Showing papers in "Methods in Experimental Physics in 1974"

8. Polarization Techniques

Abstract: Publisher Summary This chapter describes the polarization techniques for polarization of light of astronomical objects. An analyzer is a device that subdivides the incident light into two beams, one of them linearly polarized in the plane parallel to the principal plane of the analyzer, the other in the perpendicular plane. If the accuracy of measurements is limited by the photon statistics, the amount of information on wavelength dependence of polarization obtained in a given time is proportional to the number of detectors used simultaneously for different spectral regions. A pair of retarders, one made of positive crystal and the other of negative crystal, with a properly chosen thickness ratio, makes a wide-field retarder for which the retardance is almost independent of the angle of incidence unless this angle is very large. It is observed that the depolarizing action of a Lyot depolarizer can be improved if the retardance not only depends on wavelength but also changes over the surface of the depolarizer. The optimum design of an astronomical polarimeter is also elaborated in the chapter.

11. Fourier Spectrometers

Abstract: Publisher Summary This chapter examines the theory and applications of Fourier transform spectroscopy. A Michelson interferometer is ideally suited for work in high-resolution infrared spectroscopy of astronomical objects. In this application the available signal-to-noise ratio limits the number of observable objects and, therefore, any gain in signal-to-noise is of tremendous importance. The Michelson interferometer is a device for interfering two beams of light. Light entering the interferometer is divided into two separate beams by a beam splitter and is recombined after a controllable phase delay has been applied to one of the beams The use of digital computers in performing the Fourier transform of the interferogram, implies that the data be supplied in digital form. The effect of the slow-beat frequency is to make the contributions from spectral elements near the calculated element essentially positive rather than averaging to zero. The dispersive properties of the beam splitter can cause wave number-dependent phase shifts. The data recording and processing systems are as important as the optical and mechanical systems of the interferometer.

9. The Instrumentation and Techniques of Infrared Photometry

Abstract: Publisher Summary This chapter examines the instrumentation and techniques of infrared photometry. It presents the design of telescopes and modulators used in infrared. Incoherent detectors, such as bolometers and photoconductors, respond only to the incident power level or number of incident quanta, but they provide the highest possible signal/noise when observing continuous sources. Excess noise across the junction can be eliminated by reducing the photovoltaic potential to nearly zero by drawing off the charge through a low-input impedance current-measuring amplifier. One of the great advantages of the interference filters is the fact that they can usually be repeatedly cooled to low temperatures without degradation. Interference filters have been constructed in the far infrared using metal mesh grids in place of semitransparent metal films. For conventional telescopes used in the infrared, the field stops usually also serves as the defining stop in order to minimize the background from the oversized secondary and sky baffle. It is observed that when the telescope is in good collimation, extremely small stable offsets can be achieved and modulation noise remains below true sky noise for beam separations less than 30 arcs.

12. Fabry-Perot Instruments for Astronomy

Abstract: Publisher Summary This chapter elaborates the use and design of Fabry–Perot spectrometers for problems in astronomy. The ideal Fabry–Perot interferometer consists of a pair of transparent perfect optical flats having semitransparent mirrors assembled a small distance apart with the mirrored surfaces facing one another and accurately parallel. When a uniform monochromatic field is viewed through a Fabry–Perot interferometer, the field is seen as a concentric narrow-ring pattern at infinity centered at normal incidence to the interferometer, with ring positions. A scanning spectrometer is achieved by providing means for changing the wavelength of the light passing through the annulus and means for measuring and recording the light intensity as a function of wavelength, usually a photomultiplier and associated electronics. The scanning of multiple Fabry–Perot spectrometers is the same as for single etalons in the case of pressure and, for a simple series, angular scanning. The resolution of a Fabry–Perot spectrometer may be varied over a wide range by varying the plate spacing and, to a lesser extent by varying the reflective finesse.

1. Photomultipliers: Their Cause and Cure†

Abstract: Publisher Summary This chapter describes the basic physics and construction of photomultipliers. Photoelectric photometry is a fundamental technique in observational astronomy. The photo emissive detector produces a current supposedly proportional to the light intensity falling on it, thus reducing photometry to a simple electrical measurement that can be made very accurately. A thick, internal cold-box window can be helpful in lowering cathode temperature, but it increases cosmic-ray noise and so it may not reduce the total dark noise. A lower string current reduces the heat dissipated in the string, and hence the heat input and thermal gradients at the tube base. Nonlinearity can vary with the position of the illuminated spot on the cathode, and with wavelength. It is suggested that the area of the photocathode illuminated during measurements, and the wavelength of the radiation, are the same as those used during calibration. The pulse-height distribution due to thermionic or field emission from the cathode is similar to that caused by photoelectrons.

3. Observational Technique and Data Reduction

Abstract: Publisher Summary This chapter describes the different aspects of observational technique and data reduction used in photometry. The accurate extinction measurements can be made easily and without going to great air masses. A thorough discussion of the pitfalls of extinction measurement and correction is given, both from the observational side and from the point of making accurate reductions. Random changes in photomultiplier dynode gain may be caused by cesium migration within the tube. These changes are usually rather slow and may be measured by observing a standard source. Random photomultiplier temperature variations affect dynode gain and cathode spectral response. These variations are usually large and significant in dry-ice-cooled boxes, but can be much reduced by using a heat-transfer liquid such as ethyl acetate or Freon-11 on the dry ice. If the instrument is stable, individual magnitudes should be more precise than their differences by a factor of 2 1/2 , except in the important case of simultaneous measurements where cancellation of scintillation noise is possible.

