer is responsible for developing and implementing a contamination control program and creating the contamination control

WFIRST CCE*

Payload CCE

OTA CC Liason

OTA CCE*

WFI CC Lead*

WFI Detector

CCE

WOMA CCE

CGI CCE*

Spacecraft CC Lead*

Spacecraft

CCE

LS/LV CCE

Facilities CCE

Purge Engineer

Coatings Engineer

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documentation necessary to design, build, fabricate, and test an instrument so that, upon delivery, that instrument meets the cleanliness requirements specified herein. Each Instrument

Contamination Control Engineer has prime responsibility for the as-delivered cleanliness of their instrument and for assuring that contamination levels will not degrade instrument performance beyond what is allowed in the controlling Instrument Performance Specification. Where heritage designs are used, the Contamination Control Engineer is responsible for evaluating differences in mission parameters and adjusting contamination control protocols and/or the design to accommodate significant differences that would impact instrument performance. The Instrument

Contamination Control Engineer is also responsible for providing post-delivery cleaning and handling instructions for each instrument delivered to GSFC.

All contamination control discrepancies at the instrument level shall be reviewed by the

Instrument Contamination Control engineer, Instrument Systems Engineer, and the Safety and

Mission Assurance Engineer for that organization. All instrument contamination control non-conformances shall be documented. If an internal derived contamination control requirement is not satisfied, the Instrument Contamination Control Engineer may disposition the non-conformance using product assurance procedures for that organization. The Instrument

Contamination Control Engineer shall notify the WFIRST Contamination Control Engineer when the instrument contamination control non-conformance is discovered and again after it has been dispositioned. If a requirement in the WFIRST Cleanliness Specification is violated, a waiver must be approved by the WFIRST Contamination Control Board. Waivers that impact other higher level requirements must be taken to the relevant Configuration Control Board.

2.1.3 OTA Contamination Control Liaison

The OTA Liaison will provide the interface between the WFIRST CCE and the OTA

Contamination Control Engineer. The liaison will ensure that contamination requirements changes are coordinated between the OTA, WFI, and the Mission level elements.

2.1.4 OTA Contamination Control Engineer

The ultimate responsibility to control the effect of self-contamination on the on-orbit performance of the telescope lies with the Optical Telescope Assembly (OTA) Contamination

Control Engineer. The OTA vendor shall designate a Contamination Control Engineer who is responsible for the cleanliness of the delivered OTA to GSFC.

The OTA Contamination Control Engineer is responsible for developing and implementing a contamination control program and creating the contamination control documentation necessary to design, build, fabricate, and test the OTA and Aft Optics so that, upon delivery, the OTA meets the cleanliness requirements specified herein. The OTA Control Engineer has prime responsibility for the as-delivered cleanliness of the hardware and for assuring that contamination levels will not degrade OTA performance beyond what is allowed in the controlling Telescope Performance Specification. Where heritage designs are used, the

Contamination Control Engineer is responsible for evaluating differences in mission parameters and adjusting contamination control protocols and/or the design to accommodate significant differences that would impact instrument performance. The OTA Contamination Control

Engineer is also responsible for providing post-delivery cleaning and handling instructions for hardware delivered to GSFC.

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All contamination control discrepancies at the OTA level shall be reviewed by the OTA

Contamination Control engineer, OTA Systems Engineer, and the Safety and Mission Assurance

Engineer for that organization. All OTA contamination control non-conformances shall be documented. If an internal derived contamination control requirement is not satisfied, the OTA

Contamination Control Engineer may disposition the non-conformance using product assurance procedures for that organization. The OTA Contamination Control Engineer shall notify the

OTA CC Liaison or WFIRST Contamination Control Engineer when the contamination control non-conformance is discovered and again after it has been dispositioned. If a requirement in the

WFIRST Cleanliness Specification is violated, a waiver must be approved by the WFIRST

Contamination Control Board. Waivers that impact other higher level requirements must be taken to the relevant Configuration Control Board.

