Step-zoom dual-field-of-view infrared telescope (original) (raw)

Proto-Model of an Infrared Wide-Field Off-Axis Telescope

Journal of The Korean Astronomical Society, 2010

We develop a proto-model of an off-axis reflective telescope for infrared wide-field observations based on the design of Schwarzschild-Chang type telescope. With only two mirrors, this design achieves an entrance pupil diameter of 50 mm and an effective focal length of 100 mm. We can apply this design to a mid-infrared telescope with a field of view of 8 • × 8 •. In spite of the substantial advantages of off-axis telescopes in the infrared compared to refractive or on-axis reflective telescopes, it is known to be difficult to align the mirrors in off-axis systems because of their asymmetric structures. Off-axis mirrors of our telescope are manufactured at the Korea Basic Science Institute (KBSI). We analyze the fabricated mirror surfaces by fitting polynomial functions to the measured data. We accomplish alignment of this two-mirror off-axis system using a ray tracing method. A simple imaging test is performed to compare a pinhole image with a simulated prediction.

stepzoom.pdf

The design of a dual-field-of-view telescope for an 8 -12-m imaging waveband is described. Preliminary calculations are made to determine the first-order parameters of the narrow-and the wide-field modes. To achieve a switchable dual-field-of-view system, one uses an optical configuration based on the axial motion of a single lens group along the optical axis. The same lens is also used for focusing at near objects and for athermalization by small axial movement. A total of six lenses with one conic surface are used in the design, making the telescope cost effective and lightweight. The final optical design is presented, along with the aberrations curves and modulation transfer function plots, showing excellent performance in both fields of view.

A 1-degree FOV 30-meter telescope concept revisited

Ground-based Telescopes, 2004

The science case for wide fields on ELTs is well developed and justifies the implementation of 20 arc-minute and larger fields-of-view with seeing-limited performance on a 20 to 30-meter telescope. However, the practical implementation of a wide field can prove to be challenging with classical telescope design when low-thermal emissivity performance is also being optimized. Segmented mirrors assemblies need not be full aperture, axially symmetric structures. Space for secondary, tertiary, and quaternary mirror support structures that do not cross the optical path can be achieved with offaxis mirror assemblies. Barden, Harmer, Claver, and Dey 1 described a 4-mirror, 1-degree FOV 30-meter telescope. We take that concept further with an off-axis approach. Three conic mirrors are required to produce excellent image quality in the 1-degree FOV (diffraction limited across the central few arc-minutes, better than 0.3" imaging performance at the edge of the field). A flat quaternary mirror is utilized both as a beam steering mirror to different instrument ports on the lower side of the telescope and as an adaptive mirror for wind-buffeting and possible ground layer AO correction. The final f/2.2 focal ratio allows the use of an echidna-style fiber positioner for very dense target field acquisition. Extreme AO and Ground Layer AO ports can both be implemented as well. Diffraction characteristics may possibly be improved given the lack of a spider mount for the secondary mirror but will be elliptical rather than circular.

Optical design and testing of a fast, large aperture, infrared space telescope

Space Science Reviews, 1992

An optical design study for a next generation infrared space telescope has been performed, The concept is that of a passively cooled telescope of minium aperture 2.5 metre with an F/1.2 primary and wavelength coverage from A = 2 to at least 40 /zm, and possibly to 100 /~m. Compactness, low thermal emission from the optics and structure, diffraction limited imaging at A ----2/zm, and sensitivity to misalignment aberrations and manufacturing errors were the main considerations for this study. Ray tracing results are presented showing the characteristics of the various designs considered. A preliminary investigation of stray light properties is also given. Special emphasis has been placed on the testing of such a fast primary, and optical systems using a lateral shearing interferometer are described for testing both the primary and the primary/secondary combination.

Freeform Wide Field-of-View Spaceborne Imaging Telescope: From Design to Demonstrator

Sensors

Wide field-of-view imaging optics offer a huge potential for space-based Earth observation enabling the capture of global data. Reflective imaging telescopes are often favored, as they do not show chromatic aberrations and are less susceptible to radiation darkening than their refractive counterparts. However, the main drawback of reflective telescopes is that they are limited with respect to field-of-view while featuring large dimensions. We propose the use of freeform optics to maximize the field of view while maintaining diffraction-limited image quality and minimizing system dimensions. In this paper, we present a novel freeform wide field-of-view reflective telescope, starting from the optical design, and continuing to tolerancing analysis and manufacture, towards a proof-of-concept demonstrator. The novel telescope features a full field-of-view of 120° while showing an exceptional spatial resolution of 2.6 km and fitting within 1 CubeSat unit. To the best of our knowledge, thi...

