Use Of Telescopes Research Articles

High precision automated alignment procedure for two-mirror telescopes.

A significant challenge in the production of Earth observation satellites is the precise alignment of the telescope optical components. We have developed a strategy to perform automated alignment of two-mirror telescopes for use in a realistic factory-based setting. A Ritchey-Chrétien telescope was used as an example. The secondary mirror was mounted on a high precision hexapod and its misalignment inferred from the Zernike coefficients for tilt, defocus, and coma, as measured by a phase-shifting interferometer. The required corrections to the position of the secondary mirror were implemented using an integral controller and alignment was achieved within minutes, compared to within days when using a manual alignment process. The Zernike coefficient for each aberration was reduced to within one standard deviation of the fluctuations due to residual instability (48nm).

THE SPECTRAL ENERGY DISTRIBUTION OF THE COLDEST KNOWN BROWN DWARF*

ABSTRACT WISE J085510.83–071442.5 (hereafter WISE 0855–0714) is the coldest known brown dwarf (∼250 K) and the fourth-closest known system to the Sun (2.2 pc). It has been previously detected only in the J band and two mid-IR bands. To better measure its spectral energy distribution (SED), we have performed deep imaging of WISE 0855–0714 in six optical and near-IR bands with Gemini Observatory, the Very Large Telescope, and the Hubble Space Telescope. Five of the bands show detections, although one detection is marginal (S/N ∼ 3). We also have obtained two epochs of images with the Spitzer Space Telescope for use in refining the parallax of the brown dwarf. By combining astrometry from this work and previous studies, we have derived a parallax of 0.449 ± 0.008″ (2.23 ± 0.04 pc). We have compared our photometry for WISE 0855–0714 to data for known Y dwarfs and to the predictions of three suites of models by Saumon et al. and Morley et al. that are defined by the presence or absence of clouds and nonequilibrium chemistry. Our estimates of Y − J and J − H for WISE 0855–0714 are redder than colors of other Y dwarfs, confirming a predicted reversal of near-IR colors to redder values at temperatures below 300–400 K. In color–magnitude diagrams, no single suite of models provides a clearly superior match to the sequence formed by WISE 0855–0714 and other Y dwarfs. Instead, the best-fitting model changes from one diagram to the next. Similarly, all of the models have substantial differences from the SED of WISE 0855–0714. As a result, we are currently unable to constrain the presence of clouds or nonequilibrium chemistry in its atmosphere.

Optical Design of a Reflecting Telescope for CubeSat

Space telescope optics is one of the major parts of any space mission used to observe astronomical targets or the Earth. This kind of space mission typically involves bulky and complex opto-mechanics with a long optical tube, but attempts have been made to observe a target with a small satellite. In this paper, we describe the optical design of a reflecting telescope for use in a CubeSat mission. For this design we adopt the off-axis segmented method for astronomical observation techniques based on a Ritchey-Chr $\acute{e}$ tien type telescope. The primary mirror shape is a rectangle with dimensions of $8cm{\times}8cm$ , and the secondary mirror has dimensions of $2.4cm{\times}4.1cm$ . The focal ratio is 3 which can yield a 0.383 degree diagonal angle in a $1280{\times}800$ CMOS color image sensor with a pixel size of $3{\mu}m{\times}3{\mu}m$ . This optical design can capture a ${\sim}4km{\times}{\sim}2.3km$ area of the earth's surface at 700 km altitude operation.

Design of differential optical absorption spectroscopy long-path telescopes based on fiber optics

We present a new design principle of telescopes for use in the spectral investigation of the atmosphere and the detection of atmospheric trace gases with the long-path differential optical absorption spectroscopy (DOAS) technique. A combination of emitting and receiving fibers in a single bundle replaces the commonly used coaxial-Newton-type combination of receiving and transmitting telescope. This very simplified setup offers a higher light throughput and simpler adjustment and allows smaller instruments, which are easier to handle and more portable. The higher transmittance was verified by ray-tracing calculations, which result in a theoretical factor threefold improvement in signal intensity compared with the old setup. In practice, due to the easier alignment and higher stability, up to factor of 10 higher signal intensities were found. In addition, the use of a fiber optic light source provides a better spectral characterization of the light source, which results in a lower detection limit for trace gases studied with this instrument. This new design will greatly enhance the usability and the range of applications of active DOAS instruments.

