The Integration and Testing Program for the Simons Observatory Large Aperture Telescope Optics Tubes
Kathleen Harrington, Carlos Sierra, Grace Chesmore, Shreya Sutariya, Aamir M. Ali, Steve K. Choi, Nicholas F. Cothard, Simon Dicker, Nicholas Galitzki, Shuay-Pwu Patty Ho, Anna M. Kofman, Brian J. Koopman, Jack Lashner, Jeff McMahon, Michael D. Niemack, John Orlowski-Scherer, Joseph Seibert, Max Silva-Feaver, Eve M. Vavagiakis, Zhilei Xu, Ningfeng Zhu
TThe Integration and Testing Program for the SimonsObservatory Large Aperture Telescope Optics Tubes
Kathleen Harrington a , Carlos Sierra b , Grace Chesmore b , Shreya Sutariya b , Aamir M. Ali c ,Steve K. Choi d,e , Nicholas F. Cothard f , Simon Dicker g , Nicholas Galitzki h , Shuay-Pwu PattyHo i , Anna M. Kofman g , Brian J. Koopman j , Jack Lashner k , Jeff McMahon a,b,l,m , Michael D.Niemack d,e , John Orlowski-Scherer g , Joseph Seibert h , Max Silva-Feaver h , Eve M. Vavagiakis d ,Zhilei Xu g,n , and Ningfeng Zhu ga Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL, 60637, USA b Department of Physics, University of Chicago, Chicago, IL 60637, USA c Department of Physics, University of California, Berkeley, Berkeley, CA, 94720, USA d Department of Physics, Cornell University, Ithaca, NY 14853, USA e Department of Astronomy, Cornell University, Ithaca, NY 14853, USA f Department of Applied and Engineering Physics, Cornell University, Ithaca NY, 14853, USA g Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA, 19104,USA h Department of Physics, University of California, San Diego, La Jolla, CA 92093, USA i Department of Physics, Stanford University, Stanford, CA 94305, USA j Department of Physics, Yale University, New Haven, CT 06520, USA k Department of Physics and Astronomy, University of Southern California, Los Angeles CA,90089, USA l Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA m Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA n MIT Kavli Institute, Massachusetts Institute of Technology, Cambridge, MA, 02139, USADec. 15 2020
ABSTRACT
The Simons Observatory (SO) will be a cosmic microwave background (CMB) survey experiment with threesmall-aperture telescopes and one large-aperture telescope, which will observe from the Atacama Desert in Chile.In total, SO will field over 60,000 transition-edge sensor (TES) bolometers in six spectral bands centered between27 and 280 GHz in order to achieve the sensitivity necessary to measure or constrain numerous cosmologicalquantities, as outlined in The Simons Observatory Collaboration et al. (2019). The 6 m Large Aperture Telescope(LAT), which will target the smaller angular scales of the CMB, utilizes a cryogenic receiver (LATR) designedto house up to 13 individual optics tubes. Each optics tube is comprised of three silicon lenses, IR blockingfilters, and three dual-polarization, dichroic TES detector wafers. The scientific objectives of the SO projectrequire these optics tubes to achieve high-throughput optical performance while maintaining exquisite control ofsystematic effects. We describe the integration and testing program for the SO LATR optics tubes that will verifythe design and assembly of the optics tubes before they are shipped to the SO site and installed in the LATRcryostat. The program includes a quick turn-around test cryostat that is used to cool single optics tubes andvalidate the cryogenic performance and detector readout assembly. We discuss the optical design specificationsthe optics tubes must meet to be deployed on sky and the suite of optical test equipment that is prepared tomeasure these requirements.
