eLISA_AttachmentA.docx

eLISA Prototype Testbed Telescope

Telescope Information Package

Attachment A

Prepared by the eLISA Study team Aug 18, 2016

Jeff Livas, PI Goddard Space Flight Center Code 663 Greenbelt MD 20771 301-286-7289

Table of Contents

Introduction The eLISA (Evolved Laser Interferometer Space Antenna) program and the Goddard Space Flight Center will accept proposals for an all-silicon carbide (SiC) prototype – or “brassboard” – telescope, designed to meet the optical requirements of the flight telescopes, with additional elements designed to facilitate specific laboratory tests in dimensional stability and scattered light suppression. Goddard will provide an off-axis, reflective, four-element reference design baseline, and the vendor will develop and provide, in collaboration with the eLISA program, an opto-mechanical design, with thermo-mechanical analysis, and will fabricate, align, and deliver the telescope.

The eLISA telescope has unique requirements for which measurement techniques are being developed at the Goddard Space Flight Center. Early developmental work by the program has shown that a simple symmetric telescope structure of silicon carbide, with its combination of high thermal conductivity and moderate CTE, is capable of meeting the strict pathlength stability requirement. Design studies have shown that scattered light suppression requires an off-axis design. A prototype for testing the suppression of scattered light was procured last year and is being tested now. The next stage, and the object of this RFQ, will be a telescope capable of meeting all the optical requirements of the flight telescopes, including scattered light suppression and path length stability. The final telescope mechanical design will include a provision for testing the path length stability as shown in the accompanying mechanical reference design baseline and measurement concept described below. It will also include a provision for mounting small auxiliary mirrors necessary for testing path length stability of just the primary/secondary spacing as shown in the reference design. (See Figure 6.)

The successful bidder will show that they have a mastery of opto-mechanical design, fabrication and alignment of an all-SiC off-axis telescope to be operated at -70C (203K), with no potted invar inserts. Other materials that can satisfy the requirements are possible by agreement with NASA Goddard.

Interested organizations are invited to discuss the requirements described here with the study team.

Functional Description of the eLISA Flight Telescope (for reference only) The primary function of the flight telescope is to efficiently deliver laser light to the far field on axis, while maintaining optical path length [OPL] stability through the telescope.

The application is for a transceiver as part of a precision displacement measurement system. It shall simultaneously transmit 1 W of power and receive approximately 100 pW. The transmitted signal power shall back-scatter less power from the transmitter than is received into the solid angle of the receiver.

As part of a precision displacement measurement system, the optical path length [OPL] through the telescope shall be stable to interferometric tolerances of 1 pm over the time for enabling science measurements (~ 10,000 sec)]. (See Table 1 for more details.)

The telescope subsystem shall have the following high-level characteristics:

· Unobstructed reflecting telescope

· Optical transmission > 0.85 at 1064 nm, and > 0.2 at 633 nm (for testing)

· Afocal 40X magnification

· 200mm Entrance Pupil Diameter

· nominal 5 mm circular diameter collimated beam to the optical bench

· real exit pupil

· Focus adjustment to be used in a set-and-forget mode. (Active or passive.) It is anticipated that the eventual flight model will require an adjustable mechanism to optimize the focus. In order to save cost, this prototype does not necessarily require this feature, but the telescope shall be designed so that the focus may be adjusted manually to meet WFE (requirement 13), and an accommodation left for an electronic adjustment. Note that the adjustment mechanism shall not spoil the pathlength stability performance.

Requirements of the Prototype Testbed Telescope Table 1 gives the requirements of the prototype testbed telescope that is the object of this procurement. Requirements (5) and (8) are specific to the use of the telescope for metrology.

Superscripts in the table refer to the notes on the following page.

Table 1’s requirements are mostly optical and functional in nature. In particular, there are no specifications for mechanical or thermal shock, mechanical vibration, or acoustical sensitivity. This is because the telescope is a prototype intended for laboratory use and testing only at this stage of development. We expect that the final mechanical design for the prototype will be developed using experience and techniques that do not exclude flight, but we do not expect them to meet all of the eventual flight requirements.

Table 1: Summary of requirements for the Telescope Subsystem.

From

where 0.0001 < f < 1 Hz f0 = 0.003 Hz 1 pm = 10-12 m See note 5 below for the definition of “flight conditions” Add fluctuation model. See note 6 warning about materials.

> 0.2 at 633 nm 633 nm for testing Per mirror, for 4 mirrors, this is R> 0.96 at 1064 nm, and R> 0.66 at 633

noise

(Verify by model)

Notes for Table 1: [Requirements (5) and (8) are the most challenging]

1) The Field of Regard is the range of angles over which we need to be able receive signals over the full orbit. The telescope shall meet all specifications over this Field of Regard except for scattered light.

2) The Field of View [FOV] is the range of angles over which we will be making measurements under closed-loop tracking in Science Mode. The telescope shall meet all specifications over this Field Of View including scattered light.