NRAO-Exploring the Microwave Universe [MTT World

The National Radio Astronomy Observatory (NRAO) is a federally funded research and development center (FFRDC) of the United States National Science Foundation (NSF) operated under cooperative agreement by Associated Universities, Inc. (AUI) for the purpose of radio astronomy. NRAO designs, builds, and operates high-sensitivity radio telescopes for use by scientists around the world. Over 800 of the NRAO's employees are spread out across the Western Hemisphere, in partnership with the scientific community, and provide world-leading telescopes, instrumentation and expertise, train the next generation of scientists and engineers, and promote astronomy to foster a more scientifically literate society.

Computational geometry and the design of aperture-synthesis telescopes

When designing an aperture-synthesis telescope for use in astronomy it is essential to find a layout of the antennas on the site that provides good coverage of the uv-plane. Large holes in the distribution of baseline points in that plane are particularly undesirable, and so an algorithm was devised for locating all the holes in any given design and assessing whether they are unacceptably large. It finds, in each region of the uv-plane, the largest circle that does not contain any of the baseline points and then takes the properties of that empty circle, such as the radius and the position of the centre, as measures of the properties of the local hole. The algorithm is based on the Delaunay triangulation of the set of baseline points and makes use of the result that the circumscribed circle of any Delaunay triangle is always empty. The largest empty circles in the uv-plane are readily found in this way, and the algorithm selects from these circles a non-overlapping set that is a good match to the intuitive concept of the largest holes in the distribution of baseline points. Modified algorithms are also presented that rank each circle not by its absolute radius but by the radius normalized to the mean radius of the circles in the same neighbourhood of the uv-plane; these versions make proper allowance for the large-scale variation of the density of baselines caused by the taper and are therefore of greater use to the designers of a telescope. The algorithms are efficient, requiring O(NB logNB) operations where NB is the number of baselines, and may be used in several ways. One is to take the radius of the largest empty circle as a figure of demerit so that, of a set of trial configurations, the one with the smallest value may be regarded as having the best uv-plane coverage. Another is to take the baselines on the circumference of the largest circle to indicate which antennas are the most suitable for repositioning in order to improve the quality of the uv-plane coverage of a trial configuration.

Realization of a tunable crystal lens as an instrument to focus gamma rays

The Argonne/Toulouse collaboration is developing a crystal lens diffraction telescope for use as an astrophysical detector in the energy range of 200–1.3 MeV. The lens consists of eight rings of diffraction crystals that all focus a narrow band of energies on a common HPGe detector. The inclination angle of these crystals controls the energy band being focused and will need to be adjusted over a range of 0.5–1.5° with arcsecond precision to cover this energy band. At Argonne National Laboratory, a new lens frame was constructed, and the inner ring was equipped with 16 Ge crystals of 1 cm 3 size. The orientation of each crystal was adjusted using a piezo-based picomotor in combination with a noncontact eddy-current sensor. The sensors have 0.1–0.2 arcsecond resolution; the motors have a step size of 0.05–0.2 arcseconds. By changing the crystal inclination and the distance of the detector from the lens, we were able to focus the 662 keV radiation from a 137Cs source at 24.75 m, as well as line energies at 276, 303, 356, and 383 keV from a 133Ba source at 24.45 m. The sensor and system stability were demonstrated by alternately focusing different line energies. We were able to simulate scans in energy of a spaceborne instrument, as well as the enlargening of the energy response by a slight detuning of the lens crystals.

Directions for Space‐Based Low‐Frequency Radio Astronomy: 2. Telescopes

Astronomical studies of celestial sources at low radio frequencies (0.3 to 30 MHz) lag far behind the investigations of celestial sources at high radio frequencies. In a companion paper [Basart et al., this issue] we discussed the need for low‐frequency investigations, and in this paper we discuss the telescopes required to make the observations. Radio telescopes for use in the low‐frequency range can be built principally from “off‐the‐shelf” components. For relatively little cost for a space mission, great strides can be made in deploying arrays of antennas and receivers in space that would produce data contributing significantly to our understanding of galaxies and galactic nebulae. In this paper we discuss an evolutionary sequence of telescopes, antenna systems, receivers, and (u, v) plane coverage. The telescopes are space‐based because of the disruptive aspects of the Earth's ionosphere on low‐frequency celestial signals traveling to the Earth's surface. Orbiting antennas consisting of array elements deposited on a Kevlar balloon have strong advantages of nearly identical multiple beams over 4π steradians and few mechanical aspects in deployment and operation. The relatively narrow beam width of these antennas can significantly help reduce the “confusion” problem. The evolutionary sequence of telescopes starts with an Earth‐orbiting spectrometer to measure the low‐frequency radio environment in space, proceeds to a two‐element interferometer, then to an orbiting array, and ends with a telescope on the lunar farside. The sequence is in the order of increasing capability which is also the order of increasing complexity and cost. All the missions can be accomplished with current technology.

