Quantification of the expected residual dispersion of the MICADO Near-IR imaging instrument

MICADO, a near-infrared imager for the Extremely Large Telescope, is being designed to deliver diffraction limited imaging and 50 micro arcsecond (${\mu}$as) astrometric accuracy. MICADO employs an atmospheric dispersion corrector (ADC) to keep the chromatic elongation of the point spread function (PSF) under control. We must understand the dispersion and residuals after correction to reach the optimum performance. Therefore, we identified several sources of chromatic dispersion that need to be considered for the MICADO ADC. First, we compared common models of atmospheric dispersion to investigate whether these models remain suitable for MICADO. We showed that the differential dispersion between common atmospheric models and integration over the full atmosphere is less than 10 ${\mu}$as for most observations in H-band. We then performed an error propagation analysis to understand the uncertainty in the atmospheric dispersion as a function of atmospheric conditions. In addition, we investigated the impact of photometric color on the astrometric performance. While the differential refraction between stars within the same field of view can be significant, the inclusion of an ADC rendered this effect negligible. For MICADO specifically, we found that the current optomechanical design dominates the residual dispersion budget of 0.4 milli arcseconds (mas), with a contribution of 0.31 mas due to the positioning accuracy of the prisms and up to 0.15 mas due to a mismatch between the dispersive properties of the glass and the atmosphere. We found no showstoppers in the design of the MICADO ADC for achieving 50 ${\mu}$as relative astrometric accuracy.Comment: Submitted to Monthly Notices of the Royal Astronomical Society. Contains 11 pages and 12 figure

Demonstration of an imaging technique for the measurement of PSF elongation caused by Atmospheric Dispersion

Elongation of the point spread function due to atmospheric dispersion becomes a severe problem for high resolution imaging instruments, if an atmospheric dispersion corrector is not present. In this work we report on a novel technique to measure this elongation, corrected or uncorrected, from imaging data. By employing a simple diffraction mask it is possible to magnify the chromatic elongation caused by the atmosphere and thus make it easier to measure. We discuss the theory and design of such a mask and report on two proof of concept observations using the 40 cm Gratama telescope at the University of Groningen. We evaluate the acquired images using a geometric approach, a forward modelling approach and from a direct measurement of the length of the point spread function. For the first two methods we report measurements consistent with atmospheric dispersion models to within 0.5 arcsec. Direct measurements of the elongation do not prove suitable for the characterisation of atmospheric dispersion. We conclude that the addition of this type of diffraction mask can be valuable for measurements of PSF elongation. This can enable high precision correction of atmospheric dispersion on future instruments.Comment: Accepted for publication in the Monthly Notices of the Royal Astronomical Society. Contains 11 pages, 11 figures, 2 table

Gaussian beam mode analysis of standing waves between two coupled corrugated horns

Abstract—In this paper we present the theoretical analysis of the effects of standing waves between coupled horn antennas that can occur in terahertz quasi-optical systems. In particular we illustrate the approach for the case of two coupled horn antennas as the distance between them is varied. The full mode matching scattering matrix approach is based on combining a standard waveguide mode description of the horn antenna and a quasi-optical Gaussian beam description of the free space propagation. Track is kept of both the backward and forward going components of the propagating fields.We compare theoretical predictions with actual experimental test results for a quasi-optical system operating at a frequency of 0.480 THz

Gaussian beam mode analysis of standing waves between two coupled corrugated horns

Abstract—In this paper we present the theoretical analysis of the effects of standing waves between coupled horn antennas that can occur in terahertz quasi-optical systems. In particular we illustrate the approach for the case of two coupled horn antennas as the distance between them is varied. The full mode matching scattering matrix approach is based on combining a standard waveguide mode description of the horn antenna and a quasi-optical Gaussian beam description of the free space propagation. Track is kept of both the backward and forward going components of the propagating fields.We compare theoretical predictions with actual experimental test results for a quasi-optical system operating at a frequency of 0.480 THz

Applying Energy Absorption Interferometry to THz direct detectors using photomixers

Detector requirements for far infrared astronomy generally result in devices which exhibit a few-moded response to incident radiation. The sensitivity and spatial form of the individual modes to which such a detector is sensitive can be determined with knowledge of the complex valued cross-spectral density of the system, which we label the detector response function (DRF). A matrix representing the discretized cross-spectral density can be measured from the complex amplitudes of interference fringes generated by two identical sources as they are independently scanned through the field of view. We provide experimental verification of this technique using monochromatic THz beams generated by photomixers in which the relative phase is varied with fiber stretchers. We use this system to characterize the modal response of a single pixel from an array of microwave kinetic inductance detectors (MKIDs).Comment: in IEEE Transactions on Terahertz Science and Technology. 202

A novel design for a cryogenic Fabry-Pérot interferometer

The sensitivity of state-of-the-art superconducting far-infrared detectors is such that astronomical observations at these wavelengths are limited by photon noise from the astronomical source unless a method of restricting the spectral bandpass is employed. One such method is to use a high resolution Fabry-Pérot interferometer (FPI) in conjunction with a lower resolution, post dispersing system, such as a grating spectrometer. The resonant wavelength of an FPI is typically tuned by changing the spacing or medium between the parallel reflecting plates of the etalon. We present a novel design in which the wavelength is tuned by scanning the angle of incidence, which simplifies the cryo-mechanical design, actuation and metrology. The effects on the spectral response as a function of incident angle have been simulated and shown to be manageable

Cryogenic characterisation of a permanent magnet stepper motor and its impact on the MICADO atmospheric dispersion corrector

The MICADO atmospheric dispersion corrector (ADC) will be the first ADC built for an astronomical instrument that has to operate in a cryogenic environment (T = 77 K). A detailed understanding of the system behaviour is necessary to maximise the operational lifetime of the planned design concept and to design a suitable controller. The MICADO ADC design features a friction drive concept that is powered by a commercially available permanent magnet stepper motor (PMSM). Here, we report on an extensive characterisation of this PMSM. By matching the experimental results to an analytical description of PMSMs, we obtain a solid foundation to build a complete dynamical model of the ADC system. A prototype of the ADC design concept had already been built and tested at its operational temperature. The results from these tests allowed us to compare the measured and modelled response and discuss the implications. With respect to the motor characterisation, we find no significant performance difference of the tested stepper motor when operated at room temperature, compared to at 77 K. However, we do find that static friction plays a large role in the precise response of the ADC mechanism