2.1.5 Spacecraft and IC Contamination Engineer

The ultimate responsibility to control the effect of spacecraft contamination on the on-orbit performance of the instruments, radiators, and solar arrays lies with the designated Spacecraft

Contamination Control Engineer.

The Spacecraft Contamination Control Engineer is responsible for developing and implementing a contamination control program and creating the contamination control documentation necessary to design, build, fabricate, and test the Spacecraft and Instrument Carrier so that, upon delivery to I&T, they meet the cleanliness requirements specified herein. The Spacecraft

Contamination Control Engineer has prime responsibility for the as-delivered cleanliness of the spacecraft and solar arrays and for assuring that contamination levels will not degrade observatory performance beyond what is allowed in the controlling Performance Specifications.

Where heritage designs are used, the Contamination Control Engineer is responsible for evaluating differences in mission parameters and adjusting contamination control protocols and/or the design to accommodate significant differences that would impact instrument performance. The Spacecraft Contamination Control Engineer is also responsible for providing post-delivery cleaning and handling instructions for the spacecraft when delivered to I&T.

2.2 Contamination Control Board

The WFIRST Contamination Control Board shall review and disposition the WFIRST contamination control waivers for WFIRST assemblies, instruments, electronics, structures, and mechanisms. Minor system-level non-conformances can be dispositioned by the WFIRST

Contamination Control Engineer or designee. The WFIRST Contamination Control Board shall include:

• the WFIRST Contamination Control Engineer

• the Instrument Contamination Control Engineers

• the OTA Contamination Control Engineer

• the Spacecraft Contamination Control Engineer

• the WFIRST Safety and Mission Assurance Manager or Designee

• The WFIRST Mission Systems Engineer or Designee

Waivers or deviations that violate the WFIRST Cleanliness Specification will require approval of the WFIRST Contamination Control Board.

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2.3 Definitions

Bidirectional Reflectance Distribution Function (BRDF) - The scattering properties of light reflected off surfaces, expressed as the ratio of differential outputs of radiance divided by differential inputs of radiance. Surface contaminants scatter the incident radiation in all directions and with variable intensities; BRDF is a method to quantify the spatial distribution of the scattered energy.

Budget Allocation - The itemized summary of contamination accumulation for a given critical hardware item distributed over all phases from manufacture through end of performance lifetime.

Cleanroom - An environmentally conditioned area (generally a room or tent) where temperature, humidity, and airborne contaminants are controlled by design and operation. The air handling system utilizes High Efficiency Particulate Air (HEPA) filters, which have not been tested with the Dioctyl Phthalate (DOP) method, to achieve the air cleanliness level. Air particulate cleanliness is classified in accordance with ISO-14644-1.

Contamination - Any foreign matter that degrades the performance and/or function of the surface on which it resides.

Contamination Control (CC) - Organized actions including: design, operations, or mitigation techniques to control contamination levels.

Controlled Area - An area that does not require a high degree of temperature and humidity control, but where a semi-clean atmosphere is desired. Air conditioning is standard commercial design except that filtration is rated to 80-85% for 1.0 micrometer and larger particles to control airborne contaminants. These areas typically will not exceed the airborne particle concentration of Class 8.5 per ISO-14644-1.

Facility - The total real property required to accomplish the environmental control and operation of cleanrooms and controlled areas as well as administrative and personnel support. NOTE:

This includes the cleanroom proper, air locks, change rooms, parts cleaning, storage, HVAC equipment, offices, etc.

Molecular Contamination - Non-particulate matter in the form of droplets or thin films which adversely affects component or system performance.

Nonvolatile Residue (NVR) - Soluble material remaining after evaporation of a filtered volatile fluid or precipitate from a gas phase, usually reported in milligrams per unit area (or volume).

Outgassing - The release of gaseous molecular substances from solid or liquid materials.

Particulate Contamination - Small discrete mass of solid matter, usually measured in micrometers (µm), which adversely affects component or system performance.

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Precision Cleaning - Cleaning of hardware surfaces by approved engineering methods to meet specific cleanliness criteria.