THE TELESCOPE SYSTEM OF THE MACHO PROGRAM

ABSTRACT The renovation of the Great Melbourne Telescope for use as a 1.27m aperture telescope in the MACHO program is described. The current mode of telescope structure, drive systems and control are outlined and ongoing programs of dome seeing improvement are discussed.

A CdTe gamma-ray spectrometer with imaging capabilities for astrophysics: Monte Carlo simulation results

One of the critical design constraints for satellite-borne gamma-ray astronomy telescopes incorporating solid-state (mainly Ge) detectors has been the problem of keeping the detection plane at a very low temperature. This problem should be alleviated by the considerable progress being made in the technology of room temperature devices such as CdTe detectors. Furthermore, a new geometrical arrangement in the design of these particular detectors allows the use of small devices (μ-detectors) with high detection efficiency and good spectroscopic performance (E/δE ∼ 100). These μ-detectors can be mosaiced in order to form a large area position sensitive detector (PSD) with fine spatial resolution (about a few millimeters). Such a PSD could be coupled with a coded aperture to realise a telescope for use in high energy astronomy (0.05–5 MeV). Herein we give preliminary results from a Monte Carlo simulation of such a PSD concerning the detection efficiency and spatial resolution as a function of photon energy and incident angle. The results suggest that CdTe is a promising new detection medium for use in high energy astronomy.

A high resolution imaging detector for TeV gamma-ray astronomy

Details are presented of an atmospheric Cherenkov telescope for use in very high energy gamma-ray astronomy which consists of a cluster of 109 close-packed photomultiplier tubes at the focus of a 10 meter optical reflector. The images of the Cherenkov flashes generated both by gamma-ray and charged cosmic-ray events are digitized and recorded. Subsequent off-line analysis of the images improves the significance of the signal to noise ratio by a factor of 10 compared with non-imaging techniques.

Comparison Of Large Aperture Telescopes With Parabolic And Spherical Primaries

Quasi-Cassegrain-type four-mirror telescopes are compared to conventional two-mirror Cassegrain telescopes for use as high performance, very large aperture space telescopes. Spherical and parabolic primaries with continuous as well as segmented surfaces are considered. Imaging characteristics and misalignment sensitivities serve as the principal criteria of comparison. The evaluation shows that parabolic primaries yield superior wide-field performance, whereas spherical primaries hold distinct advantages regarding manufacturability and regarding certain alignment aspects in the case of segmentation.

Fast Timing of Air Shower Fronts

In order to design a telescope for use in ultra-high energy y-ray astronomy one needs to carefully consider those effects which determine the system angular resolution. This paper considers detector and discriminator designs for such a system including a consideration of the effect of the spread of particles in the shower front. An angular resolution of ~ 10 at energies of 10 15 e V can be achieved

Status and problems of semiconductor detectors

A brief review is given of the types of silicon and germanium detectors used or presently being developed for nuclear experiments. Large-area silicon and germanium detector telescopes for use in long-range particle detection and identification are emphasized. Large area position-sensitive detectors are also described. Some results are presented regarding radiation damage and damage repair by annealing. Evidence is also presented for the importance of producing large area silicon crystals of adequate quality to reduce trapping problems to negligible proportions.

<title>High Performance Extreme Ultraviolet (EUV) Telescope</title>

A nested array of Wolter type II grazing incidence telescopes for use in the extreme ultraviolet is described. It is demonstrated by an example that it is possible to design the array with all subsystems having matching focal lengths, matching field radii, the same grazing angle at the primary elements, and only slightly different grazing angles on the secondary elements.

