NASA/GSFC WFIRST Auto Collimating Flat (ACF) Request for Information
WFIRST Auto Collimating Flat (ACF) Request for Information
dated October 1, 2019
1 WFIRST Program Overview
The Wide Field Infrared Survey Telescope (WFIRST) is a mission responding to the 2010 National Research Council New Worlds, New Horizons (NWNH) Astronomy and Astrophysics Decadal Survey top priority recommendation in the large space mission category. The science program includes two dedicated investigations to tackle outstanding questions in dark energy research and exoplanet exploration, and includes a substantial General Observer program to enable targeted investigations of astrophysical phenomena to advance other goals from the Decadal Survey. A coronagraph instrument is included in the payload for purposes of advancing the present state of the art of coronagraph technology.
2 Procurement Overview
The WFIRST system is comprised of a 2.5-meter imaging system that operates at about 260K in order to allow imaging out to 2.0 microns. NASA plans to test the alignment of the imaging system using an auto collimating flat (ACF) operating near the telescope operating temperature. NASA has a limited space constraint for the test system so the space envelope for the ACF system will be limited. At this time, the total space envelope for the ACF system is 0.6m in the optical axis direction. We realize that this will drive an ACF mirror design that is thinner than would normally be expected for a mirror of this size and that this design constraint will further influence the mount design in order to insure an ACF surface figure at temperature meets the WFIRST requirements. The space envelope is very limited due to volume constraints; however, it is negotiable with NASA. We do not have a specific interface defined between the ACF system and the chamber at this time. It is expected that some three point bolted interface would be used.
This RFI is comprised of three basic areas of interest.
A) Blank Procurement
The blank procurement would normally be an integral part of the procurement of the system. But due to schedule constraints, NASA may have to procure the mirror blank prior to the ACF contract finalization. So we are asking to break this part of the response out separately so that NASA understands the cost and lead time associated with this activity.
B) Mounted ACF System (including mirror blank finishing)
NASA is currently in the process of trading between a full aperture 2.5m ACF and a sub-aperture, 1.5m ACF. Due to the size differences and space limitations, this size differential may lead to a different mount and test scheme for the fabrication and test. We also realize that the smaller ACF will be much easier to verify and would be preferable if the WFIRST requirements can be verified at the smaller size.
B.1) Mounted 1.5m Clear Aperture ACF
B.2) Mounted 2.5m Clear Aperture ACF
C) ACF Thermal Control System
The ACF system will be installed in a high vacuum chamber surrounded by LN2 shrouds. A turn-key thermal control system will be required by to allow the ACF temperature and gradients to be controlled around a set temperature of 260K.
3 Supplier input is requested in the following section of the RFI.
3.1 ACF Blank Specification
Feedback on the parameters for the ACF blank are requested.
3.1.1 Diameter
Two lead times (1.5m and 2.5m) and costs will be associated with the response to this RFI.
As discussed in Section 2, two different ACF diameters are being considered by NASA at this time.
Notionally, the diameter of the mirror blanks should be slightly oversized from the blank provider. Recommendations for this oversize would be helpful since NASA has defined a clear, coated aperture of 1.5m and 2.5m. The mount scheme and edge effects will make the mirror blank larger than these values by some amount.
3.1.2 Thickness
The thickness of the blank will be 115mm (4.53") minimum. This is also a notional value and will have to be considered within the larger system space envelope introduced in Section 2.
3.1.3 Blank CTE
The blank material shall have a max CTE of ±0.05E-6/K at operating temperature.
Preliminary analysis by NASA and Sigmadyne indicates that this CTE should be acceptable for this application. Homogeneity is critically important in order to control surface figure changes as the ACF cools to the operating temperature of 260K.
3.1.4 Surface finish
Ground surfaces are acceptable on all surfaces of the blank.
3.1.5 Surface quality
The blank surface shall be flat to 1mm or less
3.1.6 Front to back parallelism
The front to back surface of the blank shall be parallel to 5mm or less.
3.1.7 Roundness
The blank shall be round to within 10mm.
3.1.8 Break Edges
All blank edges shall have a hand break edge of 1mm minimum.
3.1.9 Inclusions and Fractures
There shall be no inclusions in the part that could create a live fracture during processing or operationally due to thermal changes.
The blank material shall not have any live fractures.
3.2 ACF Mechanical/Optical System
The response to this section will be in two parts. Part 1 will be with respect to a 1.5m clear aperture ACF and Part 2 will be with respect to a 2.5m clear aperture ACF. As noted earlier, due to the space limitations and effective stiffness of two configurations, the mounting scheme may differ between the two different sizes.
3.2.1 Nominal Configuration
Since the WFIRST system operates off-axis, the nominal configuration of the ACF will be with its surface perpendicular to gravity.
3.2.2 Space Envelope
As discussed in Section 2, the total space envelope being allocated to the ACF is 0.6m. This will include the ACF, the ACF mount, the Tip/Tilt actuation system, and the thermal control elements. Note that the space limitation is only in the axial direction.
The interface to the test configuration has not been defined at this time. It is assumed that there would be a three-point, hard bolted interface at the top of the 0.6m space envelope. Note that this interface is controlled and could impart strain into the ACF assembly during integration and cool-down. Recommendations regarding this interface would be welcomed.
3.2.3 System Mass
An allocation of 2,700kg (6,000 pounds) has been allocated to the ACF system. This is driven by the allowable chamber head loading. This is the current allocation and could be increased if needed by doing additional chamber analysis.