Surface Cleanliness - An established level of maximum allowable particulate and/or molecular contamination ranging from visibly clean to a specific IEST-CC-1246 designation (e.g., Level

100 R1, Level 400 R2, etc.)

Visibly Clean - Absence of particulate or molecular contaminants when viewed from a specified distance with normal (or corrected to normal) vision with a specified illumination level in accordance with IEST-CC-1246.

2.4 Sensitive Hardware

Table 1 describes hardware sensitivities to contamination. Each CCIP shall identify the specific hardware that is contamination sensitive which is being protected by that CCIP.

Table 1: Hardware Contamination Sensitivity

Hardware Particulate Effects Molecular Effects

Optical and detector surfaces Obscuration causing throughput loss; unwanted scattering; or increased background noise; false data

Absorption of light by thin/thick films; optical interference by thin films which moves wavelength cutoff values;

scattering by droplets; films which may be susceptible to ultra-violet (UV) darkening

Solar array (cell side) Obscuration causing power loss

Absorption causing power loss

Fiber optic connectors Misalignment of fiber ends causing throughput loss

Thermal control surfaces Change in thermal properties;

scattering of sunlight to normally shadowed areas

Change in thermal properties

Optical baffles Scattering reducing effectiveness; can result in incorrect data at the detectors

Reduction in signal due to absorption of incoming light

Thermal interfaces Reduction in thermal conductivity

Cryogenic MLI Change in thermo/optical properties of MLI layers

Water ice may create thermal shorts through MLI

Seals and valves Leaks

Heat pipe and cryo-cooler working fluid

Change in thermal/optical properties of surfaces

Impurities decrease efficiency

Electrical Connectors Failure to make contact;

Electrical shorts across pins

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Hardware Particulate Effects Molecular Effects

Electronics boards Electrical shorts or arcing Can impact adhesion of connections, chips, etc.

Mechanisms Binding or early failure from galling and wear

Corrosion

Bonded interfaces Adhesion or surface finish issues

Adhesion and loss of strength

2.5 Implementation Plan

Each hardware element vendor shall produce a Contamination Control Implementation Plan

(CCIP). The CCIP shall do the following:

1. Identify the contamination sensitive hardware

2. Contain a contamination budget for all phases of the ground and flight operations conducted by the vendor

3. Reference or include as an appendix the analyses that were used to derive the contamination budget

4. Provide methodology for achieving and maintaining the required cleanliness, including bakeouts, purge, cleaning, and bagging

5. Specify which measurement and monitoring methods are used, and provide procedures or references where the methods differ from the practices specified in this document

6. Define the sequence of cleanliness activities, frequency of verification, and the capabilities of the facilities which will be occupied

7. Define the roles and responsibilities of the organizations and individuals which will implement the requirements

8. Provide descriptions of when and how the cleanliness activities are to be reviewed

9. Provide information on design features for contamination control and materials selection criteria

10. Provide information on how to train personnel and assess them

11. Define how to deal with situations that may result in cleanliness specifications not being achieved

12. Define single-point failure modes that can affect cleanliness, including equipment failures and human errors

13. Define the risk of cross-contamination between hardware elements

14. Provide references to procedures

15.

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3 CLEANLINESS REQUIREMENTS

3.1 General

Contamination performance degradation allocations for WFIRST are specified in WFIRST-

SPEC-06862. Cleanliness requirements for sensitive surfaces are derived from functional or performance requirements, while requirements for non-sensitive surfaces are derived from their proximity to and impact on sensitive surfaces (cross contamination). Cleanliness requirements that are more stringent than necessary to meet manufacturing and system performance requirements shall not be imposed at any level of assembly; however, cross contamination concerns at higher levels of assembly must be addressed.

3.2 Derivation of Surface Cleanliness Requirements

When deriving cleanliness requirements for a sensitive surface, all performance requirements must be examined. The most stringent cleanliness requirement determined shall apply. This section provides the methods that should be used to evaluate the optical and thermal performance impact of contamination on surfaces.

