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The core of HARMONI is the Integral Field Spectrograph (IFS) which is composed of different subsystems including the IFS Pre-Optics (IPO). The IPO main objective is to take light from the focal plane relay system and reformat and condition it to be a suitable input for the rest of the instrument. The IPO in HARMONI includes the IFS Pre-Optics Fast Shutter (POFS), a mechanical cryogenic fast shutter which will be used with both the visible and infrared detectors. This mechanism has been designed to be fast and reliable and its design has already passed the Critical Design Review (CDR) but specific issues that require further analysis have been identified. The functioning of this element is a critical part in HARMONI and, consequently, a prototype has been used to analyze possible improvements in the CDR design and to perform extensive testing before the Final Design Review (FDR).
In this work we present the design of the IFS Pre-Optics Fast Shutter and the test results obtained with the prototype developed at the facilities of the Instituto de Astrofísica de Canarias (IAC).
The Instrument Pre-Optics (IPO) is one of the HARMONI subsystems. It distributes the telescope light received from the adaptative optics systems. The main objective of the IPO is to format the field for the selected spatial scales feeding the Integral Field Unit (IFU). IPO is under the responsibility of the Institute of Astrophysics of the Canary Islands (IAC). This optical subsystem implements 30 Opto-mechanical mounts working at cryogenic temperatures. The mounts may be classified into two types based on the features of the optics they support: (1) Sprung Kinematic Mount (SKM) for flat mirrors, and (2) Thermally Compensated Kinematic Sprung Mounts (TCKSM) for power mirrors (toroidal mirrors, offaxis parabolas, and cameras). Designed to maintain optical alignment at cryogenic temperatures, the mounts maintain optical surface deformation within the limits specified by the error budget, ensuring compliance with requirements even worst-case scenarios.
This work describes the verification tests performed to the engineering models of the Opto-mechanical mounts of the IPO to validate compliance with the sub-system optical and mechanical requirements at both room and cryogenic temperatures.
The focal plane mask wheel sits in the input focus of the cryostat. It provides 16 user-selectable positions for masks (28x40 mm) used in observation. The key driver for this mechanism is the high repeatability (±2.5 μm) required, equivalent to ~1mas in the input focal plane. The IAC has previously designed, manufactured, tested and put in operation cryogenic wheels with high repeatability; however, the challenge of obtaining a wheel with such repeatability requires testing new concepts of detent positioning systems.
The shutter allows for exposures shorter than the minimum read time of the near-IR detectors and is needed for any CCD observations with the visible cameras. A dual shutter design is needed to achieve the necessary open/close times (<20 ms), but this also provides some redundancy and a graceful failure mode for this critical device. To mitigate risks on the proper behaviour of a fast cryogenics shutter a prototype based on a simple concept has been manufactured. We present the design and results for the performed cryogenic tests of a mask wheel and a shutter prototypes that we have developed.
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