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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 illustratethe rewards of observing photons in space, to state our aims, and to introducethe structure and the conventions used. The title of the book reflects the history ofspace astronomy: it started at the high-energy end of the electromagnetic spectrum,where the photon aspect of the radiation dominates. Nevertheless, both the waveand the photon aspects of this radiation will be considered extensively. In this firstchapter we describe the arduous efforts that were needed before observations frompointed, stable platforms, lifted by rocket above the Earth’s atmosphere, becamethe matter of course they seem to be today. This exemplifies the direct link betweentechnical effort— including proper design, construction, testing and calibration—and some of the early fundamental insights gained from space observations. Wefurther 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. Wealso show how efforts to observe from space platforms in the visible, infrared,sub-millimetre and microwave domains developed and led to today’s emphasis onobservations 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 theirdirection at arrival, their energy, their rate of arrival, and their polarisation.

Progress in observing photons in space has largely been driven by space astro-nomy: one rises to an altitude as high as necessary in order to get an unimpededview of the rest of the Universe from above the atmosphere. The pioneers of space

IPSI—Paul Scherrer Institut, Villigen, SwitzerlandIINightsen, Inc., Tiverton RI, USA1The name ‘photon’, derived from the Greek τ o ϕως for ‘the light’, was coined by Lewis (1926),

more than 20 years after Einstein’s postulate.

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astronomy concentrated their efforts on observing at the short wavelengths that arecompletely absorbed by the atmosphere. These efforts started shortly after WorldWar II. Space observations in the visible, which would avoid the blurring effectsby the turbulent atmosphere, were considered less of a priority at the start of thespace age, but became possible when the HST was launched in 1990 (and repairedin 1993). It is worthwhile noting, however, that in the year before the launch ofHST, two missions exploiting both the absence of a disturbing atmosphere and thepossibility to scan great circles of the celestial sphere were launched, namely COBEand Hipparcos. Nowadays, infrared and sub-millimetre observations are the primeaim 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 obser-vations 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 orbitaround planets and comets— do primarily serve geophysics, they are largely sim-ilar to space astronomy. But, as applications, observing strategies and proceduresfor data handling in this field do differ from those familiar to astronomers, we willsummarise these facets in the brief Chapter 39 (Pauluhn 2010).

Given today’s efficient support and service to users of observing facilities, thenumber of astronomers whose knowledge and interest is entirely concentrated oninterpreting observations has grown substantially in the past decades. Quite theopposite 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 toinnovate in space astronomy by advanced, more sophisticated or otherwise distinc-tive observing tools. With the long gestation, high productivity and extended lifeof modern astronomical facilities, the rate of innovation—particularly for spaceinstrumentation— has slowed down. It is now essential that a record of experienceand of sound practice be passed on to future generations of instrument builders inorder to preserve knowledge and experience gained over half a century.

The authors and editors of this book have accumulated much experience inenabling space astronomy by designing, building, testing and calibrating space-qualified instruments. Some of them have been involved in building instrumentsearly in the space age, others are right now working with the first results of theirinstruments. The latter are those who have built observing facilities for the morerecently accessible infrared and sub-millimetre domains. Some authors cover meth-ods and techniques that will be extensively exploited in space in the future, namelyinterferometry and polarimetry. Another topic included in this book, is the modusoperandi of laser-aligned structures in space; this will make space astronomy evenmore powerful. Giant laser-aligned structures, moreover, will open a fundamentallynew window to the Universe, namely the hitherto untapped information carried bygravitational 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 expandour 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 wavesare accessible only in space, where they are not submerged in the geophysical noise (as is thecase for ground-based gravitational-wave detectors). Gravitational radiation eventually would letus reach further back in time, beyond the Cosmic Microwave Background (CMB) —the current

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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: spaceastronomy started with the exploration of the X- and γ-ray domains and obser-vations in the ultraviolet wavelength range, thus in these early observations, thephoton 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 aremethods used for measuring photons not only in the high-energy domain, but alsoat considerably lower energies, down to the visible range. Nonetheless, both thewave and the particle aspects of electromagnetic (EM) radiation are representedthroughout the book. In this sense we interchangeably describe the relevant spec-tral 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 arerelated through the speed of light c0 = ν λ. As far as units are concerned, we triedto enforce the rules of the Systeme International d’Unites (SI), which uses unitsthat are based on terrestrial, i.e., laboratory standards. To further encourage theuse of SI we present a short guide to the Systeme International in the Appendix.