3.2.1 Optical Losses From Molecular Films

For optical losses, water and molecular films are assumed to absorb according to Beer's law:

Eq. 1) 𝐼 = 𝐼0𝑒 −𝛼𝑡

For WFIRST analytical purposes, the term alpha shall be 1e-5 (1/Å) for molecular contaminants, and 2e-6 (1/ Å) for water. These values are worst case (highest) values over the WFIRST wavelength range.

The absorption coefficient alpha has been measured for several spacecraft contaminant materials, and an enveloping curve developed. A similar envelope for water has been created based on data published by Hale and Querry (1973).

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The data in tabular form are presented in Table 2.

Table 2: Absorption Coefficient Data

Wavelength

Absorptance

(1/Angstrom)

(nm) Molecular Water

115 4.00E-03

125 3.00E-03

150 1.00E-03

161 9.00E-04

180 1.00E-03

200 2.50E-03 1.00E-09

225 9.50E-04

300 3.50E-05 9.00E-11

400 1.30E-05 6.00E-12

500 1.00E-05 3.00E-12

100 3.00E-11

200 8.00E-11

300 4.00E-10

400 1.00E-09

1000 6.00E-06 5.00E-09

1500 6.00E-06

Figure 3: Absorption Coefficients

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2000 1.00E-05 2.00E-06

2500 2.00E-04

3000 2.00E-04 1.00E-04

3300 1.00E-04

4000 5.00E-05 3.00E-06

5000 3.00E-05 5.00E-06

6000 2.50E-05 3.00E-05

7000 2.50E-05 7.00E-06

8000 3.00E-05

9000 3.50E-05

10000 4.00E-05 9.00E-06

13000 4.00E-05

30000 1.50E-05

3.2.2 Optical losses and scattering from particles

For optical surfaces, particles are treated as opaque absorbers at all wavelengths, and percent area coverage (PAC) shall be directly specified from a throughput loss allocation.

In some areas, light scattering may be of more importance than obscuration. A rigorous calculation of the BRDF over an angular range of 0.5° to 45° was performed with particle bins fully loaded according to the IEST-CC-1246 definition. The BRDF curves were calculated and plotted for various cleanliness levels (Error! Reference source not found..)

Figure 4: Calculated BRDF Curves

BRDF Estimation

(l = 650 nm at q = 15 o

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0 5 10 15 20 25 30

Angle from Specular (degrees)

B R

D F

/S r)

Level 1000

Level 600

Level 400

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Regression analysis was performed on the resulting curves and plotted against the associated

PAC value (Table 3.)

Table 3: BRDF vs PAC

Cleanliness

Level

PAC BRDF (=X*θ-1.02)

100 0.000204 1.65E-05

200 0.003228 2.61E-04

300 0.019411 1.57E-03

400 0.075107 6.08E-03

500 0.22459 1.82E-02

600 0.566217 4.58E-02

700 1.263525 0.102

800 2.571962 0.208

900 4.872033 0.394

1000 8.709636 0.705

A second regression of the BRDF coefficient versus the PAC values provides a simplified relationship for converting between PAC and BRDF. This relationship takes the form:

Eq. 2) PAC = (1 / 0.0809)* θdeg

1.02 * BRDF

This relationship shall be used for WFIRST analyses where light scattering is of importance.

3.2.3 Thermal effects of particles

The impact of particles on the thermal properties of second surface reflectors is given by

Equation 3:

Eq. 3) s = PAC (2p - 2pm – m 2 - pm

2) –

PAC2 (p - 2pm + m - m 2 - pm

2)

Where: s = change in solar absorption

2p = absorption of the particles m = absorption of the base material

For small PAC, the PAC2 term can be neglected.

For first surface reflectors, the formulation is simpler:

Eq. 4) s = PAC (p – m)

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This formulation was calculated for common spacecraft thermal control materials with the particle absorption value (p) arbitrarily set at 0.5 for the purpose of this evaluation. This enables the estimation of particle contamination limits from thermal property requirements

(change or delta) by the simple solution of:

Eq. 5) PAC = delta / coefficient

For WFIRST analyses, the values in Table 5 shall be used with equation 5 to determine the PAC from delta alpha requirements. For materials that have not been listed in Table 5, the preceding method shall be used to determine the effect.

