Off-axis Cassegrain Research Articles

Optical Design and Analysis of the Submillimeter-Wave Instrument on JUICE

The submillimeter-wave instrument on the Jupiter icy moons explorer spacecraft is a passive dual-beam heterodyne radiometer operating in the frequency bands 530–625 and 1080–1275 GHz. The instrument will observe Jupiter's atmosphere as well as the atmosphere and surface properties of its moons. This paper presents the optical design and analysis of the instrument that has been carried out using Gaussian beam mode analysis and physical optics simulations. The optics consists of a 29-cm mechanically steerable off-axis Cassegrain telescope, relay optics, and two feedhorns. Frequency-independent operation of the 600-GHz channel is predicted by the simulations. The 1200-GHz channel shows some frequency dependency because of the selected feedhorn type. The steering of the telescope affects mainly its cross-polarization level. The manufacturing and mounting tolerances, as well as distortion and misalignment by thermoelastic effects, will cause the performance of a real instrument to deviate from that of an ideal one. Physical optics simulations combined with data from finite-element method simulations and measurements of optical surface profiles show that these factors affect, in particular, the instrument pointing. The induced pointing error is up to several arcminutes, whereas the specification is less than 0.5 arcmin. The pointing error can be reduced by adjusting the alignment of the telescope secondary mirror and the feedhorns.

Optical design of off-axis Cassegrain telescope using freeform surface at the secondary mirror

Freeform surfaces enable imaginative optics by providing abundant degrees of freedom for an optical designer as compared to spherical surfaces. An off-axis two-mirror–based telescope design is presented, in which the primary mirror is a concave prolate spheroid and the secondary mirror is freeform surface-based. The off-axis configuration is employed here for removing the central obscuration problem which otherwise limits the central maxima in the point spread function. In this proposed design, an extended X−Y polynomial is used as a surface descriptor for the off-axis segment of the secondary mirror. The coefficients of this extended polynomial are directly related to the Seidel aberrations, and are thus optimized here for a better control of asymmetric optical aberrations at various field points. For this design, the aperture stop is located 500 mm before the primary mirror and the entrance pupil diameter is kept as 80 mm. The effective focal length is 439 mm and covers a full field of view of 2 deg. The image quality obtained here is near diffraction limited which can be inferred from metrics such as the spot diagram and modulation transfer function.

Theoretical analysis of a hollow laser beam transmitting in an off-axis Cassegrain optical antenna system

The optical model of a Cassegrain optical antenna with a confocal double-parabolic reflector structure has been designed, and the propagation characteristics of a hollow laser beam, which could avoid the loss of energy caused by the subreflector center reflection in the optical antenna, has been researched in this paper. By detailed analysis and numerical calculations of a receiving Cassegrain antenna with different deflection angles, the coupling efficiency curve and 3-D distributions of the receiving light intensity for different inclined angles have been obtained.

Mie lidar observations of lower tropospheric aerosols and clouds

Mie lidar system is developed at Laser Science and Technology Centre, Delhi (28.38°N, 77.12°E) by using minimal number of commercially available off-the-shelf components. Neodymium Yttrium Aluminum Garnet (Nd:YAG) laser operating at 1064 nm with variable pulse energies between 25 and 400 mJ with 10 Hz repetition rate and 7 ns pulse duration is used as a transmitter and off-axis CASSEGRAIN telescope with 100 mm diameter as a receiver. Silicon avalanche photodiode (Si-APD) module with built-in preamplifier and front-end optics is used as detector. This system has been developed for the studies of lower tropospheric aerosols and clouds. Some experiments have been conducted using this set up and preliminary results are discussed. The characteristics of backscattered signals for various transmitter pulse energies are also studied. Atmospheric aerosol extinction coefficient values are calculated using Klett lidar inversion algorithm. The extinction coefficient, in general, falls with range in the lower troposphere and the values lie typically in the range 7.5 × 10 −5 m −1 to 1.12 × 10 −4 m −1 in the absence of any cloud whereas this value shoots maximum up to 1.267 × 10 −3 m −1 (peak extinction) in the presence of clouds.

