Design optimization of a lightweight secondary mirror assembly for a 1-m class ground telescope for satellite laser ranging

Launched December 25th 2021 to its final destination at the Sun-Earth Lagrange Point 2, JWST will revolutionize the way the universe is seen back to the early universe beginnings. During the six month commissioning phase of the mission, there were various activities performed which aligned the segmented telescope from eighteen unique Point Spread Functions (PSFs) to a single PSF. One of these steps is to globally align the Secondary Mirror (SM) Assembly (SMA) to the 18 Primary Mirror Segment Assemblies (PMSA) in the primary mirror. This alignment process happens in two phases, and they are named Global Alignment 1 (GA1) and Global Alignment 2 (GA2). A successful completion of GA1 entails coarsely aligning the SM and each PMSA to achieve an RMS Wavefront Error (WFE) less than 200nm. This is accomplished by generating PMSA wavefront maps which are used to determine a correction in the SM using Phase Retrieval and Decomposition. One of the requirements is that the Fine Guidance Sensor (FGS) can guide on a segment during the exposures, minimizing the impacts of motion blur on the images. This paper details a contingency method on how global alignment can be achieved using the observatories star trackers for guiding. The star trackers line of sight (LOS) stability and pointing stability performance are intended for coarse guiding whereas FGS provides fine guiding. It is expected that motion blur will increase should the observatory use coarse guiding during the GA1 exposures. Within the paper, we detail the maximum motion thresholds needed to achieve the desired placement of the SM, as well as exposure methods which can be used to handle motions that exceed the thresholds.

For its compact size and light weight, space telescope with deployable support structure for its secondary mirror is very suitable as an optical payload for a nanosatellite or a cubesat. Firstly the realization of a prototype deployable space telescope based on tape springs is introduced in this paper. The deployable telescope is composed of primary mirror assembly, secondary mirror assembly, 6 foldable tape springs to support the secondary mirror assembly, deployable baffle, aft optic components, and a set of lock-released devices based on shape memory alloy, etc. Then the deployment errors of the secondary mirror are measured with three-coordinate measuring machine to examine the alignment accuracy between the primary mirror and the deployed secondary mirror. Finally modal identification is completed for the telescope in deployment state to investigate its dynamic behavior with impact hammer testing. The results of the experimental modal identification agree with those from finite element analysis well.

The James Webb Space Telescope (JWST) has a primary mirror, made of 18 segments, and a secondary mirror (SM) that are used to direct the light of desired targets. After launch, the secondary mirror assembly (SMA) is stowed for approximately 10 days and is subject to molecular contamination outgassing from the cavity of the secondary mirror support structure (SMSS) in-board hinge (IBH) which contains cables, motors, resolvers, and coatings. The main concern during this period before SMA deployment is the accumulation of ice due to the lack of a heater on the SMA. The temperature differentials between the IBH surfaces and SMA could cause redistribution of water vapor contamination. To address this concern, single layer insulation (SLI) was reconfigured to direct the vent path of IBH outgassing sources away from the SM. Two separate thermal vacuum (TVAC) tests were performed to quantify this contamination: a Z307 ASTM E 1559 materials test of the radiator paint used on the motor of the IBH and a separate test on the hinge motor from the primary mirror backplane assembly (PMBA) qualification engineering test unit (ETU). The PMBA ETU hinge was similar in design to the IBH. These tests approximately followed the predicted SMA predeployment thermal environment. To quantify source rates in case of a leak in the new SLI enclosure or baffle, the motor and resolver sides were separated, and quartz crystal microbalances (QCM) were used to measure the deposition of water. The SLI redesign and implementation and outgassing measurements to understand leak effects from the IBH were essential to mitigate the deposition of contamination on the SMA.