Table 4: Alpha Change Due toParticles

Material Description Absorption Resulting Coefficient

Silver Teflon 0.07 6.14 Aluminum, Gold Anodize 0.27 0.85 MS94 Black Paint 0.96 -0.48 Z93 White Paint 0.16 2.13 Betacloth 0.28 0.79 Kapton 0.45 0.11 Solar Cell 0.65 -0.23 GE Black Kapton 0.52 -0.04

The process used for the change in solar absorption was also applied to the changes in emissivity. This approach is valid since emissivity is commonly measured as one minus the absorptance. The particle emissivity value (p) was arbitrarily set at 0.5 for the purpose of this evaluation. For WFIRST analyses, the values in Table 5 shall be used with equation 5 to determine the PAC from delta emissivity requirements.

Table 5: Emissivity Change From Particles

Material Description Emissivity Resulting Coefficient

Silver Teflon 0.78 -0.36 Aluminum, Gold Anodize 0.12 3.17 MS94 Black Paint 0.91 -0.45 Z93 White Paint 0.91 -0.45 Betacloth 0.88 -0.43 Kapton 0.78 -0.36 Solar Cell 0.85 -0.41 GE Black Kapton 0.8 -0.38

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3.3 Derivation of Contamination Budget

3.3.1 Particle accumulation

A numerical evaluation of the particle fallout, normalized to area coverage, performed by R.

Perry (2010) demonstrates a correlation that is directly proportional to both cleanroom class and exposure time. As shown in Figure 5, the fallout rate is proportional to the height of the air column above the hardware.

Figure 5: Correlation of fallout rate with ceiling height

The rate of accumulation of PAC per Class-Hour (CH) is given by equation 6.

Eq. 6) PAC/CH = 1.8 x10-9(height in feet)0.7

A Class-Hour is one hour spent at an average Fed-Std-209 air classification. Converted into metric units and ISO-14644-1 classifications, the fallout rate per hour is given by equation 7.

Eq. 7) PAC/hour = (4.13x10-12) x 10(ISO Class) x (height in meters)0.7

WFIRST fallout accumulation predictions shall use this formula for calculation. For typical cleanrooms, the air class is close to zero when at rest (no activity in cleanroom); therefore, the number of hours exposed should be based on the expected number of active hours. For example, one month might contain 21 working days of 8 hour shifts; resulting in 168 hours of accumulation per month. The actual average particle counts (rather than the room certification level) should be used as the basis for calculation when data relevant to the expected operations are available.

1.00E-11

1.00E-10

1.00E-09

1.00E-08

1 10 100 1000 10000

Ceiling Height (ft)

F a ll o u t R a te

A

C /C la s s -H o u r)

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In addition to fallout within a facility, some operations may increase the contamination level on the hardware, and cleaning operations may reduce the contamination level. As the complexity of the hardware increases, the ability to remove contaminants decreases. Hardware that is bagged may be assumed to have a constant contamination level that is not affected by surrounding operations (assuming that the bagging material is non-shedding).

3.3.2 Molecular Accumulation

The contamination budget for molecular contamination shall reflect increments due to contact transfer and deposition from warm sources (especially outgassing sources in vacuum), but should not include a monthly accumulation due to residing within a cleanroom. The ordinary monthly non-volatile residue (NVR) accumulation rate in cleanrooms reaches an equilibrium with the environment, and is largely removed by exposure to vacuum. During the monitoring phase of the contamination program, cleanroom NVR deposition monitoring may reveal sources of NVR that are not removable by exposure to vacuum, such as the residue created during curing of silicone caulk or hot roof tar. These increments should be noted and the remaining contamination budget adjusted to reflect the new condition of the hardware.

