Here We Go

An alternative title of this material could be “The Data Everyone Would Like to Get for their Research!” The first thing we seem to do in astronomy is ‘see’ something, be it simply looking in the sky, using a big telescope, or helping ourselves with sophisticated adaptive optics or space probes. But the very next thing we want to do is get that light into a spectrograph! We might get spectral information from colors, energy distributions, modest resolution or real honest high resolution spectroscopy, but we desperately need such information. Why? Well, because that's where most of the physical information is, and higher spectral resolution means access to more and better information. High resolution implies actually resolving the structure of the spectrum. Naturally we want to do this as precisely as possible, not only pushing toward good spectral resolution and high signal-to-noise, but also by understanding how the equipment has modified the true spectrum and by weeding out problems and undesirable characteristics. The main focus here will be on the machinery of spectroscopy, but oriented toward optical spectrographs and the spectral lines they are best suited to analyze. I do not concentrate on the specific instruments, but rather on the techniques and thought patterns we need. These are the fundamental things you can take with you and apply to any spectroscopic work you do. Of course, you will always have to fill in specific details for the particular machinery and tools you use.

Astronomy is entering a new observational era with the advent of several Large Telescopes, 8 to 10 metre in size, which will shape the kind of Astrophysics that will be done in the next century. Scientific focal plane instruments have always been recognized as key factors enabling astronomers to obtain the maximum performance out of the telescope in which they are installed. Building instruments is therefore not only a state of the art endeavour, but the ultimate way of reaching the observational limits of the new generation of telescopes. Instruments also define the type of science that the new telescopes will be capable of addressing in an optimal way. It is clear therefore that whatever instruments are built in the comming years they will influence the kind of science that is done well into the 21st century.

The goal of the 1995 Canary Islands Winter School of Astrophysics was to bring together advanced graduate students, recent postdocs and interested scientists and engineers, with a group of prominent specialists in the field of astronomical instrumentation, to make a comprehensive review of the driving science and techniques behind the instrumentation being developed for large ground based telescopes. This book is unique indeed in that it combines the scientific ideas behind the instruments, something at times not appreciated by engineers, with the techniques required to design and build scientific instruments, something that few astronomers grasp during their education.

Chapter 1 reviews the image restoration/reconstruction problem in its general setting. We first discuss linear methods for solving the problem of image deconvolution, i.e. the case in which the data is a convolution of a point-spread function and an underlying unblurred image. Next, non-linear methods are introduced in the context of Bayesian estimation, including Maximum-Likelihood and Maximum Entropy methods. Finally, the successes and failures of these methods are discussed along with some of the roots of these problems and the suggestion that these difficulties might be overcome by new (e.g. pixon-based) image reconstruction methods.

Chapter 2 discusses the role of language and information theory concepts for data compression and solving the inverse problem. The concept of Algorithmic Information Content (AIC) is introduced and shown to be crucial to achieving optimal data compression and optimized Bayesian priors for image reconstruction. The dependence of the AIC on the selection of language then suggests how efficient coordinate systems for the inverse problem may be selected. This motivates the selection of a multiresolution language for the reconstruction of generic images.

Chapter 3 introduces pixon-based image restoration/reconstruction methods. The relationship between image Algorithmic Information Content and the Bayesian incarnation of Occam's Razor are discussed as well as the relationship of multiresolution pixon languages and image fractal dimension. Also discussed is the relationship of pixons to the role played by the Heisenberg uncertainty principle in statistical physics and how pixon-based image reconstruction provides a natural extension to the Akaike information criterion for Maximum Likelihood estimation.

This paper reviews near infrared instrumentation for large telescopes. Modern instrumentation for near infrared astronomy is dominated by systems which employ state-of-the-art infrared array detectors. Following a general introduction to the near infrared wavebands and transmission features of the atmosphere, a description of the latest detector technology is given. Matching of these detectors to large telescopes is then discussed in the context of imaging and spectroscopic instruments. Both the seeing-limited and diffraction-limited cases are considered. Practical considerations (e.g. the impact of operation in a vacuum cryogenic environment) that enter into the design of infrared cameras and spectrographs are explored in more detail and specific examples are described. One of these is a 2-channel IR camera and the other is a NIR echelle spectrograph, both of which are designed for the f/15 focus of the 10-m W. M. Keck Telescope.