The James Webb Space Telescope is a large, deployable telescope that will operate at cryogenic temperatures at the Earth-Sun Lagrange 2 point. The Webb Optical Telescope Element (OTE) consists of 18 actively controlled Primary Mirror Segment Assemblies (PMSAs), an actively controlled Secondary Mirror Assembly (SMA), and an Aft-Optics Subsystem (AOS) that contains a fixed Tertiary Mirror and a Fine Steering Mirror. The OTE is combined with the Integrated Science Instrument Module (ISIM) to create the full optical train called OTIS (OTE and ISIM). OTIS has recently undergone cryogenic vacuum testing in Chamber A at Johnson Space Center in Houston, TX. A key outcome of this test was to verify there is adequate range of motion in PMSA and SMA actuators to align them to AOS/ISIM under flight-like conditions. The alignment state of the PMSAs and SMA was measured using photogrammetry and cross-checked optically using a variation of a classical Hartmann test. In the “Pass-and-a-Half” (PAAH) configuration, fiber sources near the Cassegrain focus propagate light through the full optical train and small tilts on the PMSAs create an array of spots on the science instrument detectors, mimicking the effect of a Hartmann mask. Comparison of measured and modeled spot arrays provides the alignment state of the SMA and the global tilt of the primary mirror. This paper will discuss the methodology, testing, and analysis performed to measure the alignment state of OTIS using the Hartmann method and verify the primary and secondary mirrors can be successfully aligned on orbit to meet performance requirements.

The James Webb Space Telescope Primary Mirror Segment Assemblies (PMSAs) and Secondary Mirror Assembly (SMA) were cleaned at the Johnson Space Center (JSC) in January 2018. In order to quantify the effectiveness of the cleaning, the same cleaning process was performed on the PMSA and SMA traveling witness wafers. These wafers have accompanied their respective mirror segments from their arrival at the Goddard Space Flight Center, through transport to JSC, and ultimately their exposure in Chamber A for cryogenic testing. The traveling wafers were analyzed using an Image Analysis automated microscope both prior to and after the cleaning. The resulting data showed that the PMSA wafers' Percent Area Coverage (PAC) reduced by 83.5% on average, from 0.1524 PAC to 0.0251 PAC. The SMA wafer's PAC decreased by 97.2%, from 0.1194 PAC to 0.0034 PAC. Further analysis of the particle size bins was completed in order to calculate their particle distribution slopes. The slope of the PMSA wafers increased by 0.025 on average, and the SMA wafer slope increased by 0.066. This indicates that the ratio of large to small particles slightly increased after the cleaning across all mirror segments. Visual inspections of the wafers and the flight PMSAs and SMA showed considerable and comparable particulate coverage improvements, thus leading to the conclusion that the average PAC on the PMSAs and SMA improved by the same factor as their respective wafers.

Several variations of large space-based observatories have been hypothesized using different approaches to deploying the primary and secondary mirrors on orbit. Careful consideration must also be given to the design and implementation of the shield that protects these observatories from thermal extremes, micro-debris, and controls stray light entry into the optical train. One approach to the shield architecture is use of an Optical Barrel Assembly (OBA), such as that used on the Hubble Space Telescope (HST). For space telescopes much larger than the HST, an OBA will need to be deployed or assembled to form an adequately large structure to fully shield both the primary mirror and secondary mirror. This paper describes the design, prototyping, characterization tests, and test results from two different OBA development efforts. The first design is a combined barrel and secondary mirror support structure. This system was designed for a fixed primary mirror and deploys straight upward along the optical axis, carrying the Secondary Mirror Assembly (SMA) with it. The second OBA design is of a structurally independent OBA that deploys out from behind the Primary Mirror Assembly (PMA) (itself deployed or assembled) and extends forward along the optical axis to completely enclose the optical train, pulling along the shroud material. Examples of both systems were built out of prototype materials, tested, and the test results were compared against modeled predictions of system performance. The designs, test procedures, and test results are presented along with recommendations for future work.