At the mission level, the contamination budget must include an allocation for on-orbit molecular accumulation and an allocation for launch/ascent particle redistribution. The analysis of outgassing contributions to molecular contamination may result in stricter bakeout requirements that must be imposed on hardware at lower levels of assembly.

The contamination requirement for water deposition may drive limits on retained water within bulk materials or on hardware surfaces. In these cases, an accumulation budget for water is not required; however, a purge analysis shall be performed to determine the required flow rate and effect of flow interruptions before launch after the hardware has been dried through bake out, long term purge, or thermal vacuum testing.

The supplier for each hardware element shall perform an analysis of contaminant transport and deposition for all mission phase operations for which they are responsible, and will develop a contamination budget for the contamination sensitive surfaces. Assumptions and supporting data shall be documented along with the contamination budget. Where the cleanliness would exceed the hardware requirements, and the surfaces are accessible, cleaning steps should be added to the schedule.

3.4 Contamination Transport Analysis

Molecular transport analyses shall be performed for the instrument and observatory thermal vacuum conditions, and for on-orbit conditions. Separate cool-down (transient) analyses and operational (steady state) analyses should be run.

Where actual outgassing rates for materials or historical data are not available, ASTM E1559 or

E595 data may be used. When scaling outgassing rates to different temperatures, an Arrhenius constant of 10820.3 K should be used for molecular contaminants and 6039.255 K should be used for water. To scale the deposition rate for molecular contaminants, the constant is 0.104 K between -20 C and + 25 C. From -20 C to -90 C there is a further 25% increase in the deposition rate due to the capture of increasingly light species. Below -90 C, no additional molecular

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contaminants are captured. These scaling rules should also be followed when setting bakeout parameters that differ from the flight temperatures. Water deposition rates can be determined from the vapor pressure curve.

For on-orbit steady state analyses, the outgassing rate should be set to the average rate over the mission lifetime. Where depletion rates are unknown, the rate should be assumed to decay as

1/sqrt(time) with the initial rate set at time x, where x is the ground vacuum exposure time.

Particle redistribution during launch may be modeled as uniform redistribution or by using computational means.

3.5 Requirements for Non-Sensitive Surfaces

Non sensitive surfaces shall have their contamination requirements set such that they are consistent with the assumptions in the contamination transport analysis performed during contamination budget derivation. However, if there are cross-contamination concerns, these non sensitive surfaces may be assigned stricter contamination requirements.

3.6 Communication Between Organizations

A method shall be outlined by each element vendor for imposing contamination control requirements on sub-tier contractors.

For most items, cleanliness requirements will be included in purchase orders, the statement of work, or specifications for sub-tier vendors. Where the hardware may be adequately cleaned and verified after receipt, the items may be ordered without cleanliness requirements; please note that the materials used in all parts, components, and surfaces must be approved for space flight hardware. Hardware manufactured in-house will have cleaning instructions added to the manufacturing work instruction, or to the final inspection work instruction.

Some surfaces become inaccessible during assembly of hardware. The verification and acceptance plan for hardware elements should include the verification of these surfaces prior to loss of access. Cleanliness requirements for these surfaces may be specified on assembly drawings or in assembly procedures.

Transportation and environmental test plans should include contamination monitoring and verification steps.

The status of the contamination and cleanliness control program shall be reported at the monthly project reviews. The presentation should demonstrate that suitable activities are planned, are being implemented or have been successfully achieved. Any unexpected issues or problems should be brought to the attention of the CCE immediately for full review and resolution.

A contamination working group will be formed consisting of the WFIRST Contamination

Control Engineer (CCE) and representatives from the hardware vendors. This working group will meet as frequently as needed based on the level of hardware development effort.

Contamination anomalies will be reported to the working group within 48 hours of occurrence.

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4 CONTAMINATION CONTROLS

4.1 Materials Selection

Materials selection criteria are based on best practices developed over decades of spacecraft and instrument construction, and designers should comply with these criteria. A complete materials list for each vendor item should be supplied to the CCE and Materials Engineer (ME). Non-flight material selection criteria apply to materials that will be used in cleanrooms, in vacuum chambers, or in contact with flight hardware. Despite passing the selection criteria, some materials may not be appropriate in all locations. In addition, some materials that do not pass the screening criteria may be appropriate for use in certain locations or quantities. The

Contamination Engineer will review material selections and recommend alternatives or processing steps to ensure the cleanliness of sensitive hardware will not be compromised.

