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Spectroscopic observations in the far and extreme ultraviolet (FUV/EUV, 40-200nm) is of great interest in various scientific fields, such as in Solar Physics, in physics of interstellar medium and in planetary exospheres studies. Microchannel plates-based detectors have been for a long time the detectors of choice for astronomical applications in this range of wavelength, due to their photon counting capability (since the expected photon flux are low) and the possibility of solar blindness (photon flux in the visible range are order of magnitudes higher and filtering may be an issue).
However, the spectral features observed in the targets are characterized by a high range of intensities, which can cover several orders of magnitude. Response of MCP detectors at high flux is limited ultimately by the MCP itself, but generally the readout system introduces further restrictions, thus the technique of lowering the efficiency in the area interested by the most intense lines is often adopted.
In the framework of technological R&D for future astronomical FUV/EUV spectrograph, we are developing a photon counting, solar blind, UV detector with readout system based on a 2D anode array integrated in a custom designed Read Out Integrated Circuit (MIRA - Microchannel plate Readout ASIC), with the aim of achieving high performances characteristics, in particular unprecedented performance in terms of dynamic range combined with spatial resolution close to 30 μm. This detector will allow to measure, simultaneously and without the necessity of filters, spectral lines with different intensities of orders of magnitude, exploiting the maximum Signal to Noise Ratio provided by the statistical limit.
A first prototype has been realized, based on a demonstrator of the MIRA ASIC, 32×32 pixels, 35×35 μm2 size, for a total chip area of 2×2 mm2, to be integrated with a standard demountable MCP intensifier.
A simple derivation of the cyclic error due to optical mixing is proposed for the cancelable circuit design. R and M beatings are collected by two photodiodes and conveniently converted by transimpedance amplifiers, such that the output signals are turned into ac-coupled voltages. The detected phase can be calculated as a function of the real phase (a change in optical path difference) in the case of zero-crossing detection. What turns out is a cyclic non-linearity which depends on the actual phase and on the amount of optical power leakage from the R channel into the M channel and vice versa. We then applied this result to the prototype of displacement gauge we are developing, which implements the cancelable circuit design with wavefront division. The splitting between R and M is done with a double coated mirror with a central hole, tilted by 45° with respect to the surface normal. The interferometer features two removable diffraction masks, respectively located before the merging point (a circular obscuration) and before the recombination point (a ring obscuration). In order to predict the extent of optical mixing between R and M, the whole layout was simulated by means of the Zemax ® Physical Optics Propagation (POP) tool. After the model of our setup was built and qualitatively verified, we proceeded by calculating the amount of optical leakages in various configurations: with and without the diffraction masks as well as for different sizes of both the holey mirror and the diffraction masks. The corrisponding maximum displacement error was then calculated for every configuration thanks to the previously derived formula. The insertion and optimization of the diffraction masks greatly improved the expected optical isolation inside the system.
Data acquisition from our displacement gauge has just started. We plan to experimentally verify such results as soon as our prototype gauge will reach the desired sub-nanometer sensitivity.
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