As we intend to preserve the experience of today’s generation of scientists whohave 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 Frohlich 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 polarimetricanalysis.

Throughout the book we emphasise the principles of the methods employed,rather than details of the techniques being used. Overall, the subject is treatedfrom the point of view and to the benefit of the practitioner: potential pitfalls arementioned and good practice is stressed. We believe that this concept provides auseful guide not only to students, but also to professionals who want to informthemselves 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 spec-trum is probably the most powerful means to gain an overall understanding of ourenvironment. This applies to the nearest objects as well as to the early phasesof the Cosmos that can be observed in the distant Universe. Much of the matterwhich 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. Byextending the sensitivity of our instruments into the infrared and sub-millimetreregions, we gain access to the broad-band spectral features that provide informa-tion about the composition of solid objects, such as comets, planets, asteroids and

limit to photon observations. Moreover, when direct observation of gravitational radiation fromastrophysical sources begins, new tests of general relativity, including strong gravity, will becomepossible (Will 2009).

6 1. Observing photons in space

dust. And by going into the microwave region, we can reveal the ‘structure’ of theearly Universe, which is contained in the anisotropy and polarisation of the CosmicMicrowave Background (CMB) radiation.

If we remain on the ground, the Earth’s atmosphere limits our ‘view’ to theso-called ‘optical’ window, i.e., to near-ultraviolet, visible and near-infrared light,whose wavelengths reach from λ ≈ 310 nm to λ ≈ 1 µm.3 Ground-based observa-tions are also possible in the so-called ‘radio’ window, i.e., for wavelengths of about5 mm to 10 m, the latter corresponding to a frequency of ν ≈ 30 MHz. At theother end of the spectrum, galactic photons of extremely high energy (> 1 TeV)can be indirectly observed from the ground by studying the secondary events theycause in the atmosphere. In order to directly observe the impinging photons in allspectral domains, however, one has to go to space. The same is true if one wantsto overcome the distorting influence of the turbulent atmosphere, the seeing, whichblurs our view in the optical window. This deleterious effect, however, can be signi-ficantly reduced by the indirect method of interferometry, and in direct imaging byso-called adaptive optics, albeit up to now at infrared wavelengths only.

Another advantage of space astronomy that has already been mentioned, butis pointed out less frequently, is the undisturbed and simultaneous access to oppo-site parts of the celestial sphere. This was of fundamental importance for COBE(Boggess et al 1992) and the astrometry mission Hipparcos (Perryman 2009).

The absorption of EM radiation by the terrestrial atmosphere is shown schemat-ically in Figure 1.1. Note that the interstellar medium, in addition, blocks a signi-ficant part of the vacuum-ultraviolet (VUV) spectral region, even if the observinginstrument is above the atmosphere. This absorption is dominated by the Lymancontinuum (λ < 91.2 nm) of hydrogen, the neutral species of the most abundantelement in the Universe. Owing to the proximity of the Sun this absorption is notnoticeable in front of our daylight star. In fact, the Sun provides the main exam-ple of a stellar spectrum in this domain (cf., Figure 2.1 in Wilhelm and Frohlich2010).4 The onset of atmospheric absorption at the long-wavelength end of the ra-dio window is subject to diurnal variations: it depends on the electron density in theionosphere, plotted for day- and night-time in Figure 1.2. The figure also shows thevarious subdivisions of the Earth’s atmosphere—troposphere, stratosphere, meso-sphere, thermosphere and exosphere— and the general course of temperature andelectron density. The atmosphere expands with increasing solar activity throughthe activity-related increase of the solar irradiance at VUV wavelengths. Thus, thelevel of the solar VUV irradiance controls the exospheric temperature and hencethe extent and density of the upper atmosphere. The change in the thermo- andexospheric temperatures with solar activity is shown schematically in Figure 1.3.Increasing activity causes both an increased irradiance and subsequent absorptionof VUV radiation and an increase in the density at a given altitude in the terrestrialatmosphere. Earth-orbiting satellites thus experience an increased drag and a more

3The atmosphere also has some windows and stretches of only moderate absorption in thewavelength range between ca. 1 µm and 30 µm (cf., Figures 1.1 and 1.7).

4There is, in addition, a sample of nearby stars, where the column density of the interstellar gasis exceptionally low along the lines of sight, and where, consequently, studies in this wavelengthregion are possible as well (cf., EUVE, Bowyer and Malina 1991).

