KEYWORDS: Modulation transfer functions, Optical transfer functions, Phase transfer function, Point spread functions, Mirrors, Telescopes, Wavefront errors, Segmented mirrors
For a segmented mirror telescope, the desired resolution can only be achieved when all the segments are precisely co-aligned, co-focused and co-phased. The process of minimization of the piston errors is called co-phasing, which is one of the most stringent tasks in handling the segmented mirror telescope. The OTF based phasing technique samples light from the center of the segments, which is re-imaged on the detector plane, forming a fringed PSF. The Fourier transform of the PSF gives the OTF, which carries information about the piston and tip/tilt errors of the segments. The phase part (PTF) of the OTF can be primarily utilized to extract segment tip-tilt and piston errors lesser than ±λ/4, whereas, the magnitude part (MTF) can provide piston measurements up to a few hundreds of micron. To study the OTF technique in detail, we have developed a Python-based simulation code, that can generate realistic PSF images for the OTF based phasing system under practical conditions. It takes into consideration all the external effects, such as the stellar magnitude and spectral type, atmospheric turbulence and extinction, losses due to optics, and various noises. The OTF is also affected by the segment tip/tilt errors, and hence we have studied the effects of cross-talk between piston and tip/tilt errors on the measurements. In addition to these simulations, we have also conducted laboratory experiments so that the simulation results can be validated. In this paper, we present the results of our extensive simulations as well as experimentation.
Differential optical transfer function (dOTF) is a model-independent image-based wavefront sensor for measuring the complex field. This sensor-less wavefront sensing technique is efficient for precise non-common path aberrations (NCPA) measurement and the phasing of segmented telescopes. In this communication, we report on a thorough exploration of the dOTF method with the SPEED facility for NCPA and cophasing optics application.
We examine several focal plane wavefront sensing (FPWFS) approaches from a theoretical perspective, specifically comparing their fundamental origins and similarities. We consider various techniques, including the differential optical transfer function wavefront sensor, the self-coherent camera wavefront sensor, the fast-modulated self-coherent camera wavefront sensor, and other variants, as well as pair-wise probing. They are evaluated within a unified framework based on precise optical transfer function analysis and their implementation on the high-contrast imaging SPEED test bed. These concepts are highly similar in essence, even serving as mere extensions of one another. Indeed, we emphasise that these FPWFS can be seen as different expressions or embodiment of the same underlying principles.
The Prototype Segmented Mirror Telescope is a 1.3 m aperture, seven segment telescope, being developed as a technology demonstrator for India’s large optical-IR telescope project. For this segmented mirror telescope, a Shack Hartmann sensor based alignment device has been designed and developed. The device not only precisely captures the segment misalignment but also measures the segment focus error with an accuracy of a few microns and hence helps in the tip-tilt correction and co-focusing of the mirror segments. The device is designed to work primarily in two different modes: the Shack–Hartmann mode and the imaging mode. After completion of the alignment procedure, the final image quality can be checked in the imaging mode. The device is designed in such a way that it also has a provision to conduct the Keck kind of phasing experiment with one pair of mirror segments. To make the device cost effective, only off-the-shelf components are used. The optical design and opto-mechanical analysis of the device were carried out using Zemax and SolidWorks software. Then, the device was realized and its extensive testing was carried out in the laboratory. Here, we have presented the details of the opto-mechanical design and analysis as well as the preliminary results of performance tests conducted in the laboratory.
The Prototype Segmented Mirror Telescope (PSMT) is a 1.3m segmented mirror telescope that aims to develop and demonstrate the segmented mirror technology indigenously. The telescope design includes a spherical primary with seven hexagonal segments of size 500mm each and an ellipsoidal secondary. Since the telescope has a spherical primary, hence it suffers from spherical aberrations as well as large off-axis aberrations, thus limiting its field of view. In order to improve the image quality over relatively larger field, an aberration corrector is required. The Faint Object Spectrograph and Camera (FOSC) is a widely used back-end instrument for any telescope and PSMT is also supposed to be equipped with such an instrument. Therefore, we have designed the optics of the FOSC in such way that it meets dual requirements i.e. it works as a science instrument as well as an aberration corrector. The FOSC instrument consists of multi-element collimator and camera lenses and a grism is used as a dispersive element. The FOSC optics design is optimized for the visual wavelength range of 4500-8500A° and up to 10 arc-min field of view. Here, we present the optical design of the FOSC and outcome of the analysis carried out using the ZEMAX optical design software.
The segment support system of any segmented mirror telescope is considered to be one of the most critical subsystem. The segment support not only holds the mirror without altering its figure, but also features mechanism which facilitate active alignment of the segments with the help of three linear actuators. We have designed and a developed a segment support system for a proposed prototype segmented mirror telescope (PSMT). The baseline design of the PSMT segment support comprises of nine point axially supporting whiffletree coupled with a moving frame and a central diaphragm for the radial support. Our design uses large number of flexural components including flex pivots which make it friction-less system, requiring no lubrication. In this paper we present the details of our design as well as results of very extensive finite element analysis carried out to explore effect of variable gravity as well as temperature on the performance of the support system. During the course of telescope movement from zenith to horizon, interplay between axial and radial support system has also been studied in great detail. The modal analysis is also carried out to determine different natural frequencies/modes the support system is subjected. Functional and operational aspect of the segment support is also tested by conducting experiments on one fully realized system. The segment support which is primarily designed for 0.5m size PSMT segment can be easily scaled up to 1 m size segment and hence can be used for any large telescopes aimed to utilize segmented primary mirror.
Indian astronomers are aiming to build a large 10m class optical-NIR telescope, equipped with state-of art instruments. After exploring many potential design options, we ended up with two mirror Ritchey-Chretien (RC) type design, which provide diffraction limited performance over a sufficiently large field and delivers decent image quality over fairly extended field. The segmented primary mirror is a natural choice for the proposed 10m class telescope. However, unlike monolithic primary mirror, various factors linked with the segmentation plays very critical role to decide the performance of the telescope. In great detail, we have also studied the effect of the segment piston, tip and tilt, clocking, the radius of curvature, the shear, the segment size, inter-segment gap as well as figuring error on the telescope performances. All these studies are conducted using a custom developed generic python-based tool that can be used along with ZEMAX ray-tracing software. In this paper we present the optical design of proposed 10m class telescope as well as our extensive study on segmentation and alignment related effects.
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