Further author information: (Send correspondence to K.H: [email protected]) a r X i v : . [ a s t r o - ph . I M ] F e b . INTRODUCTION Observations of the cosmic microwave background (CMB) continue to revolutionize our understanding of thecontents and evolution of the universe and Simons Observatory (SO) is one of the next observatories dedicatedto extending these observations to new regimes. SO consists of one Large Aperture Telescope (LAT) optimizedto target smaller angular scale fluctuations in the CMB temperature and polarization and three Small ApertureTelescopes (SATs) targeting the larger angular scales. The LAT will observe over 40% of the sky across sixfrequency bands from 27 to 280 GHz, where the bands are chosen to span the Galactic foreground minimaand avoid regions of high atmospheric emission. The high resolution maps made by the LAT will contain thedamping tail of the CMB power spectra, enabling measurements of cosmological parameters such as the sum ofthe neutrino masses. These maps will also measure the gravitational lensing of the CMB as it traverses theuniverse and observe the thermal and kinematic Sunyaev–Zeldovich effects from thousands of galaxy clusters;observations of the late time universe which can shed light on the current tension between early and late-timemeasures of the Hubble parameter. The LAT has two 6 m mirrors in a cross-Dragone configuration that focus light into the Large ApertureTelescope Receiver (LATR) cryostat. This 2.4 m diameter cryostat is designed to hold up to 13 optics tubes,seven of which will be populated for the initial phase of SO. Each optics tube uses three silicon lenses, anti-reflection coated with meta-material layers diced into the surfaces, to reimage the incoming light onto a focalplane comprised of three Universal Focalplane Modules (UFMs), that house the detectors and coupling opticsfor each frequency band. Each pixel in the UFM is dichroic and dual-polarization, meaning three flavors of UFMsat low, mid, and ultra-high frequency (LF, MF, and UHF, respectively) are required for the six frequency bandsused in SO. Each optics tube will contain three UFMs of the same flavor with AR coatings and low-pass edgefilters matched to those particular bands. In total, the seven optics tubes in the initial phase of SO will containnearly 32,000 transition edge sensor (TES) detectors
14, 15 that will be read out using microwave multiplexing( µ Mux) SMuRF electronics.
16, 17
The scientific goals of SO require high sensitivity maps with low levels of systematics effects. Achieving theseunprecedented requirements necessitates quality control at all levels of the design and integration process. Herewe discuss the integration and testing program for the SO LATR optics tubes. In Section 2 we describe thesingle optics tube test cryostat that has been developed using an SAT cryostat to decouple the optical testingfrom the development of the LATR cryostat and in Section 3 we provide an overview of the identified criticalmetrics that require optics tube level testing before deployment to the SO site.
2. OPTICS TUBE TESTING PLATFORM: LATR TESTER
The optical testing of the LATR optics tubes was decoupled from the LATR integration and testing to optimizethe personnel and time resources before deployment. A fully loaded LATR cryostat is expected to take 35 daysto cool down to 100 mK. A quick turn around single-tube test cryostat will perform optical validation mea-surements in parallel to LATR integration and allows for more and varied cool downs of optics tubes if designchanges are necessary. In addition, decoupling Optics Tube testing from LATR integration means Optics Tubetesting can continue after the LATR has been shipped to Chile and LAT commissioning has begun.This LATR tester (LATRt), shown in Figure 1, was built out of one of the SO SAT cryostats that is slated forlater deployment after LATR optics tube testing is completed. The design of the SAT cryostats was presentedin Galitzki et al. 2018. Each SAT cryostat has four temperature stages nominally at 40, 4, 1, and 0.1 Kelvin.The “backend” of these cryostats contains the cryocoolers and the housekeeping and detector readout paths andthe “frontend” holds the different optical elements. Cooling power is provided by a Bluefors SD400 DilutionRefrigerator ∗ backed by a Cryomech PTC-420 cryocooler † and an additional PTC-420 mounted directly to thebackend 40 K and 4 K shells. In total, each SAT is expected to have 110 W of cooling power at 40 K, 4 W at4 K, 10 mW at 1 K, and 400 µ W at 100 mK.The mechanical design of the LATR was presented in Zhu et. al. 2018. When compared to the SAT, thelength of the LATR optics tubes is significantly longer. Converting an SAT into the LATRt required extending ∗ https://bluefors.com/products/sd-dilution-refrigerator/ † ix G10 Support Tabs80K pseudo-stage with shield Two-piece LATR Front EmulatorHeat strapping between SAT DR and OT Cold Fingers 4K Adapter Plate for OT Mounting40K Front