3) The intended application of the prototype telescope subsystem includes the role of a scattered light test bed. The reference optical design meets the scattered light specification but without a detailed structural model for scattered light. The final optical design must meet the scattered light specification at a similar level of detail. The mechanical design must include a clearly defined mirror mounting scheme that does not introduce stress into or contamination onto the mirrors during alignment, transport, etc Note that the contamination requirement is driven by the scattered light performance. Stress is limited by degradation of the WFE and by damage to the mirrors.

4) Optical path length [OPL] is the net total path length through the telescope as experienced by either the transmitted or received beam from input pupil to exit pupil, which can be defined as the accumulated phase divided by the wavenumber (2), where lambda is the design wavelength, 1064 nm, although, as mentioned, the measurement wavelength may be in the visible region with the proper conversion. The key specification is that the telescope subassembly OPL shall be stable with time over the thermal operating temperatures.

5) Flight conditions for the purposes of this prototype are defined as a soak temperature T ~ 203 +/- 3 K (-70 +/- 3 C), with temperature fluctuations of 10-6 K/√Hz at a frequency of 10-4 Hz. The supplier shall directly test compliance with the WFE requirement at room temperature, defined as T = 298K +/- 3K, and at the soak temperature by analysis, as approved by NASA Goddard – including the stability of the focus adjustment mechanism, if any. Optical Path Length (OPL) stability at both room temperature and the soak temperature, shall be demonstrated by analysis.

6) The materials must have demonstrated capability to achieve the level of stability required and must not have any known materials creep or abrupt length change issues such as Invar. In addition, most composites have a coefficient of moisture expansion (CME) and long out-gassing timescales. In analogy with the specification of a surface figure to include mid-spatial frequency variations, materials that have long term creep or abrupt length changes as internal material stresses slowly relax can generate optical path length changes within the measurement band of 0.1 to 0.0001 Hz. Two relevant references for both notes 5 &6:

Sanjuan, J., Preston, A, Spector, A., Korytov, D., Freise, A.,Dixon, G., Mueller, G., Livas, J. “Note: silicon carbide telescope dimensional stability for space-based gravitational wave detectors”, Rev. Sci. Instrum. 83(11), 116107 (2012).

Sanjuan, J., Preston, A, Spector, A., Korytov, D., Freise, A.,Dixon, G., Mueller, G., Livas, J. “Carbon fiber reinforced polymer dimensional stability investigations for use on the laser interferometric space antenna mission telescope”, Rev. Sci. Instrum. 82(12), 124501 (2011); doi: 10.1063/1.3662470

7) The Telescope Subsystem input and output shall have an alignment feature to enable the alignment of the input and output optical axis to an outside instrument, e.g. interferometer, theodolite, or similar device. Such a feature shall be proposed to and approved by NASA Goddard.

8) A clean structure is required: low volatiles, including epoxies, lubricants, and materials consistent with vacuum use (this is a prototype for space, so we need to be sure we can meet the requirements with materials that could be used in a flight unit)

9) A sample optical design that may be used follows the prescription in the attached Code V sequence file and the mirror specs are from the accompanying spreadsheet. This design meets the requirements in Table 1, but may be modified or improved in consultation with GSFC.

10) The telescope should be assembled and aligned in a clean room consistent with the particulate contamination specification needed to meet the scattered light requirements.

Reference Optical Design Attached is a reference optical design [Code V seq file] that meets optical specifications. Functionally the design is an afocal beam expander with a 200 mm diameter large aperture and a 5 mm output small aperture, for a magnification of 40X.

Figure 1 shows an optical layout. The basic design is an optimized off-axis Cassegrain with an off-axis nearly parabolic primary mirror (M1), and an off-axis nearly hyperbolic aspherical secondary mirror (M2). These two mirrors bring the incoming beam to an intermediate focus, which is recollimated by the M3/M4 mirror pair. The mirrors are mounted in a compact sub-housing (see Figure 4) such that the pair may be moved together to keep the telescope properly focused.

The reference design may be used “as is” or as a starting point for a custom design. In any case, the Telescope Subsystem specifications shall be met regardless of the design used. A new or modified design shall be presented to NASA Goddard for approval.

Figure 1: Reference design optical layout. The desgin is an afocal 40X telescope with a 200 mm large aperture and a 5 mm small aperture. Design is an optimized Cassegrain with a Schwarzshild pupil extender. M3 and M4 move together as a pair to maintain focus. Please see attached CodeV sequence file for details.

Reference Mechanical Design Figures 2-5 below shows a reference mechanical design. The CAD drawings for this design are in three files that accompany this document. Since the telescope will be fabricated at room temperature, but used at 203K (-70C), the intent is to either make an athermal design, or to fabricate the entire telescope out of a material such as silicon carbide.

