Observing Photons in Space

—1— Observing photons in space For the truth of the conclusions of physical science, observation is the supreme court of appeals Sir Arthur Eddington Martin C.E. HuberI, Anuschka PauluhnI and J. Gethyn TimothyII Abstract This first chapter of the book ‘Observing Photons in Space’ serves to illustrate the rewards of observing photons in space, to state our aims, and to introduce the structure and the conventions used. The title of the book reflects the history of space astronomy: it started at the high-energy end of the electromagnetic spectrum, where the photon aspect of the radiation dominates. Nevertheless, both the wave and the photon aspects of this radiation will be considered extensively. In this first chapter we describe the arduous efforts that were needed before observations from pointed, stable platforms, lifted by rocket above the Earth’s atmosphere, became the matter of course they seem to be today. This exemplifies the direct link between technical effort — including proper design, construction, testing and calibration — and some of the early fundamental insights gained from space observations. We further report in some detail the pioneering work of the early space astronomers, who started with the study of γ- and X-rays as well as ultraviolet photons. We also show how efforts to observe from space platforms in the visible, infrared, sub-millimetre and microwave domains developed and led to today’s emphasis on observations at long wavelengths. The aims of this book This book conveys methods and techniques for observing photons1 in space. ‘Observing’ photons implies not only detecting them, but also determining their direction at arrival, their energy, their rate of arrival, and their polarisation. Progress in observing photons in space has largely been driven by space astronomy: one rises to an altitude as high as necessary in order to get an unimpeded view of the rest of the Universe from above the atmosphere. The pioneers of space IPSI—Paul Scherrer Institut, Villigen, Switzerland IINightsen, Inc., Tiverton RI, USA 1The name ‘photon’, derived from the Greek τò ϕως˜ for ‘the light’, was coined by Lewis (1926), more than 20 years after Einstein’s postulate. 3 4 1. Observing photons in space astronomy concentrated their efforts on observing at the short wavelengths that are completely absorbed by the atmosphere. These efforts started shortly after World War II. Space observations in the visible, which would avoid the blurring effects by the turbulent atmosphere, were considered less of a priority at the start of the space age, but became possible when the HST was launched in 1990 (and repaired in 1993). It is worthwhile noting, however, that in the year before the launch of HST, two missions exploiting both the absence of a disturbing atmosphere and the possibility to scan great circles of the celestial sphere were launched, namely COBE and Hipparcos. Nowadays, infrared and sub-millimetre observations are the prime aim of observational space astronomy; for several reasons, including technology, they took a long time to come to the fore. Along with space astronomy, and by use of most of its technology, space observations of the Earth have gained importance and momentum over the past decades. Although Earth observations — as well as observations during a fly-by or from orbit around planets and comets — do primarily serve geophysics, they are largely sim- ilar to space astronomy. But, as applications, observing strategies and procedures for data handling in this field do differ from those familiar to astronomers, we will summarise these facets in the brief Chapter 39 (Pauluhn 2010). Given today’s efficient support and service to users of observing facilities, the number of astronomers whose knowledge and interest is entirely concentrated on interpreting observations has grown substantially in the past decades. Quite the opposite has happened to the number of scientists who are familiar with, and capa- ble of dealing with, instrumentation. This kind of scientist is, however, needed to innovate in space astronomy by advanced, more sophisticated or otherwise distinc- tive observing tools. With the long gestation, high productivity and extended life of modern astronomical facilities, the rate of innovation — particularly for space instrumentation — has slowed down. It is now essential that a record of experience and of sound practice be passed on to future generations of instrument builders in order to preserve knowledge and experience gained over half a century. The authors and editors of this book have accumulated much experience in enabling space astronomy by designing, building, testing and calibrating space- qualified instruments. Some of them have been involved in building instruments early in the space age, others are right now working with the first results of their instruments. The latter are those who have built observing facilities for the more recently accessible infrared and sub-millimetre domains. Some authors cover methods and techniques that will be extensively exploited