The James Webb Space Telescope (JWST) Secondary Mirror Assembly (SMA) is a circular 740mm diameter beryllium convex hyperboloid that has a 23.5nm-RMS (λ/27 RMS) on-orbit surface figure error requirement. The radius of curvature of the SMA is 1778.913mm±0.45mm and has a conic constant of -1.6598±0.0005. The on-orbit operating temperature of the JWST SMA is 22.5K. Ball Aerospace & Technologies Corp. (BATC) is under contract to Northrop Grumman Aerospace Systems (NGAS) to fabricate, assemble, and test the JWST SMA to its on-orbit requirements including the optical testing of the SMA at its cryogenic operating temperature. BATC has fabricated and tested an Aspheric Test Plate Lens (ATPL) that is an 870mm diameter fused silica lens used as the Fizeau optical reference in the ambient and cryogenic optical testing of the JWST Secondary Mirror Assembly (SMA). As the optical reference for the SMA optical test, the concave optical surface of the ATPL is required to be verified at the same 20K temperature range required for the SMA. In order to meet this objective, a state-of-the-art helium cryogenic testing facility was developed to support the optical testing requirements of a number of the JWST optical testing needs, including the ATPL and SMA. With the implementation of this cryogenic testing facility, the ATPL was successfully cryogenically tested and performed to less than 10nm-RMS (λ/63 RMS) surface figure uncertainty levels for proper reference backout during the SMA optical testing program.

The James Webb Space Telescope’s (Webb’s) deployable primary and secondary mirrors are actively controlled to achieve and maintain precise optical alignment on-orbit. Each of the 18 primary mirror segment assemblies (PMSAs) and the secondary mirror assembly (SMA) are controlled in six degrees of freedom by using six linear actuators in a hexapod arrangement. In addition, each PMSA contains a seventh actuator that adjusts radius of curvature (RoC). The actuators are of a novel stepper motor-based cryogenic two-stage design that is capable of sub-10 nm motion accuracy over a 20 mm range. The nm-level motion of the 132 actuators were carefully tested and characterized before integration into the mirror assemblies. Using these test results as an initial condition, knowledge of each actuator’s length (and therefore mirror position) has relied on software bookkeeping and configuration control to keep an accurate motor step count from which actuator position can be calculated. These operations have been carefully performed through years of Webb test operations using both ground support actuator control software as well as the flight Mirror Control Software (MCS). While the actuator’s coarse stage length is cross-checked using a linear variable differential transformer (LVDT), no on-board cross-check exists for the nm-level length changes of the actuators’ fine stage. To ensure that the software bookkeeping of motor step count is still accurate after years of testing and to test that the actuator position knowledge was properly handed off from the ground software to the flight MCS, a series of optical tests were devised and performed through the Center of Curvature (CoC) ambient optical test campaigns at the Goddard Space Flight Center (GSFC) and during the thermal-vacuum tests of the entire optical payload that were conducted in Chamber A at Johnson Space Center (JSC). In each test, the actuator Fine Step Count (FSC) value is compared to an external measurement provided by an optical metrology tool with the goal of either confirming the MCS database value, or providing a recommendation for an updated calibration if the measured FSC differs significantly from the MCS-based expectation. During ambient testing of the PMSA hexapods, the nm-level actuator length changes were measured with a custom laser deflectometer by measuring tilts of the PMSA. The PMSA RoC fine stage characterization was performed at JSC using multi-wave interferometric measurements with the CoC Optical Assembly (COCOA). Finally, the SMA hexapod fine stage characterization test was performed at JSC using the NIRCam instrument in the “pass-and-a-half” test configuration using a test source from the Aft-Optics System Source Plate Assembly (ASPA). In this paper, each of these three tests, subsequent data analyses, and uncertainty estimations will be presented. Additionally, a summary of the ensemble state of Webb’s actuator fine stages is provided, along with a comparison to a Wavefront Sensing and Control (WFSC)-based requirement for FSC errors as they relate to the optical alignment convergence of the telescope on-orbit.

The Space Infrared Telescope Facility (SIRTF), a NASA "Great Observatory" to be launched in the late 1990s, is a superfluid helium-cooled one meter class IR telescope with a sophisticated chopping facility provided by a dynamic two-axis tilt control capability of the secondary mirror. This paper describes the pointing performance analysis of the Prototype Secondary Mirror Assembly (PSMA) design. The two-axis PSMA tilt control system employs four linear actuators and four pairs of position eddy current sensors, and a reaction mass system to isolate the servo loops from the structural modes. The actuators and sensors operate in the 4 K environment of the secondary mirror assembly, while the control electronics reside in the "warm" electronics box outside the dewar. The PSMA design meets stringent pointing performance requirements over a range of discrete chop amplitudes and frequencies, for three different dynamic chop modes. The analysis of the pointing performance utilizes a detailed dynamic model of the PSMA in time and frequency domains, including the discretization and quantization effects in the servo-controllers, the sensor noise, and the structural modes. The servo stability issues are also addressed.