Imaging with multi-aperture optical telescopes and an application

Comptes Rendus de l'Académie des Sciences - Series IV - Physics, 2001

The two main types of Multi-Aperture Optical Telescopes (MAOTs) (so-called Michelson and Fizeau) and the two possible modes of optical beam combination are reviewed. Widefield imaging with a Michelson instrument is studied and the constraints are identified. An example of application to Earth observation is given. Then, we address the optimization of the aperture configuration, a key issue in the design of a MAOT. We also stress the image restoration, a necessary component of such an instrument because of the shape of its point spread function. Finally, a MAOT seems to be a promising technical solution for high resolution Earth observation from Space on a high orbit such as a geostationary one.  2001 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS optical interferometry / imaging / synthetic aperture optics / phased array telescope / high angular resolution

The double Prime Focus camera for the Large Binocular Telescope

Ground-based Instrumentation for Astronomy, 2004

The Prime Focus for the Large Binocular Telescope are a couple of Prime Focus stations each equipped with four 4kx2k CCDs and a six lenses corrector with an aspheric surface and the first lens as large as roughly 800mm in diameter. These cameras will cover almost half degree of Field of View on 8m-class telescopes with unprecedented velocity of F/1.4. The two units are optimized for the Red and Blue portions of the visible wavelength and additionally an extension to J and H bands is foreseen. An overview of the project, including the optomechanics, the cryogenics, the electronics, and the software is given along with a preliminary account of lessons learned and on how much the second unit, the Red one, the schedule of which is shifted with respect to the Blue one by several months, will take advantage from the experience gained in the Blue unit assembly and integration.

Design and analysis of a compact wide-field unobscured zoom mirror system

Zoom Lenses II, 1997

The evolution ofan unobscured all-reflective zoom optical system for utilization in the infrared spectrum Is presented. The objective is to develop a system which has a flat Image surface, wide fieldof-view, spatially remote entrance pupil, 2:1 zoom range, and Is spatially compact. The optical system comprises three aspheric mirrors sharing a common optical axis where the primary mirror Is spatially fixed with respect to the entrance pupil, and secondary and tertiary mirrors move during zm. The field-of-view ranges from 2.2 degs by 2.2 degs to 4.4 degs by 4.4 degs. The focal ratio varies from F/4 to F/S. The Inherent characteristics ofthIs type optical system are discussed, as are design methods to control aberrations, distortion and anamorphic error over the zoom range. The baseline design configuration Is presented along with MTF performance data. The results of tolerance sensitivity analysis are also discussed.

A wide-field near-infrared camera and spectrograph for the Mt. Abu 1.2 m telescope

Ground-based and Airborne Instrumentation for Astronomy II, 2008

We describe the design and optimization of a wide-field near-infrared camera and spectrograph (NICAS) for Mt Abu 1.2 m, f/13 Cassegrain telescope of Physical Research Laboratory. The principal science goals include photometric mapping of star forming regions and medium resolution spectroscopy of Young Stellar Objects, evolved stars and transient sources. The design goals are to achieve seeing-limited angular resolution in an un-vignetted field of view of ~ 8'x8' with 0.5′′ per pixel (of 18.5 µm) on a HgCdTe 1024x1024 infrared array, requiring a two-fold Cassegrain focal reduction. In addition to the imaging, the instrument is required to have spectroscopic capability with a resolving power of 10 3 in the 0.85 -2.5 µm region, needing a dispersion of 1 nm per pixel. Finally, since our telescope has a moderate aperture, the throughput losses need to be minimized. The specifications are achieved by an optical design using 9 singlet lenses. Only those lens materials are chosen for which measured values are available for refractive indices at 77 K (detector operating temperature), changes of indices with temperature, and thermal coefficients of expansion. The design is optimized to give sharpest images at 77 K. The optical path is folded by 90° after collimation by a fold-mirror and reimaged on the detector. The fold-mirror is replaced by a diffraction grating for spectrograph mode. In order to minimize the reflection losses, all the lenses will be anti-reflection coated for the full operating wavelength range. Details of the design are presented.