in space in the future, namely interferometry and polarimetry. Another topic included in this book, is the modus operandi of laser-aligned structures in space; this will make space astronomy even more powerful. Giant laser-aligned structures, moreover, will open a fundamentally new window to the Universe, namely the hitherto untapped information carried by gravitational waves or — to remain in the parlance of this book — by gravitons.2 2LISA, a projected giant laser-aligned structure with a 5 Gm extent, would be able to expand our knowledge of the Universe by sensing the low-frequency gravitational radiation that is contin- uously emitted by massive objects throughout the Universe. Low-frequency gravitational waves are accessible only in space, where they are not submerged in the geophysical noise (as is the case for ground-based gravitational-wave detectors). Gravitational radiation eventually would let us reach further back in time, beyond the Cosmic Microwave Background (CMB) — the current 5 One may wonder why this book’s title is not simply ‘Observing Electromag- netic Radiation in Space’, instead of the shorter but perhaps somewhat gauche ‘Observing Photons in Space’. As mentioned, our title reflects the history: space astronomy started with the exploration of the X- and γ-ray domains and observations in the ultraviolet wavelength range, thus in these early observations, the photon as an ‘energy packet’ was the dominating concept. Up to today, disper- sionless, i.e., detector-based spectroscopy, or the timing of photon arrival time are methods used for measuring photons not only in the high-energy domain, but also at considerably lower energies, down to the visible range. Nonetheless, both the wave and the particle aspects of electromagnetic (EM) radiation are represented throughout the book. In this sense we interchangeably describe the relevant spectral domains by the energy of the photon, Eν = h ν, by the wavelength (in vacuo), λ, or by the frequency, ν; where h stands for Planck’s constant, and ν and λ are related through the speed of light c0 = ν λ. As far as units are concerned, we tried to enforce the rules of the Système International d’Unités (SI), which uses units that are based on terrestrial, i.e., laboratory standards. To further encourage the use of SI we present a short guide to the Système International in the Appendix. As we intend to preserve the experience of today’s generation of scientists who have been involved with instrumentation for future generations, we present the ma- terial at a level that is accessible to interested undergraduate students. For them, Chapter 2 (Wilhelm and Fröhlich 2010) on ‘Photons — from source to detector’ provides an introduction to the physical processes involved in the generation, trans- mission and imaging of photons, as well as in their spectroscopic and polarimetric analysis. Throughout the book we emphasise the principles of the methods employed, rather than details of the techniques being used. Overall, the subject is treated from the point of view and to the benefit of the practitioner: potential pitfalls are mentioned and good practice is stressed. We believe that this concept provides a useful guide not only to students, but also to professionals who want to inform themselves about methods being used in a field far from their own. Why observe photons — and why from space? To date, observing photons over the entire range of the electromagnetic spectrum is probably the most powerful means to gain an overall understanding of our environment. This applies to the nearest objects as well as to the early phases of the Cosmos that can be observed in the distant Universe. Much of the matter which we see in this way emits or absorbs photons in a gaseous or plasma state. Spectroscopy of the line spectra of matter in these aggregate states can yield tem- perature, composition and motions of (and in) the objects under investigation. By extending the sensitivity of our instruments into the infrared and sub-millimetre regions, we gain access to the broad-band spectral features that provide information about the composition of solid objects, such as comets, planets, asteroids and limit to photon observations. Moreover, when direct observation of gravitational radiation from astrophysical sources begins, new tests of general relativity, including strong gravity, will become possible (Will 2009). 6 1. Observing photons in space dust. And by going into the microwave region, we can reveal the ‘structure’ of the early Universe, which is contained in the anisotropy and polarisation of the Cosmic Microwave Background (CMB) radiation. If we remain on the ground, the Earth’s atmosphere limits our ‘view’ to the so-called ‘optical’ window, i.e., to near-ultraviolet, visible and near-infrared light, 3 whose wavelengths reach from λ 310 nm to λ 1 ñm. Ground-based observations are also possible in the so-called≈ ‘radio’ window,≈ i.e., for wavelengths of about 5 mm to 10 m, the latter corresponding to a frequency of ν 30 MHz.

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