We describe the design of a liquid helium temperature prototype secondary mirror assembly (PSMA) under development for the NASA Space Infrared Telescope Facility (SIRTF) program. The SIRTF assembly must operate below 4 K and provide the functions of highly precise two-axis periodic tilting (“chopping”) in addition to the conventional functions of focus and collimation adjustment. The PSMA design employs a fused quartz mirror kinematically attached at its center to an aluminum cruciform. The mirror/cruciform assembly is driven in tilt about its combined center of mass using an aluminum flexure pivot of unique design and a four-actuator control system with feedback provided by pairs of differential position sensors. The voice coil actuators are mounted on a second flexure-pivoted mass to enhance servo system stability and isolate the telescope from vibration-induced disturbances. The mirror/cruciform and reaction mass are attached to opposite sides of an aluminum mounting plate whose position relative to the outer housing is controlled by a six degree of freedom focus and centering mechanism using pivoted actuation levers driven by lead screw/harmonic drive/stepper motor assemblies.

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5m infrared telescope built into a Boeing 747 SP. In 2014 SOFIA reached its Full Operational Capability milestone and nowadays takes off about three times a week to observe the infrared sky from altitudes above most of the atmosphere’s water vapor content. An actively controlled 352mm SiC secondary mirror is used for infrared chopping with peak-to-peak amplitudes of up to 10 arcmin and chop frequencies of up to 20Hz and also as actuator for fast pointing corrections. The Swiss-made Secondary Mirror Mechanism (SMM) is a complex, highly integrated and compact flexure based mechanism that has been performing with remarkable reliability during recent years. Above mentioned capabilities are provided by the Tilt Chopper Mechanism (TCM) which is one of the two stages of the SMM. In addition the SMM is also used to establish a collimated telescope and to adjust the telescope focus depending on the structure’s temperature which ranges from about 40°C at takeoff in Palmdale, CA to about −40◦C in the stratosphere. This is achieved with the Focus Center Mechanism (FCM) which is the base stage of the SMM on which the TCM is situated. Initially the TCM was affected by strong vibrations at about 300 Hz which led to unacceptable image smearing. After some adjustments to the PID-type controller it was finally decided to develop a completely new control algorithm in state space. This pole placement controller matches the closed loop system poles to those of a Bessel filter with a corner frequency of 120 Hz for optimal square wave behavior. To reduce noise present on the position and current sensors and to estimate the velocity a static gain Kalman Filter was designed and implemented. A system inherent delay is incorporated in the Kalman filter design and measures were applied to counteract the actuators’ hysteresis. For better performance over the full operational temperature range and to represent an amplitude dependent non-linearity the underlying model of the Kalman filter adapts in real-time to those two parameters. This highly specialized controller was developed over the course of years and only the final design is introduced here. The main intention of this contribution is to present the currently achieved performance of the SOFIA chopper over the full amplitude, frequency, and temperature range. Therefore a range of data gathered during in-flight tests aboard SOFIA is displayed and explained. The SMM’s three main performance parameters are the transition time between two chop positions, the stability of the Secondary Mirror when exposed to the low pressures, low temperatures, aerodynamic, and aeroacoustic excitations present when the SOFIA observatory operates in the stratosphere at speeds of up to 850 km/h, and finally the closed-loop bandwidth available for fast pointing corrections.