Theoretical analysis and test for off-axis Cassegrain optical antenna

By detailed analysis of Cassegrain optical antenna with inclined optical axis, the receiving antenna power and the curve of power attenuation are obtained for different deflection angles. Three-dimensional images which describe the power distribution of the receive laser beam have been obtained. The coupling efficiency of antenna system is obtained from specific experiment. If the deflection angle is less than 0.1519 rad, the coupling efficiency is beyond 80%. In this case, the optical antenna system can be viewed as in alignment approximately. This research will provide a theoretical base and broad application for achieving precise alignment of optical axis in the field of optical communications and three-dimensional laser radar images.

Geometrical theory of aberrations near the axis in classical off-axis reflecting telescopes

A geometrical theory of aberrations for the vicinity of the focus of arbitrary off-axis sections of conic mirrors is derived. It is shown that an off-axis conic mirror introduces linear astigmatism in the image. However, in classical two-mirror telescopes this aberration can be eliminated by tilting the secondary parent mirror axis. It is also shown that the practical geometrical-optics performance of a classical off-axis two-mirror telescope with no linear astigmatism is equivalent to the performance of an on-axis system, proving that both systems have identical third-order coma. To demonstrate the applicability of the theory developed in a practical system, a fast (i.e., f/2), compact, obstruction-free classical off-axis Cassegrain telescope is designed.

Submillimeter Wave Astronomy Satellite Performance on the ground and in orbit

The Submillimeter Wave Astronomy Satellite (SWAS), which was launched in 1998 December, is a NASA mission dedicated to the study of interstellar chemistry and star formation. SWAS is conducting pointed observations of molecular clouds throughout our Galaxy in either the ground state or a low-lying transition of five astrophysically important species: O2, C I, H218O, 13CO, and H216O at approximately 487, 492, 548, 551, and 557 GHz, respectively. The SWAS instrument is comprised of a 54 cm × 68 cm off-axis Cassegrain telescope feeding two independent heterodyne receivers with Schottky barrier diode mixers, passively cooled to about 175 K. An Acousto-Optical Spectrometer (AOS) provides ~1 MHz (0.6 km s-1) frequency resolution and 1400 MHz (840 km s-1) total bandwidth with 350 MHz (210 km s-1) per line for spectral analysis. SWAS was fully characterized during ground-based testing, and all performance parameters were verified on-orbit. During its on-orbit operation, SWAS observed more than 200 astronomical objects with more than 5000 lines of sight. This paper describes the tests conducted and compares the ground-based test results with the on-orbit test results.

The submillimeter wave astronomy satellite: instrument & mission description

The Submillimeter Wave Astronomy Satellite ( SWAS), launched in December 1998, is a NASA mission dedicated to the study of star formation through direct measurements of: (1) molecular cloud composition and chemistry; (2) the cooling mechanisms that facilitate cloud collapse; and, (3) the large-scale structure of the UV-illuminated cloud surfaces. To achieve these goals, SWAS is conducting pointed observations of dense ( n(H 2)>10 3 cm −3) molecular clouds throughout our Galaxy in either the ground-state or a low-lying transition of five astrophysically important species: H 2O, H 2 18O, O 2, CI, and 13CO. By observing these lines SWAS is: (1) testing long-standing theories that predict that these species are the dominant coolants of molecular clouds during the early stages of their collapse to form stars and planets; and (2) supplying previously missing information about the abundance of key species central to the chemical models of dense interstellar gas. SWAS carries two independent Schottky barrier diode mixers — passively cooled to ∼ 175 K — coupled to a 54 × 68-cm off-axis Cassegrain antenna. During its baseline three-year mission, SWAS is observing giant and dark cloud cores with the objective of detecting or setting a 3σ upper limit on the water and molecular oxygen abundance of 3 × 10 −6 (relative to H 2). In addition, advantage is being taken of SWAS's relatively large beamsize of 3.3 × 4.5 arcminutes at 553 GHz and 3.5 × 5.0 arcminutes at 490 GHz to obtain large-area (∼1° × 1°) maps of giant and dark clouds in the 13CO and CI lines. With the use of a 1.4 GHz bandwidth acousto-optical spectrometer, SWAS has the ability to simultaneously observe either the H 2O, O 2, CI, and 13CO lines or the H 2 18O, O 2, and CI lines with a velocity resolution ∼- 1 km s −1.