The Near Infrared Waveband

In the last ten years there has been tremendous growth in the field of Infrared Astronomy. This growth has been stimulated in large part by the development of very sensitive imaging devices called infrared arrays. These detectors are similar, but not identical, to the better-known silicon charge-coupled device or CCD, which is limited to wavelengths shorter than 1.1 µm. In particular, near infrared array detectors are now sufficiently sensitive that images of comparable depth to those obtained with visible-light CCDs can be achieved from 1.0 µm to 2.4 µm and high resolution IR spectrographs are now feasible.

This lecture introduces the opportunities presented by ground-based telescopes for new discoveries in the thermal infrared, and discusses techniques used to make sensitive observations in an environment with high background flux levels from atmospheric emission and from the telescope structure and mirrors.

Mid-IR astronomy—opportunities and problems

The capability now exists to observe mid-IR astronomical objects with spatial resolution of a third of an arcsecond and sensitivities reaching well below a mJy. Both imaging and spectroscopy with new array instruments on optimized large telescopes are producing new data on sources from comets, to active galactic nuclei. With sensitivity to emission from cool dust, diagnostic lines from ionized gas and molecular species, and the capability to look through clouds opaque in the visible, many new results are appearing, and many more can be anticipated. In particular, our understanding of the star formation process should improve significantly in the next decade. Yet all of this is achieved operating through the earth's atmosphere which absorbs and distorts the signals, and which, together with the telescope structure itself, radiates into the beam up to a million times the power detected from the source. The problems encountered, and the techniques used to make ground based mid-IR observations will be discussed here.

IRAS (Infrared Astronomical Satellite) revealed how fascinating and complex the IR sky is at wavelengths of 12, 25, 60 and 100 µm. The IRAS mission lasted for 300 days in 1983 completing an all sky survey with a 57-cm diameter cooled telescope.

In Chapter 3, we studied how single charged particles move in specified electric and magnetic fields, and we then applied our knowledge of single particle motion to the radiation belt and ring current plasma. However, the fields in some situations depend too much on the particle distributions to be readily specified and must be found self-consistently using the charged particle distribution functions. Often, it is not necessary to have complete information about the distribution functions in a system. In fact, it is usually sufficient to know only a few of the velocity moments of the distribution function, as derived in Chapter 2. In Chapter 4, we will adopt the “fluid” picture of a plasma, introduced in Chapter 2, and further refine it to obtain an analytical tool useful for studying space plasma phenomena. This analytical tool is called magnetohydrodynamics (or MHD for short). We cannot adequately cover in one chapter all the material that would be desirable to know about this subject and so the reader is encouraged to consult one or more of the references listed in the bibliography at the end of this chapter.

Two-fluid plasma

Let us consider a plasma consisting of two species: electrons (e) with mass me and a single ion species (i) with mass mi.

Most of the visible matter in the universe exists as a fluid composed of electrically charged particles rather than as a gas made of neutral atoms or molecules. Gas mixtures of electrically charged particles, such as electrons and ions, are called plasmas. Plasmas are found in the following solar system environments: the solar atmosphere, the interplanetary medium, planetary magnetospheres, and planetary ionospheres. Most of the interstellar medium is also plasma, as are most other regions of our galaxy.

Most of the plasma found in our own solar system is accessible to in situ measurements made by instruments onboard spacecraft. Since the advent of the space age in the late 1950s, space probes have visited Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and comets Giacobini–Zinner, Halley, and Grigg–Skjellerup. The space environment surrounding the Earth has also been extensively studied by experiments onboard rockets and satellites. The Sun and astrophysical plasma environments outside our own solar system are not subject to direct measurements but must be observed remotely with sophisticated instruments located either at ground-based observatories or on orbiting observatories. An exception to this are the very energetic particles called cosmic rays, which can be observed using Earthbased or balloon-borne experiments. Solar cosmic rays have energies up to about 100 million electron volts (100 MeV) and originate in the solar corona.