To build a long-wave infrared catadioptric optical system for deep space low-temperature target detection with a lightweight and wide field of view, this work conducted a study that encompasses a local cooling optical system, topology optimization-based metal mirror design, and additive manufacturing. First, a compact catadioptric optical system with local cooling was designed. This system features a 55 mm aperture, a 110 mm focal length, and a 4-degree by 4-degree field of view. Secondly, we applied the principles of topology optimization to design the primary mirror assembly, the secondary mirror assembly, and the connecting baffle. The third and fourth modes achieved a resonance frequency of 1213.7 Hz. Then, we manufactured the mirror assemblies using additive manufacturing and single-point diamond turning, followed by the centering assembly method to complete the optical assembly. Lastly, we conducted performance testing on the system, with the test results revealing that the modulation transfer function (MTF) curves of the optical system reached the diffraction limit across the entire field of view. Remarkably, the system’s weight was reduced to a mere 96.04 g. The use of additive manufacturing proves to be an effective means of enhancing optical system performance.

We describe our concept for a liquid helium temperature prototype secondary mirror assembly (PSMA) for the Space Infrared Telescope Facility. SIRTF, a NASA "Great Observatory" to be launched in the 1990's, is a superfluid heliumcooled 1-meter class telescope with much more stringent performance requirements than its precursor the Infrared Astronomical Satellite (IRAS). The SIRTF secondary mirror assembly must operate near 4 K and provide the functions of 2-axis dynamic tilting ("chopping") in addition to the conventional functions of focus and centering. The PSMA must be able to withstand random vibration testing and provide all of the functions needed by the SIRTF observatory. Our PSMA concept employs a fused quartz mirror kinematically attached at its center to an aluminum cruciform. The mirror/cruciform assembly is driven in tilt about its combined center of mass using a unique flexure pivot and a four-actuator control system with feed-back provided by pairs of eddy current position sensors. The actuators are mounted on a second flexure-pivoted mass providing angular momentum compensation and isolating the telescope from vibration-induced disturbances. The mirror/cruciform and the reaction mass are attached to opposite sides of an aluminum mounting plate whose AL/L characteristics are nominally identical to that of the aluminum flexure pivot material. The mounting plate is connected to the outer housing by a focus and centering mechanism based upon the six degree of freedom secondary mirror assembly developed for the Hubble Space Telescope.

In this study, a finite-difference model of a radiometer was established to perform heat transfer analysis. To start with, the computational methodology was introduced along with suitable assumptions. The temperature distribution was calculated based on the advanced finite-difference method with a control volume approach, which can deal with the radiation boundary conditions efficiently. The Oppenheim method was used to calculate the radiation heat transfer inside the radiometer, and the calculation of radiation view factor of the boundary surface was based on the hemicube method. The heat flux calculation was performed based on the Gebhardt method for nonspecular elements and the ray-tracing method for specular elements. The effect of the midnight solar intrusion on the temperature of the second mirror assembly was investigated. Several full disk frame cases and CEI initialization cases were analyzed to determine if the secondary mirror assembly temperatures exceed the mission allowable temperature. The thermal test of the secondary mirror assembly was used to validate the reliability of the thermal model. The results show that the maximum temperature of the secondary housing exceeds the allowable temperature, which will produce a measurable change in instrument performance.

The Stratospheric Observatory for Infrared Astronomy (SOFIA) is a 2.5m infrared telescope built into a Boeing 747 SP. In 2014 SOFIA reached its Full Operational Capability milestone and nowadays takes off about three times a week to observe the infrared sky from altitudes above most of the atmosphere’s water vapor content. Despite reaching this major milestone the work to improve the observatory’s performance is continuing in many areas. This paper focuses on the telescope’s current pointing and chopping performance and gives an overview over the ongoing and foreseen work to further improve in those two areas. Pointing performance as measured with the fast focal plane camera in flight is presented and based on that data it is elaborated how and in which frequency bands a further reduction of image jitter might be achieved. One contributor to the remaining jitter as well as the major actuator to reduce jitter with frequencies greater than 5 Hz is SOFIA’s Secondary Mirror Assembly (SMA) or Chopper. As-is SMA jitter and chopping performance data as measured in flight is presented as well as recent improvements to the position sensor cabling and calibration and their effect on the SMA’s pointing accuracy. Furthermore a brief description of a laboratory mockup of the SMA is given and the intended use of this mockup to test major hardware changes for further performance improvement is explained.