STUDY SUMMARY - IPMU

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<strong>SUMMARY</strong> REPORT WIDE FIELD FIBER-FED OPTICAL MULTI-OBJECT SPECTROMETER (WFMOS) 3.4 High-Level Assumptions This section describes the high-level assumptions used to finalize the proposed design of WFMOS (Table 3.4-1). A full list of assumptions appears in the Detailed Technical Design. We received documents defining the performance of the Subaru Wide Field Corrector (WFC); however, we do not have design details that can be used in the system-level analysis. Based on the conceptual design provided, which shows the number of optical elements and qualitative curvature of each optical surface, we developed a working model of the WFC consistent with the provided performance specifications. The mechanical tolerances in our positioner design and the design of the field element are based on our assumed WFC model. We assumed that a portion of the PFU would be shared between HSC and WFMOS, that the instruments would need to be de-mated from the PFU before the other instrument can be mated, and that this process would take approximately a day. If it takes substantially longer than a day, it will affect the observing timelines. We also assumed that it would only take three people to remove or install WFMOS from the telescope and move it to the TUE room. We further assumed that Subaru could staff this task without hardship. A related assumption is that we have two interfaces between WFMOS and the PFU: one at the rotator and one at the top of the hexapod. Another important assumption concerns the stiffness and mechanical design of the HyperSuprimeCam PFU. We assumed that the instrument hexapod can use a static look-up table to compensate for the flexure in the PFU structure. We further assumed that there will be non-negligible differential flexure between the WFC mounting interface and the interface where the fiber positioning system must mount. Further details are given in Section 3.9. We assumed the performance parameters for some aspects of the Subaru telescope and the elements common to HSC and WFMOS would not dominate the error budgets and assumed values were used as shown in Table 3.4-2. As a measure of caution, whenever we encountered an unknown parameter, we increased the level of margin held in the budget. We assumed that the HSC wavefront sensor will produce lookup tables that will control the PHSC hexapod accurately enough for WFMOS, as suggested in the Interface Control Documentation (90.00.00.01_15.10.00.00_ICD_PHSC). Our computations using the reversed engineered WFC model showed that we need the focus of the WFC to be accurate to 50 μm. We assumed that we will be able to send focus updates to the hexapod to adjust the focus of the WFC relative to the Subaru primary. We likewise assumed we will be able to send updates and offsets to the Instrument Rotator with a high speed communications link. Our Acquisition & Guide System will modify both focus and rotation. 25
<strong>SUMMARY</strong> REPORT WIDE FIELD FIBER-FED OPTICAL MULTI-OBJECT SPECTROMETER (WFMOS) Table 3.4-1: Major Assumptions Assumption Basis Technical Impact Differential flexure between WFC interface and positioner interface HSC design and WFC mass Need for a passive flexure compensation mechanism. WFC vignetting and telecentricity WFC performance provided, JPL Field corrector design model of WFC PFU interface HSC design Fiber placement accuracy PHSC hexapod performance and HSC design Fiber placement accuracy and stability stability Ability to send focus updates to the HSC design Optical coupling efficiency to fibers hexapod to adjust the focus of the WFC relative to the Subaru primary. Ability to send updates and offsets to the instrument Rotator HSC design Fiber placement accuracy and stability Table 3.4-2: Assumed Telescope Performance Throughput Values 420 nm 440 nm 550 nm 640 nm 790 nm 970 nm Component / Subcomponent Component Throughput Component Throughput Component Throughput Component Throughput Component Throughput Component Throughput Atmosphere 0.83 0.83 0.89 0.90 0.90 0.90 Secondary Obscuration 0.95 0.95 0.95 0.95 0.95 0.95 Primary Reflectance 0.87 0.87 0.87 0.85 0.82 0.82 WFC Throughput 0.88 0.88 0.88 0.88 0.88 0.88 WFC Vignetting 0.83 0.83 0.83 0.83 0.83 0.83 Uncorrected Atmospheric Dispersion 0.995 0.995 0.995 0.995 0.995 0.995 Misalignment between WFMOS/HPFU 0.95 0.95 0.95 0.95 0.95 0.95 We assumed we will get the following documents at the start of any contract. These assumptions have driven our cost and schedule estimates for the build phase. • Complete Subaru Interface Control Documents. • Optical Prescription to the Wide Field Corrector. • Complete Subaru and Gemini requirements. • Finite Element Analysis for the WFC/PHSC interface. • Dynamic environment from telescope movement at spectrograph room and at Prime Focus During the course of the build phase we assumed we will get: • SOSS Software Interfaces by Control System PDR. • The Gemini Pipeline Reduction System CDR Documentation by Data Reduction Pipeline PDR. • The Gemini Pipeline Reduction System Software by Data Reduction Pipeline CDR. 26
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