The [ITAL]Submillimeter Wave Astronomy Satellite[/ITAL]: Science Objectives and Instrument Description

The submillimeter wave astronomy satellite (SWAS) mission is dedicated to the investigation of star formation and interstellar chemistry. In order to perform the mission, SWAS will survey dense molecular clouds within the Milky Way Galaxy in either the ground state or a low-lying transition of five astrophysically-significant species: H2O, H2(18)O, O2, C I and (13)CO. The observation of these lines will: test theories that predict that these species are dominant coolants of molecular clouds during early stages of their collapse to form stars and planets, and supply information concerning the abundance of species central to the chemical models of dense interstellar gas. The SWAS will use two independent Schottky barrier diode mixers and a 53 x 68 sq cm, off-axis Cassegrain antenna.

The submillimeter wave astronomy satellite: Mission science objectives

The Submillimeter Wave Astronomy Satellite (SWAS) mission is dedicated to the study of star formation and interstellar chemistry. To carry out this mission, SWAS will survey dense ( n H 2 > 10 3 cm −3) molecular clouds within our galaxy in either the ground-state or a low-lying transition of five astrophysically important species: H 2O, H 2 18O, O 2, CI, and 13CO. By observing these lines SWAS will: (1) test long-standing theories that predict that these species are the dominant coolants of molecular clouds during the early stages of their collapse to form stars and planets and (2) supply heretofore missing information about the abundance of key species central to the chemical models of dense interstellar gas. SWAS will employ two independent Schottky barrier diode mixers, passively cooled to ∼150 K, coupled to a 54 × 68-cm off-axis Cassegrain antenna with an aggregate surface error ≤ 11 μm rms. During its three-year mission, SWAS will observe giant and dark cloud cores with the goal of detecting or setting an upper limit on the water abundance of 3 × 10 −6 (relative to H 2) and on the molecular oxygen abundance of 2 × 10 −6 (relative to H 2). In addition, advantage will be taken of SWAS's relatively large beamsize of 3.2 × 4.0 arcminutes at 551 GHz and 3.6 × 4.5 arcminutes at 492 GHz to obtain large-area (∼ 1° × 1°) maps of giant and dark clouds in the 13CO and CI lines. With the use of a 1.4 GHz bandwidth acousto-optical spectrometer, SWAS will have the ability to simultaneously observe the H 2O, O 2, CI, and 13CO lines. All measurements will be conducted with a velocity resolution of less than 1 km s −1.

An Off-Axis Cassegrain Optimal Design for Short Focal Length Parabolic Solar Concentrators

The present work addresses an off-axis Cassegrain optical concentration system. The specific primary collector analyzed, a short focal length parabolic concentrator, is at the University of Florida’s Energy Park. A secondary hyperbolic reflective element was designed to redirect the solar radiation from the primary focal plane to an off-axis target on the polar axis of the primary concentrator. This ground level target will be required for planned experimental work. The analysis was performed using a numerical ray tracing procedure that incorporates both random and systematic errors due to slope and surface irregularities. The optimization process varied secondary element size, curvature, and offset angle, and yielded information required for optimum design. As a single secondary element was found impractical, three elements were designed for use at various times of the year. The numerical analysis predicts that typically 70 to 75 percent of the solar flux incident on the primary concentrator aperture was focused within a 0.5-meter radius. During the design, it was found that this type of compact concentration system is a practical alternative. The optical system is also shown to have advantages that are generally applicable for problems involving short focal length primary concentrators, or when the solar apparatus is to be placed outside the primary collector aperture.