Generation Large Telescope Research Articles

Transit Light Curves for Exomoons: Analytical Formalism

The photometric transit method has been the most effective method to detect and characterize exoplanets as several ground based as well as space based survey missions have discovered thousands of exoplanets using this method. With the advent of the upcoming next generation large telescopes, the detection of exomoons in a few of these exoplanetary systems is very plausible. In this paper, we present a comprehensive analytical formalism in order to model the transit light curves for such moon-hosting exoplanets. In order to achieve analytical formalism, we have considered circular orbit of the exomoon around the host planet, which is indeed the case for tidally locked moons. The formalism uses the radius and orbital properties of both the host planet and its moon as model parameters. The coalignment or noncoalignment of the orbits of the planet and the moon are parameterized using two angular parameters and thus can be used to model all the possible orbital alignments for a star–planet–moon system. This formalism also provides unique and direct solutions to every possible star–planet–moon three circular body alignment. Using the formula derived, a few representative light curves are also presented.

“Slow-scanning” in Ground-based Mid-infrared Observations

Abstract Chopping observations with a tip-tilt secondary mirror have conventionally been used in ground-based mid-infrared observations. However, it is not practical for next generation large telescopes to have a large tip-tilt mirror that moves at a frequency larger than a few hertz. We propose an alternative observing method, a “slow-scanning” observation. Images are continuously captured as movie data, while the field of view is slowly moved. The signal from an astronomical object is extracted from the movie data by a low-rank and sparse matrix decomposition. The performance of the “slow-scanning” observation was tested in an experimental observation with Subaru/COMICS. The quality of a resultant image in the “slow-scanning” observation was as good as in a conventional chopping observation with COMICS, at least for a bright point-source object. The observational efficiency in the “slow-scanning” observation was better than that in the chopping observation. The results suggest that the “slow-scanning” observation can be a competitive method for the Subaru telescope and be of potential interest to other ground-based facilities to avoid chopping.

New Methods for Arc Permanent Magnet Linear Synchronous Motor to Decrease Torque Ripple

In this paper, we propose two new methods to decrease the torque ripple for an arc permanent magnet linear synchronous motor (Arc PMLSM), which will be applied on the next generation large telescope. By connecting different phase windings of different stators, the problem of unbalanced three-phase winding was solved. Compared with the primary three-phase winding, there were fewer harmonics in the current. Besides, by changing the angle between two stators, the end cogging force of different stators was counteracted. After these methods were adopted on the optimized Arc PMLSM, the torque ripple was decreased from 3.2% to 0.65%, which can satisfy the requirement of tracking accuracy for telescope.

Design and Optimization of Arc Permanent Magnet Synchronous Motor Used on Large Telescope

In this paper, we propose to analyze the cogging force of an arc permanent-magnet synchronous motor (Arc PMSM), which will be applied on the next generation large telescope. Finite element analysis (FEA) is carried out to analyze the cogging force of Arc PMSM by using planar cross sections. By adopting optimization methods, such as adjusting the length of stator and adopting fractional slot, the end cogging force decreased from 6.2 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{N}\cdot\hbox{m}$</tex></formula> to 2 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\hbox{N}\cdot\hbox{m}$</tex></formula> , and the slot cogging force decreased from 6.0 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{N}\cdot\hbox{m}$</tex> </formula> to 0.67 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{N}\cdot\hbox{m}$</tex> </formula> accordingly. A geometric optimization in regards to the relative positions of stators along the rotor is also contrived to reduce the end cogging force. Both simulation and experimental results show that the total toque ripple can be as little as 2%, which can satisfy the tracking accuracy of the telescope.

Distributed H∞ control of dynamically coupled segmented telescope mirrors: Design and simulation

Next generation telescopes are to employ segmented mirrors to realize extremely large primary mirror surfaces. For example, one of the current next generation large telescope projects, the Thirty Meter Telescope (TMT), will employ a 30m primary mirror with 492 segments. With the high degree of segmentation, control of these segments against external disturbances to maintain the parabolic mirror shape becomes a challenging task. In this paper, a preliminary study of H∞ control of the segmented primary mirror is presented. Both spatially-invariant distributed and centralized H∞ controllers are designed for a proof-of-concept 37-segment system model. In our proposed mirror design, the segments are dynamically coupled through a uniform support structure, which is modeled using the finite element method. In addition, the segments have the coupled control objective of maintaining the overall mirror shape to allow the highest possible image resolution. A spatially-invariant distributed model is obtained by using a finite element model of the 7-segment system. The resulting model is then used to synthesize a spatially-invariant distributed controller, and is expanded to obtain the 37-segment system by interconnecting the distributed model in a honeycomb-like pattern. The simulation results of the distributed controller is presented and compared with the results from the centralized H∞ controller applied to the 37-segment model. It is shown that both centralized and spatially-invariant distributed controllers satisfy the imaging performance requirements.

Lyα absorbers in motion: consequences of gravitational lensing for the cosmological redshift drift experiment★

The evolution of the expansion rate of the Universe results in a drift in the redshift of distant sources over time. A measurement of this drift would provide us with a direct probe of expansion history. The Lyman α (Lyα) forest has been recognized as the best candidate for this experiment, but the signal would be weak and it will take next generation large telescopes coupled with ultrastable high-resolution spectrographs to reach the cm s−1 resolution required. One source of noise that has not yet been assessed is the transverse motion of Lyα absorbers, which varies the gravitational potential in the line of sight and subsequently shifts the positions of background absorption lines. We examine the relationship between the pure cosmic signal and the observed redshift drift in the presence of moving Lyα clouds, particularly the collapsed structures associated with Lyman limit systems and damped Lyα systems. Surprisingly, the peculiar velocities and peculiar accelerations both enter the expression, although the acceleration term stands alone as an absolute error, whilst the velocity term appears as a fractional noise component. An estimate of the magnitude of the noise reassures us that the motion of the Lyα absorbers will not pose a threat to the detection of the signal.

Imaging polarimetry as a diagnostic tool

AbstractSome of the earliest polarimetric measurements made in astronomy were concerned with the polarization of the interstellar medium resulting from dust grains aligned in the Galactic magnetic field. More than 50 years later, polarimetry continues to be an important diagnostic of field structure on size scales ranging from planetary to galactic. The use of both linear and circular polarimetry at optical and infrared wavelengths can provide additional insights into the nature of dust particles, their alignment in magnetic fields and the field topology. Given the science benefits that polarimetry offers it is perhaps surprising that the continued existence of polarimetric facilities on current and next generation large telescopes needs to be ensured.

Probing HI in the Universe with SKA

This paper briefly describes the capabilities of a new generation large telescope such as the Square Kilometer Array (SKA) for imaging HI in a one degree field out to z ≍ 3