Effects of diffraction in multiple-grid telescopes for x-ray astronomy

We describe the theory of diffraction in multiple-grid x-ray telescopes for use in astronomy. Convenient numerical procedures for estimating the effects of diffraction are given and illustrated for typical grid telescopes; in general diffraction limits the long-wavelength performance of such telescopes at the present state of technology. Comparisons with the measurements of Blake et al. [ Rev. Sci. Instrum.47, 889 ( 1976)] show favorable agreement for wavelengths of order 50 Å. The effects of diffraction are similar in magnitude to the results of convolving the geometrically derived response profile with the Fraunhofer diffraction pattern expected from a slit the same size as the grid openings. The possibility of diffraction causing significant leaks which are not allowed geometrically does not seem to occur. Blake et al. measured the response of their collimators by looking at the collimated output of one diffusely-illuminated collimator with another similar collimator. We suggest a variation on their technique which entails effectvely averaging over the relative translational positions between the two collimators so as to eliminate the effect of the variable overlap of the facing grids from consideration.

Spacelab Infrared Telescope Facility (SIRTF)

NASA has completed a preliminary design study of a 1.2-m, cooled infrared telescope for use on 7 to 28 day Spacelab missions. Beryllium optics, cooled to below 20° K with supercritical helium, are used. Noise in the infrared telescope will be less, by a factor of 10 , than that of ground-based telescopes operating at 10 /*m (when the telescopes are operated over comparable bandwidths and fields of view). The Spacelab infrared telescope facility will enable astronomers to see more than 30 times deeper into the universe than is now possible over most of its 5-200 jim optimum spectral range. Such capability will initiate a spectacular advance in our knowledge and understanding of the cool regions of the universe where molecules and dust are the predominant radiators and absorbers.

Negative Affinity X-Ray Photocathodes

True image-forming telescopes for use in X-ray astronomy are available now with arc-second resolution for A ≥3A. Such telescopes have been very successfully applied on rockets and on the recent Skylab mission for solar observations. However, their application to observations of extra-solar X-ray sources has been severely limited due to a lack of sensitive X-ray imaging detectors. We describe here a new type of detector using a GaAs photocathode with quantum efficiency approaching 100%. This detector allows us to detect sources 10 to 20 times fainter than previously possible, while preserving spatial resolution on the several micron scale. We present data on the X-ray photoelectron yield for GaAs (Cs-O2) negative affinity cathodes both in reflection mode using a thick cathode and in the transmission mode using a 3.7 micron wafer. A theoretical model is developed and compared with the data to determine the electron diffusion length, the front surface electron reflection coefficient and the back surface electron escape probability for the transmission cathode. There is good agreement between the crystal parameters obtained from X-ray data and from optical data. Application to X-ray telescope imaging in a practical device is discussed.

A focusing X-ray telescope for use in the study of extraterrestrial X-ray sources in the energy range 20–140 keV

We have designed and tested a focusing X-ray telescope with an angular resolution (FWHM) of 1.0° for the study of extraterrestrial X-ray sources in the energy range 20–140 keV. The telescope's lens is an array of rock salt crystals (each of which is approximately one inch square), mounted on a 6-ft-diameter paraboloidal frame. The lens is located 9.5 ft above a 2-in.-diameter NaI crystal and PM tube (which measures the energy of the incident X ray). X rays passing through the salt crystals undergo a 2θ deflection if the Bragg condition for reflection, nλ = 2d sin θ, is approximately satisfied. Optimum thickness and effective energy bandwidths for such reflections have been determined for energies in the telescope's operating range. The area-efficiency product of the lens is approximately 8 times the NaI crystal area from 20 to 40 keV, giving a real improvement in the signal-to-background ratio. Crystals more nearly perfect than rock salt offer the possibility of increased gain over narrow energy intervals. Polarization measurements of the incident X rays may also be made.

A charged particle telescope for use in the measurement of the cosmic ray electron spectrum in the energy range 10 MeV-2 GeV

Recent observations near the top of the earth's atmosphere have indicated the presence of a component of extra-terrestrial electrons. This paper will describe the mating of a low pressure gas Čerenkov detector and a lead glass Čerenkov total energy spectrometer into a particle telescope that is suitable for measurements of this cosmic ray electron spectrum in the energy range 10 MeV-2 GeV near the top of the atmosphere. Various studies of the efficiency and calibration of the instrument have been made and will be discussed. In addition, tests of the response of this system to sea level cosmic ray muons and electrons and the measurement of the sea level electron spectrum will also be reported.