A gas consisting of charged particles is called a plasma, although the use of the term is often restricted to charged particle gases in which collective phenomena, such as plasma oscillations, are more important than collisional phenomena. Collisions generally involve the short-range interactions of discrete particles, whereas collective phenomena involve large numbers of particles working in unison. The charged particle species in most plasmas are positive ions and negative electrons, although negative ions are also present in the D-region of the terrestrial ionosphere. Fully ionized plasmas contain only charged particles, whereas partially ionized plasmas also contain neutral gas. The solar wind plasma – that is, the interplanetary medium – is a fully ionized plasma; the ionosphere is a partially ionized plasma. A variety of methods have been developed to describe plasmas. Kinetic theory uses particle distribution functions to describe plasmas, whereas fluid theory (which includes magnetohydrodynamics or MHD) only uses a few macroscopic quantities derived from the full particle distribution functions. Because the subject of kinetic theory is largely outside the scope of an introductory book on space physics, this book will primarily use fluid theory to explain plasma phenomena in the solar system. However, a short introduction to kinetic theory and the derivation from kinetic theory of the fluid equations is provided in this chapter. More detailed treatments of kinetic theory can be found in the references listed at the end of the chapter.

We learned in the previous chapter that the solar wind is an almost collisionless plasma consisting mainly of protons and electrons flowing outward from the Sun supersonically and super-Alfvénically at several hundred kilometers per second. The interplanetary magnetic field is carried out into the solar system by the solar wind. Planets and other solar system bodies act as obstacles to the flow of the solar wind, but the nature of this interaction strongly depends on the characteristics of the planet. Chapter 7 deals with the solar wind flow around planets and other objects. A very brief introduction to this topic was given in Chapter 1. Further reading material on this topic can be found in the bibliography at the end of this chapter. Chapter 8 will deal with the internal dynamics of the terrestrial magnetosphere as well as with the magnetospheres of the outer planets.

Types of solar wind interaction

Nature of the obstacle

The manner in which the solar wind interacts with objects, or bodies, in the solar system depends, naturally, on the characteristics of that object. Relevant characteristics include its heliocentric distance (r), its size, whether or not it has an atmosphere and ionosphere, and the strength of its intrinsic magnetic field. Table 7.1 lists some relevant characteristics for all the planets and for other solar system bodies.

The Sun is a star. As stars go, the Sun is rather cool and small and has the gross characteristics listed in Table 5.1. The Sun is the source of virtually all energy in our solar system, including the Earth. Solar radiation heats our atmosphere and provides the light needed to sustain life on our planet. The Sun is also the source of space plasmas throughout the solar system. For example, solar extreme ultraviolet (EUV) radiation is largely responsible for the existence of planetary ionospheres via the photoionization of atoms and molecules in the upper atmospheres of the planets. The solar wind plasma is really an extension of the solar corona out into interplanetary space. The Sun is also, naturally, the source of solar activity. Solar activity refers to both short-term and long-term temporal variations in the solar atmosphere (and hence in the solar wind) that create changes in the Earth's plasma environment (i.e., geomagnetic activity). We will deal with the effects of solar activity on the Earth later.

The field of solar physics has advanced dramatically during the past few decades, due to observations made by increasingly sophisticated ground- and space-based observatories, including NASA's OGO, Skylab, and Solar Maximum missions and the NASA/ESA SOHO (Solar and Heliospheric Observatory) mission, and due to theoretical developments in the areas of stellar nuclear physics, stellar radiative transfer, and solar MHD.

The intrinsic magnetic field of the Earth acts as an obstacle to the solar wind and shields a volume of space, called the magnetosphere, from direct access of the solar wind. In Chapter 7, we considered the role of the magnetosphere as an obstacle to the solar wind and were mainly concerned with the region “external” to the magnetopause. The details of the internal dynamics of the magnetosphere do not seriously affect, at least to about the 95% level, the external solar wind plasma flow, but the solar wind does strongly affect the internal dynamics of the magnetosphere and ionosphere, as we will see in this chapter. This chapter will strongly emphasize macroscopic or fluid aspects of magnetospheric physics rather than the microscopic physics operating in the magnetosphere. Some aspects of the inner magnetosphere (i.e., the ring current and radiation belts) were already considered in Chapter 3.

The terrestrial magnetosphere has been extensively studied over the past 35 years with dozens of Earth orbiting satellites. The International Sun Earth Explorer (ISEE), Dynamics Explorer (DE), and AMPTE missions have been especially important, and in the near future we can expect useful information from recently launched spacecraft such as Geotail and Polar. The volume of observational and theoretical literature that exists, mainly in the Journal of Geophysics Research–Space Physics, has become immense. Much has been learned about how the magnetosphere works, although many key processes remain poorly understood.