SPIRE Design Description - Research Services

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Draft SPIRE Design Description Document 3.5.4 Optical alignment Alignment of the SPIRE FPU will be performed according to a philosophy based on high-precision machining and pre-assembly 3-D measurements and a program of optical alignment checks during and after assembly. Nominally no adjustments will be necessary but, if a serious misalignment is detected, its compensation will be possible by re-machining of the M6 mirror stand. Alignment of the instrument with respect to the telescope axis and pupil will be performed using a Herschel optical bench simulator consisting of a set of reference mirrors accurately located with respect to the instrument interface points. Verification of image quality and internal alignment stability will be effectuated using a set of alignment tools (sources, reticules, theodolites etc) mounted in strategic positions in the optical train (object plane, cold stop, image plane). The detailed alignment procedures are described in the SPIRE Alignment Plan. Details of the calculation of the optical alignment budget are given in the SPIRE Optical Error Budgets document, here we give an overview of the requirements on the alignment. An optical sensitivity study shows that with an alignment tolerance of 0.1 mm and 1′ applied to all the mirrors in the photometer, the alignment-related relative pupil displacement will be less than ∆R/RI = 4.1%. This compares favourably with the contribution from telescope alignment errors, budgeted to ∆R/RT = 6%, and the theoretical design contribution of ∆R/RD = 5%. The total instrument budget is estimated by squaresumming of the random alignment errors and summing of the deterministic design error: 2 I 2 T ∆ R / R = ∆R / R + ∆R / R + ∆R / R , D giving a total of 12.3%. For an exactly sized pupil this gives a loss of 8% in telescope transmission factor. For the spectrometer, the predominant alignment criterion is interferometer contrast, calculated from the misalignment-induced lateral separation of the interfering images using the van Cittert-Zernike theorem. Note that this only concerns mounting tolerances of the fixed optical components within the interferometer (beamsplitters and collimator/camera mirrors) since the interferometer design with its back-to-back corner cube reflector leaves the interferogram contrast insensitive to errors in the scanning movement. Again, tolerances of 0.1 mm and 1′ have been found appropriate, offering a contrast in the interferogram of 87%. Including mirror surface quality and differential aberrations a total contrast greater than 80% is expected. 3.6 Thermal design The thermal design of the SPIRE instrument is predicated on the following constraints: (i) the need to reduce the temperature of the structure; mechanisms and optics to below a temperature where they can radiate significant optical power onto the detectors; (ii) the need to get the bolometers to 300 mK or less and keep them at this temperature in a stable manner for all instrument observing modes; (iii) the need to minimise the thermal load into the Herschel cryostat to make the mission last as long as possible given the amount of liquid helium available; 3.6.1 Instrument temperature levels Given these drivers and the structural layout of the SPIRE instrument, three basic temperature levels have been defined within the SPIRE FPU. The various optical components (mirrors, filters, beam splitters etc.) and other instrument sub-systems (structure, mechanisms, harnesses etc.) must not be allowed to radiate significant IR power in these wavelengths. The temperature of all components that can either directly or indirectly (i.e. via multiple reflections) irradiate the detectors must therefore be below 5 K. The main 48
Draft SPIRE Design Description Document structure of the instrument and most of the optics are therefore held at 4 K (termed Level-1 of the cryostat); the structure that houses the detectors is held at 2 K (Level-0), and, in order to achieve a sensitivity matched to the photon noise limit from the telescope thermal background, the detectors must be at 300 mK – this temperature is generated internally by the sorption cooler which is mounted from the 4-K optical bench. The detector JFET amplifiers must run at a temperature of ~120 K so they are mounted on silicon nitride membranes which isolate them thermally from the surrounding structure. These membranes are then housed in a JFET units that is hard mounted to the Herschel optical bench at the cryostat Level-2 temperature of between 9 and 12 K. Herschel L0 He Tank 1.7 K Herschel L1 He Vent Pipe 4K Herschel L2 He Vent Pipe 10 K L0 Evap Strap L0 Pump Strap L0 Box Strap L1 Strap L2 Straps Cooler Evaporator 290 mK Kevlar Support Cooler Pump 1.8 K Cooler 4-K Structure Kevlar Support 300-mK Copper Straps Kevlar Support Mechanisms 49 Kapton /Constantan Harness Photometer / Spectrometer JFET Enclosures 10 K FIRST Optical Bench 10 K 300-mK Detector Assembly Kevlar Supports 1.8-K Detector Flange 1.8-K Photometer and Spectrometer Detector Enclosures FPU 4-K Structure Stainless Steel Harness Stainless Steel Harness Stainless Steel Blade Supports Figure 3-25 - Summary of the thermal analysis model for the subsystems within the CVV. Stainless Steel Cone and A-Frame Supports 3.6.2 Cryogenic heat loads To maximise the life of the mission and therefore the quantity of science data, the rate of consumption of helium must be minimised. This is done by (i) minimising the power dissipated in the various sections of the instrument, and (ii) by minimising the flow of heat from warmer stages to stages requiring lower operating temperatures. Stringent thermal budgets have been applied to all dissipating elements in the focal plane, and to the mechanical mounts that support the FPU from the Herschel optical bench; the mechanical mounts for detector cold boxes from the SPIRE optical bench and the support system for the various elements that have to be maintained at 300 mK. The electrical harnesses for the mechanisms and the detectors also contribute significantly to the thermal loads and these will have to be carefully designed to ensure that they meet both the electrical and thermal requirements placed upon them. The heat flows within the instrument are represented in an ESATAN model and shown schematically in Figure 3-25. A detailed description of the SPIRE thermal mathematical model is found in the SPIRE Thermal Configuration Control Document.
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