3.2.4 Surface Quality at the operating temperature
The mirror will operate at 260K. The requirements stated apply at this temperature including a maximum axial gradient of 0.3K.
The surface figure and figure stability are as follows:
Power, Peak to Valley WFE (nm)
Absolute 600
Knowledge 100
Stability NA: budget under figure
Wavefront Error, RMS (static power removed)
Absolute 25 Goal, 40 Required
Knowledge
Nominal Temp 15
Delta Over Temp Range 10
Total Over Angle and Temperature 20
Repeatability
Shipping/reconfiguring 10
Notes:
• The thermal control system will limit the axial gradient in the mirror to less than 0.3K
• Alternate thermal control schemes are acceptable as discussed in Section 3.3.
3.2.5 Coating
The ACF will be coated with a protected silver coating over the full 1.5m or 2.5m aperture (minimum).
It should be noted that the coating may extend to the OD of the mirror for thermal control reasons but there is no surface figure requirement outside the 1.5m or 2.5m diameter. A non-contact apodizer could be used to effectively mask off this area of the mirror surface.
3.2.6 Mounting and Repeatability
The ACF mount is the major design consideration for this procurement. The mount should provide the following key features:
• Provides an interface between the mirror and the chamber
• The mirror mount interface should be repeatable such that the mirror can be installed, removed, and replaced with high confidence that the surface figure created during calibration at the supplier's facility is unchanged.
• The interface between the mirror and the mount should be capable of high vacuum environment operations over a large temperature range (survival 313K to 250K; operational 293K to 260K)
• If needed, it is acceptable to have through holes in the mirror within the clear aperture. These holes and resulting mount obstructions should be minimized in size. (Note: A proof of concept analysis has been completed by Sigmadyne that demonstrates a 9-point whiffletree design.)
• The surface figure requirements apply to the mirror in the mount at temperature.
• Repeatability should be demonstrated by test. It should be noted that this is repeatability of the assembly. This could be the ability to remove the mirror from the mount or the mirror/mount system removal from the support system. As discussed in Section 3.2.7, the system must be shipped to NASA and reconfigured with a demonstrated repeatability for surface figure.
• Some initial ideas/concepts are provided in Appendix A of this RFI. These are for reference purposes and should not be used as design direction.
3.2.7 Tip/Tilt Control and Range
The mirror mount shall provide the ability to remotely tip/tilt the mirror over a range of ±2.5 degrees with an incremental motion of 2.75 arcsec or less. In order to assess plate-scale, the commanded tip/tilt should be predictable to 2.75 arcsec and verified during verification testing.
No rotation or decenter adjustment is required.
3.2.8 Packing and Shipping
The mirror system will need to be transported from the supplier's facility to Greenbelt, MD and installed into the NASA Goddard vacuum chamber. As discussed earlier, this activity should not impact the surface figure verification as previously measured.
3.3 Thermal Control System
The thermal control system is critical to the optimum performance of the ACF system. It can be assumed that cable feedthroughs will be provided as required by NASA in order to operate the system during optical testing.
The thermal control system should provide enough zones to allow the ACF performance to be tuned in real time as the thermal environment is further understood. An estimated number of harnesses and bundle sizes to be provided as part of the response.
The system should:
• Provide a thermal shroud for the back and sides of the mirror. Note that the shroud could extend over the front face, outside the clear aperture in order to minimize gradients.
• Likely the design will only have the low emissivity front face of the mirror exposed to the chamber environment. In Section 3.2.5 is was stated that the mirror could be coated outside the clear aperture.
• In addition to controlling the bulk temperature of the ACF, the real key to providing optimum and predictable performance is controlling the gradients in the ACF. The axial gradient will be the most important. Given this design constraint, are there other control algorithms that could be incorporated to minimize these gradients? For example, the 260K operating temperature is notional. Initial analysis by NASA shows that there is little heat transfer between the mirror and the telescope. So the actual bulk temperature of the ACF is not critical. A control law for the thermal control system may be to drive the axial gradient to near zero and allow the mirror temperature to float downward over time. The control scheme would require high homogeneity in the ACF material since CTE variations would cause surface figure errors over due to the transient nature of the bulk temperature.
4 RFI Response Overview
The response to this RFI should be approximately 10 pages. The specific information is listed below:
• Comments regarding the mirror blank section. Material selection is expected to be a low CTE glass or glass ceramic material. Further insight into specific material(s) are welcomed.
• Comments regarding the mounting scheme envisioned. Note that the eventual RFP will not specify the mount, only the performance requirements. The proposed mounting technique will help inform NASA regarding the feasibility and mount sensitivity that would be expected. We realize that the space limitation will create a challenge for manufacturing and long term stability/repeatability.
One of the areas of concern is the repeatability of the mount. The ability to ship the system from the contractor's facility to Greenbelt, Maryland and install in the vacuum chamber while still relying on the original surface figure metrology.
The other area of concern is the thermal performance of the ACF. As the mirror cools in the mount, will strains be created that are not repeatable.
• A list of technical risks that potentially need to be addressed should be provided along with a mitigation plan.
• A cost and schedule WAG for the following:
o Mirror blank procurement of a 1.5m or 2.5m blank as described in Section 3.1
o ACF System Procurement
1.5m clear aperture system
2.5m clear aperture system
o Thermal control system (Note that it is assumed that the TMS would be completed in parallel with the ACF system procurement and would be off the critical path)
Responses should be marked proprietary. Cost and schedule information can be provided in an addendum to the technical response.
Julie Anne Janus, Senior Contracting Officer, Phone 3012864931, Email julie.a.janus@nasa.gov