Shown are the essential pieces of the off-axis telescope, and plus two additional features that are part of the testing configuration. These are the three legs, and a 12” flat mirror, shown partially transparent and on the lower right. The 12” flat test mirror is not part of the baseline telescope design to keep costs low. We will try to find an existing mirror at Goddard that may be used, but a separate bid for supplying an appropriate mirror would be welcome.

The secondary mirror is supported with a single post as shown in Figure 3. The concept with all of the mirrors is to have the mirror mount reference the front surface of the mirror so that the metering between surfaces is controlled directly by the structure and does not vary with temperature changes. The concept for alignment of the secondary mirror is to lap an interface piece between the post and the flat plate that holds the mirror, and bolt it into place with pre-loaded bolts.

Figures 4 and 5 show the compact mounting arrangement for M3 and M4, designed so that the pair of mirrors may be moved together to keep the telescope in focus. To keep cost low, it is not necessary to actually mount an actuator on the prototype, but some accommodation should be made for manual motion of the mirror pair, and space allocated for a mechanism. NASA Goddard should be consulted for requirements and a recommendation for a mechanism.

Figure 2: Basic reference mechanical design. The blue lines trace rays through the telescope to guide the eye. The brown mirror at the lower right is an oversized reference flat that is not part of the telescope design, but would be used for testing. See Figures 8 and 9 for more details. The three legs with round bottoms are also not part of the baseline telescope design, but instead for the basis for supporting the telescope above the reference flat for testing. See Figures 8 and 9. CAD files are attached to this document.

Figure 3: Details of the secondary mirror mount. The green structure shown in the right hand view is a mounting piece that can be lapped to position the secondary mirror correctly, and then the structure is held in place with pre-loaded bolts.

Figure 4: Details of the M3/M4 mount with provision for focusing adjustments. M3 and M4 move together as a pair. A focus adjustment mechanism would be mounted in the structure at the lower left. The same structure on the upper right holds the base of the boom that supports the secondary mirror.

Figure 5: Cutaway section view of the M3/M4 mirror mount shown in Figure 4, showing baffle at the intermediate focus.

Alternate Mechanical Design Concept Figures 6 and 7 show an alternate conceptual mechanical design with more symmetry than the design shown in Figure 2-5. Here the metering structure and secondary mirror mount is a cylinder, and as with the design show in Figures 2-5, would be best made out of a single material such as silicon carbide to allow us to fabricate and align the telescope at room temperature, but operate at -70C (203K).

Figure 6: Section view of an alternative mechanical design. Metering between M1 and M2 is provided by a cylindrical structure rather than a single boom.

Figure 7: Section view of the same alternative mechanical design as shown in Figure 6 from a slightly different vantage point.

Optical Pathlength Test Configuration One of the most challenging requirements is the optical pathlength stability. We do not expect a direct test, but we do require some analysis to show why the mechanical design should be stable. The figure below shows the proposed test configuration.

The basic idea is to turn the optical path through the telescope into part of a Fabry-Perot cavity. A laser is then frequency stabilized to this cavity, and the measured frequency stability is compared with that of an identical laser locked to a reference cavity that we have previously characterized. The frequency fluctuations are used to estimate the length fluctuations. The temperature of the telescope and the test environment is measured carefully at the same time, and the effect of the temperature fluctuations is calculated and compared against the estimated length fluctuations.

The suggested analysis to demonstrate path length stability is to use a thermal model with the expected temperature fluctuations to demonstrate mechanical motion consistent with the optical pathlength requirements. The expected temperature fluctuations are 3 x 10-7 K/√Hz at a frequency of 10-4 Hz on the primary mounting structure through conduction, and an all over temperature fluctuation of the same magnitude due to radiation.

For further details, this measurement process has been described here:

Sanjuan, J., Note: Silicon carbide telescope dimensional stability for space-based gravitational wave detectors, in Rev. Sci. Instrum. 83, Issue 11, 28 November 2012.

Figure 8: Reference optical layout for optical pathlength stability testing. The basic idea is to form a Fabry-Perot cavity with the telescope, a reference flat, and an additional external mirror. A laser is frequency-locked to the cavity, and the stability of the frequency is compared against the stability of the laser locked to a single reference cavity.

The mechanical design should have a provision to allow the mounting of small (6 or 12.7 mm diameter as appropriate) auxiliary mirrors that are supplied separately and independently from the telescope. These small mirrors could be used to form a Fabry-Perot cavity to allow testing of the primary to secondary mirror spacing as a diagnostic should it proved difficult to measure the optical path length through the complete telescope. As shown in Figure 9, these mirrors would be placed at either end of the metering structure for the primary to secondary distance, shown here in cross section as a single boom.

Figure 9: Reference design mechanical layout showing the cross section of the secondary mirror support arm. The support arm is hollow and has provisions for small auxiliary mirrors (supplied separately) to be mounted at either end to allow making a Fabry-Perot cavity out of the support arm.

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