Extension
Vacuum ExtensionMeta-material Black Tiles in OT Front End 1025 mm
NDF Mount
Figure 1. The upper figure shows a cross-section of the LATR-Tester cryostat while the lower figure shows a more detailedsection of the front-end developed to emulate the optical setup of the LATR cryostat. The various alterations made tothe SAT cryostat to enable LATR optics tube testing are indicated.igure 2. Load curves taken with the LATRt Bluefors SD400 Dilution Refrigerator (left) held vertically before installationin the LATRt cryostat and (right) held at 27 . ◦ after installation in the LATRt but with no connections on the still ormixing chamber. both the frontend vacuum shell and the 40 K frontend. This was accomplished by replacing the vacuum shellintended for the SAT half-waveplate with a longer version that placed the window the correct distance from theUFMs. The LATR window plate is about 6 cm thick, a thickness that is required due to its 2.3 m diameter.This thickness is unnecessary in the LATRt, so a second aluminum piece is attached to the vacuum side of theLATRt window plate to emulate that thickness and hold the 300 K double-sided IR (DSIR) filters in positionsto match the LATR.In addition to the extra required length, the LATR has an addition temperature stage, nominally at 80 K,cooled by single-stage Cryomech P90 pulse-tube coolers. This stage holds additional DSIR filters and a meta-material AR coated alumina IR absorbor. Six G10 standoffs are used to build a pseudo-80 K stage in the LATRt,this stage is thermally connected to the 40 K stage via four two-layer heat-straps of 5N Aluminum. In practice,this stage is completely thermally coupled to the 40 K stage below. Lastly, a 4 K optics tube mounting platewas installed on the 4 K backend and the SAT frontend 4 K shell was removed to further emulate the LATRcryogenic setup.Housekeeping and detector readout is achieved with very few changes compared to the SAT and optics tubearchitecture. This is due to the interchangeability between the LATR and SAT systems. The warm and coldthermometry breakout boards and wiring for both cryogenic systems are nearly identical; only one additionalcryogenic cable is required to incorporate LATR optics tube housekeeping into the SAT system. Similarly, the µ Mux readout for the LATR and SAT cryostats use identical universal readout harnesses (URHs) for readoutcomponents between 300 K and 4 K. Connecting the LATR optics tube cold readout assembly to the SATURH only required 12 additional runs of isothermal hand-flexible coaxial cables.The LATRt completed several integration and calibration cooldowns before the first optics tube installation.The installations of the pulse-tube cryocooler and the dilution refrigerator were validated. Load curves at allstages were taken before the optics tube was installed in the LATRt. Figure 2 shows the Bluefors SD400 DRcapacity map before it was installed in the LATRt (left) and after it was installed in the LATRt (right). Inthe LATRt it operates at a 27 . ◦ angle with respect to gravity, ‡ which reduces its cooling capacity. The basetemperature before installation was below 7 mK and it was about 12 mK in the LATRt without an optics tubeattached. Even tilted at 27 . ◦ and set inside a 4 K shell, the DR mixing chamber has 400 µ W of available cooling ‡ The tilting of the DR is due to the in-lab setup, the deployment configuration for the SAT DRs is close to 0 ◦ . ower at 100 mK. This level of cooling power is nearly equal to that of the LATR DR and completely sufficientfor running one LATR optics tube.In subsequent tests, the LATRt has been used to measure the cryogenic optical loading of the front-endfiltering scheme for the LATR. The combined optical loading on the 80 K and 40 K stage was 9.7 W for the setof filters tested in the LATRt. § This was higher than the expected loading of 3.9 W and was traced to a higherthan expected blue-leak above the DSIR cutoff frequency. The optics tube and URH were also installed and theparasitic cryogenic loading from the optics tube was measured to be 4 . ± . µ W on the 100 mK stage, in goodagreement with expectations and measurements in the LATR. With this final set of validations complete, theLATRt is ready to proceed with the LATRt optics tube testing program.