4. Reshaping and Stabilization of Astronomical Images

Abstract: Publisher Summary This chapter examines the reshaping and stabilization of astronomical images. The simplest image stabilizer is a connection to the slow-motion controls of the main telescope. Much faster response can be obtained by rotating or translating a small optical element. The image stabilizer for a single object offers a different problem, for fast response is useful but at the same time the light for the measurement should not be wasted. The Richardson slicer resembles a confocal cavity or a White cell. The mirrors can be adjusted in two different ways. Multiple strips are formed along the slit, in much the same way as a Bowen slicer and each beam is superposed on the grating. Addition of a Bowen slicer to an existing coude spectrograph will shorten the exposure only to the extent that it is no longer necessary to widen the spectrum by trailing the image. The most direct method is to divert some 10% of the light by a beamsplitter. IA small part of the telescope aperture may be used to provide an offset tracking beam.

2. Other Components in Photometric Systems

Abstract: Publisher Summary This chapter elaborates optical systems, calibration problems, and standard sources in photometric systems. In measuring the light of a star, a small diaphragm is placed in the focal plane of a telescope to exclude, as far as possible, unwanted light from neighboring stars and from the sky. The photometric error due to decentering is the difference between the light shifted out of the diaphragm on one side and the light shifted in on the other. The transmission of absorbing filters decreases with increasing temperature as the absorption bands widen out. The transmission of interference filters may decrease in the passband and increase in the wings if the temperature changes produce a mismatch in the resonant structure at the peak wavelength, but this effect is usually small. If the lamp is placed farther away to reduce the foregoing problems, the horizontal extinction rises proportionally. The usual method of estimating horizontal extinction has been to measure the vertical extinction astronomically and scale the result by the amount of air in each path. It is suggested that all photometers should be checked for linearity, especially at the higher light levels.

10. Diffraction Grating Instruments

Abstract: Publisher Summary This chapter presents the general properties of gratings and grating spectrometers and a consideration of a number of grating instrument designs, as they are used in astronomy. The important parameters of any spectrometer include spectral purity spectral resolution, throughput, throughput-resolution product, efficiency, and spectrographic speed. A summary of some of the pertinent properties of diffraction gratings is provided. The intensities for wavelengths other than the blaze wavelengths are determined by the single slit diffraction pattern of the individual groove. When a plane grating is used in non-collimated light, aberrations heretofore absent are introduced. These aberrations may either be compensated by other optical elements within the spectrometer or may be negligible as compared with limitations imposed by the seeing conditions. A concave grating cross disperser also serving as the spectrometer collimator should be more efficient than the transmission grating arrangement, and can be selected to give adequate order separation.

6. Two-Dimensional Electronic Recording

Abstract: Publisher Summary This chapter describes two-dimensional electronic recording using image tubes. Phosphor screens can convert the kinetic energy of an incident electron into light with an efficiency of about 10%. In order to achieve very high photon gain, several tubes must be cascaded or electron-multiplying sandwiches must be inserted between the photocathode and the phosphor screen. The simplest tube is one that uses proximity focusing. The phosphor screen is located very close to the photocathode. Photoelectrons released by the photocathode are accelerated to the phosphor screen by a uniform axial electric field. The P 829D uses 5 KCl transmission dynodes on alumina support films to provide an average electron gain of about 3000. It is observed that when used with fast emulsions, this is sufficient to expose a number of photographic grains for each event, even following the light losses introduced by the transfer lens. Tubes with electronic magnification can be used to match the optical scale of a telescope or spectrograph camera to the resolving capability of the detector.

7. X-Ray and Gamma-Ray Detection by Means of Atmospheric Interactions: Fluorescence and Čerenkov Radiation

Abstract: Publisher Summary This chapter describes the detection of cosmic X-ray and gamma-ray by atmospheric fluorescence and atmospheric Cerenkov radiation. In air, the fluorescence process in the visible and near-infrared regions of the spectrum occurs almost entirely in the nitrogen molecule and the nitrogen molecular ion. For an X-ray beam of given energy and incident in a normal direction on the atmosphere, the altitude of maximum fluorescence radiation is presented. An important parameter in the detection of atmospheric fluorescence is the time duration of the radiation received at the ground-based detector when an X-ray burst. The primary contribution to the Cerenkov light from an air shower is from the electromagnetic cascade, since the nuclear particles contribute only a small fraction to the total number. In the first approximation, a gamma-ray-initiated shower cannot be distinguished from a proton shower of similar energy by ground-based detection of Cerenkov light. The lateral distribution of Cerenkov light from a 1011-eV photon-initiated shower as a function of the detector's field of view, the temporal characteristics of the light pulse and the light intensity as a function of distance from the shower core are also shown.

5. Detective Performance of Photographic Plates

Abstract: Publisher Summary This chapter describes the detective performance of photographic plates. The detective properties of the three types of Kodak Spectroscopic Plates; 103a-O, IIa-O, and IIIa-J are presented. One of the first steps in a photometric determination is the measurement of the degree of blackening of various positions on the photograph. This is usually accomplished with an instrument that measures the relative fraction of light that is transmitted by a small area. The output noise-to-signal for the same five plates depends only on the density above fog and has essentially no dependence on developing time at a given density. It is observed that the noise-to-signal performance of IIa-O is virtually constant for densities greater than about 0.4 above fog. If IIa-O plates are being used for photometry of very faint objects, the developer type and developing time can be chosen to bring up the density of the faintest images. It is suggested that the developer type and developing time can be chosen for low contrast in order to increase the dynamic range of the plate.