Flight material selection criteria:

1. Materials shall pass ASTM E-595 vacuum stability with <= 1% Total Mass Loss (TML) and <= 0.1% Collected Volatile Condensable Materials (CVCM).

2. Materials used on bearing or wear surfaces shall not shed particles during normal operations. Dry lubricants must be properly burnished to remove loose material before use.

3. Metallic materials that produce whiskers or have high vapor pressures shall not be used.

This includes pure tin, zinc, and cadmium.

4. Silicone based oils and release compounds shall not be used because of the difficulty in controlling contact transfer.

5. Closed cell foams shall not be used. Although closed cell foams generate fewer particles when cut or abraded, they are not amenable to bakeout on a reasonable time scale.

6. Non-optical surface finishes shall be compatible with 15 minute exposure to IPA and/or

DI water to permit ultrasonic cleaning of small parts and wiping of large parts.

7. Optical coatings selection shall be compatible with 15 minute exposure to acetone (to ensure proper cleaning may be performed).

Non-flight material selection criteria:

1. Materials that will be used in vacuum shall pass ASTM E-595 vacuum stability with <=

1% TML and <= 0.1% CVCM.

2. Materials shall not shed particles during normal operations. Materials known to generate particles include bare aluminum, wood, cardboard, non-cleanroom paper, dry lubricant

(unless burnished on), some acrylic adhesives, and rusted tools. Painted items must be in good condition, and should not be used where wear is expected.

3. Polymeric materials containing volatile plasticizers or electro static discharge (ESD) additives shall not be used. This includes vinyl, ethylene propylene diene monomer

(EPDM), neoprene, and soft silicone rubber.

4. Silicone based oils and release compounds shall not be used because of the difficulty in controlling contact transfer. Note that many molded items may be contaminated with silicones on arrival and require cleaning before use in a clean room.

5. Surface finishes shall be compatible with 15 minute exposure to isopropyl alcohol (IPA) and deionized (DI) water.

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4.2 Design Features

The flight hardware will require repeated cleaning during Assembly, Integration and Test

(AI&T) to maintain the required surface cleanliness levels. The design of the hardware should be capable of enduring multiple cleanings, using typical cleaning solvents and methods, without damage to the hardware surfaces. Harnesses and instrumentation wires should be routed such that strips of SLI may be used to cover it and prevent particulate accumulation. Narrow hogged out areas, blind holes, and small gaps in the vicinity of optics should also be closed out with SLI or tape. Smooth finishes should be selected over rough or no-contact finishes.

The design should also accommodate protection of critical surfaces. Optical mounts should include attachment points for hard covers to be used during ground handling (or deployable covers during on orbit operations). The attachment should be designed for ease of access to permit use and cleanliness verification after installation and alignment. Larger items may require locations that are safe for taping protective bags. Consideration should be given to whether or not standoffs may be required to prevent bags from abrading painted or delicate hardware.

Where appropriate, on orbit ‘warm-up’ periods should be scheduled to allow for materials outgassing of the observatory prior to exposure of optics for initial data accumulation.

Hollow structures and MLI shall have filtered vents. A structure wrapped in MLI may use the filter built into the MLI rather than a filter directly mounted to the structure. These filtered vents should be oriented away from critical surfaces.

In addition, accessibility for monitoring and the inclusion of witness samples should be considered to support the contamination control program. Removable panels sized for visual inspection or witness sample installation should be considered where structural requirements do not preclude them.

Hardware with critical humidity requirements should have accommodations for an internal dewpoint sensor. Purge lines and manifolds should incorporate sampling ports and point of use particle filters.

The design of ground support equipment that is in the same room as, in close proximity to or in contact with, flight hardware shall also be similarly…