3. REQUIREMENTS FOR INTEGRATION TESTING
The LATRt testing program is designed to ensure the LATR Optics Tube design achieves, at a minimum, thebaseline noise requirements laid out in Simons Observatory Collaboration et al. Discounting survey parameters,such as scan pattern and integration time, the noise levels in SO maps will be determined by the array NoiseEquivalent Temperatures (NETs) of each deployed optics tube. As discussed in Hill et al., if every detector inan array has the same properties, the array NET isNET arr = Γ √ (cid:113) NEP + NEP + NEP (cid:16) dPdT CMB (cid:17) √ Y N det . (1)In this equation, the Noise Equivalent Power (NEP)s are the contributions to the total single-detector NEP(in W / √ Hz units) from photons, thermal carrier noise, and readout noise, respectively, dPdT
CMB is the end-to-endoptical efficiency that is used to convert between units of power and temperature, N det is the number of detectorsin the array, Y is the yield, the fraction of working detectors, and Γ is the level of optical white noise correlations.The LATRt testing program focuses on the aspects of this equation that depend on the performance of the LATROptics Tubes and cannot be tested at smaller scales (i.e. testing of individual detector modules). The testingprogram will validate the designs of the different flavors (LF, MF, and UHF) of optics tube, meaning one tubeper flavor will undergo the tests laid out below. The denominator of Equation 1 has a few different factors that affect the overall sensitivity of the array. The
Y N det combination represents the number of working detectors in any array. This will be measured for individualdetector modules (UFMs), and again once the the UFMs have been integrated into the Optics Tube with allthe associated readout components. Each MF and UHF UFM will contain 864 optical detectors per frequencyband while each LF UFM will have 74 optical detectors per band. The SO baseline noise projections assume a70% yield across all arrays.Each Optics Tube tested in the LATRt will be cooled down with a blank-off plate and a cold load at the endof the optics tube at 4 K. IV curves, saturation powers, and dark noise levels will be measured for all detectorsin this configuration to ensure the arrays have been successfully integrated into the optics tube.The blank-off plate will be removed and replaced with up to three different neutral density filters (NDFs)designed to cut the optical loading on the detectors to less than 1 / ∼
293 K environment is much brighter than the ∼
10 K atmospheric signalexpected during normal operations at the Atacama site. The NDFs used will be primarily absorptive, made outmachinable Eccosorb ¶ of various loss levels and thicknesses which will be tuned to the frequency bands undertest. The 4 K NDF mounting plate is designed to allow changes between different NDF options in situ, withoutremoval of the entire optics tube. Successive cool-downs with different NDF properties can be used to further § A similar level of loading was observed for this filter set in the LATR ¶ alibrate the transmission of the NDFs or to optimize the optical loading between the upper and lower bands oneach UFM.The end-to-end optical efficiency, the dP/dT cmb factor in the denominator of Equation 1, is a componentthat cannot be tested without a fully integrated optics tube. As described in Hill et. al., the end-to-end opticalefficiency accounts for the transmission through every optical element as well as the stop efficiency and thedetector efficiency. This factor will be measured using chops of beam filling thermal loads ranging from 293 to350 Kelvin. These thermal loads are 1 m by 1 m plates covered with meta-material absorbing black tiles (see Ref.23) mounted on sliders positioned above the window of the cryostat. Heater elements on the back of these plateswill actuate the temperatures of the tiles and an infrared thermometer will be used to measure the temperatureson the side of the plates facing the cryostat window. These measurements will be performed with multiple NDFoptions in front of the same detector/optical setup to simultaneously calibrate the NDF transmission and theoptical efficiency of the rest of the system. The combined in-lab measurements of detector yield and end-to-endoptical efficiency must be sufficient to achieve the baseline array NETs required for the SO science goals. The numerator of Equation 1 contains the different contributions to the single-detector NEPs for the array.Simons Observatory, as with most CMB experiments, aims to be photon noise limited, meaning the NEP dueto photons is larger than the thermal carrier noise and readout noise. For the wavelengths of interest here, bothshot noise and wave noise contribute to NEP ph , but both of these factors depend on the total optical loading onthe detectors.Due to the nature of in-lab characterizations, including the need for NDFs and absence of the 6 meter LATmirrors, the optical loading during optics tube testing will not match the expected on-sky loading. However,several instrument performance parameters that are expected to impact the on-sky optical loading can be verifiedin-lab. The two of these that are most important for the science goals of the LAT are end-to-end bandpass andspill to 300 K. The end-to-end bandpasses of a CMB instrument affects both the overall sensitivity of the instrument and moresystematic effects such as the separation of galactic foregrounds emission from the CMB signals. Investigating thegalactic foreground separation requirements leads to a final on-sky bandpass calibration requirement of betterthan 0.5% for the SO instruments. However, the requirements for in-lab validation measurements are lessstringent because these measurements are primarily concerned with ensuring the sensitivity of the instrument.A Fourier Transform Spectrometer (FTS) in the style of the PIXIE instrument with refractive couplingoptics will be used to measure the end-to-end bandpasses for the optics tubes. The band-edges for the integratedoptics tube and UFMs must be within 2% of the designed values, ensuring the edges are far enough fromatmospheric absorption features to achieve the desired on-sky optical loading. The measured bandpasses will bepropagated through the SO instrument sensitivity tracking, which must result in projected noise levels consistentwith those in Simons Observatory Collaboration et al. High precision bandpass calibrations will be performedon-site where NDFs are unnecessary.
The fraction of the beam that spills to 300 Kelvin has a significant effect on the optical loading on the detectors.During the design of the LAT instrument, it was found that spill to 300 K was one of the main drivers in theprojected sensitivity overall and that reducing this spill could significantly reduce the noise in the resulting SOmaps.As described in Gudmundsson et al., a substantial effort has gone into controlling the scattering inside theoptics tubes to reduce this warm spill, including the design of meta-material absorbing black tiles that willbe used instead of baffles blackened with carbon-loaded Stycast ‖ in the upper region of the optics tubes. TheLATRt will be verifying that these efforts were successful before the optics tubes are deployed to the site. ‖ igure 3. (Left) The instrument mounting structure designed to hold the different sets of optical test equipment androll over the LATRt cryostat during operation. Sliders, shown in blue, will be used to measure the end-to-end opticalefficiency through chopping of beam-filling thermal sources at different temperatures. (Right) The XY-stage with 150 cmof travel in both directions which mounts to the underside of the instrument mounting structure and will be used for boththermal and holographic beam mapping. The upgraded optics tube baffling design reduces the expected scattering at large angles ( (cid:38) ◦ ) from -25 dBto -50 dB levels. Similarly, constraining the spill outside of the secondary mirror to a 1% level will requiremapping the beam down to about -33 dB to measure a significant enough fraction of the total beam.The LATRt will use two different types of beam mapping to verify the efficacy of the black tiles and constrainthe expected warm spill. First, we use a 600 ◦ C IR source mounted behind a spinning chopper wheel with anadjustable aperture. The source will be mounted on an XY stage with 150 cm of travel in both directions. Withthis source, a 25 mm aperture, and a 5% transmissive NDF, we expect to require 1 second of integration time tomeasure a -36 dB signal at a signal-to-noise ratio of one. This sensitivity is expected to be sufficient to verify thatthe black tiles perform better than the carbon-loaded stycast simulations and to map the beam to low enoughlevels to constrain the warm spill to a 1% level.Measuring the amount of scattering expected from the black tiled optics tube requires measuring the beamsdown to a -50 to -60 dB level. This is unlikely to be possible with thermal sources in lab but may be possiblewith holographic measurements of the detector beams. This measurement uses a coherent source and receiverto measure the amplitude and phase of the electric fields across a two-dimensional surface in front of the opticstube window. A coherent receiver will be mounted on a feedhorn array at the optics tube focal plane, withsignals transmitted through the cryogenic coaxial cables used for the SMuRF µ Mux system. The source will bemounted on the same XY stage used for the thermal beam mapping. These beam maps will be compared tosimulations of the expected beams at the measurement plane to verify the effectiveness of the black tiles.
4. STATUS AND OUTLOOK
At the time of these proceedings, the LATRt cryostat, pictured in Figure 4, has been integrated and cryogenicallytested with all major components except UFMs (detectors) and the optics tube IR filters. Those will be installednext and the LATRt Testing program will begin characterizing the performance of the first MF optics tube. Indepth testing is currently planned for one of each flavor (LF, MF, UHF) of optics tube to verify the design of igure 4. The backend of the integrated LATRt cryostat, showing the installed optics tube, universal readout harness,housekeeping, and Bluefors DR insert. Custom heat straps are used to connect the DR stages to the optics tube coldfingers that provided a cooling path the the 1 K and 100 mK stages. ach of type of optics tube. Testing on the other four optics tubes that are part of the nominal-SO plan willdepend on hardware availability and deployment schedules.
ACKNOWLEDGMENTS
This work was funded by the Simons Foundation (Award
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