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Figure 1.1: Lower panel: Absorption in the terrestrial atmosphere plotted vs. alogarithmic wavelength scale in Angstrom. Upper panel: The Lyman series and theionisation continua of H and He atoms absorb radiation in the interstellar medium,and thus block a part of the spectrum even for instruments carried above theEarth’s atmosphere. This latter absorption applies to the radiation of almost allstars, but not to the Sun (from Aller 1963).

Table 1.1: Discriminating spectral domains according to the sub-disciplines of ob-servational space astronomy.

Sub-discipline of observational astronomy Energies, wavelengths and frequenciesHigh-energy astrophysics > 100 keVX-ray astronomy from 100 eV to 100 keVExtra-terrestrial/vacuum-ultraviolet range from 300 nm to 10 nmVisible and near-infrared spectral domains from 30 µm to 0.3 µmMid- and far-infrared range > 30 µmCosmic Microwave Background from 30 GHz to 300 GHz

rapid decay of their orbits. This eventually leads to the destruction of a satelliteas it enters the denser lower layers and burns up.5

The sub-division of the spectrum into energy, wavelength, or frequency domainsvaries among research communities.6 In this book we simply define them throughthe sub-disciplines of observational space astronomy (cf., Table 1.1).

As we shall see at the beginning of the next section, even balloons providecertain advantages to astronomical observations. More recently still, observing fa-cilities are also placed in orbits around the Sun rather than the Earth.

5In the exceptional case of the HST, this fate has been postponed several times by re-boosts.As the HST was designed for in-orbit servicing, it can be connected to the Shuttle and, by useof the Shuttle propulsion engines, can be lifted into a higher orbit. This possibility also allowedastronauts, who visited Hubble five times, to repair and upgrade HST ; in the first instance byauxiliary optics, which compensated the accidental spherical aberration of the prime mirror. TheISS is regularly boosted, too.

6The International Standard Organisation (ISO) has issued a highly-detailed list of definitionsof wavelength ranges. This list, however, is accessible only to paying customers.

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Figure 1.2: The general structure of the terrestrial atmosphere and the diurnalvariation of the ionosphere (from the Space Weather Prediction Center www.swpc.noaa.gov).

Enabling technologies

High-altitude balloons floating above 30 km permit access to wavelengths be-low 300 nm, the atmospheric absorption limit,7 but measurements at shorter wave-lengths require the use of sounding rockets or satellites operating at altitudes above160 km. However, even at these altitudes residual absorption and emission by thegeocorona and by terrestrial airglow emission, respectively, can affect the measure-ments. The latter effects can be eliminated, if the observing instrument is placedat L1 or L2, i.e., at Lagrange points of the Sun-Earth system, and thus in a helio-centric orbit. L1 and L2 lie on the Sun-Earth line 1.5 Gm away from the Earth,8 infront of and behind the Earth, respectively. In general the spacecraft are, however,not placed in the Lagrange points themselves, but guided along halo orbits aroundL1 or L2, where their station keeping is energetically less demanding than if theirposition were maintained in the Lagrange points themselves.9

The first sounding-rocket measurements were made in the mid 1940s with Ger-man V2 rockets that had been captured at the end of the Second World War.10

7High-altitude balloons do not only extend the accessible wavelength range towards shorterwavelengths, but also permit to overcome the seeing caused by turbulence in the lower, denseratmosphere. This was exploited by the ‘Stratoscope’ project, a telescope (with a diameter D =30.5 cm), which took photographs of the solar granulation at a wavelength λ ≈ 505 nm at analtitude of 24 km, i.e., above the tropopause (cf., Figure 1.3). With proper optical and thermaldesign, the theoretical diameter of the diffraction disk, given by the diameter of the primary mirrorof the telescope, 1.12 (λ/D) ≈ 1.85×10−6 rad = 0.35′′ was actually achieved (Schwarzschild 1959).

81.5 Gm correspond to 1 % of the Earth-Sun distance.9For L1 this also avoids disturbances between the radio downlink of the spacecraft and radio

emission from the Sun.10Even before the end of World War II, instrumentation had been prepared in Germany for

flights with an A4 rocket. The initiative came from Erich Regener, and was supported by Wernhervon Braun. A barrel-shaped container (the ‘Regener Tonne’) was equipped with barometers andtemperature sensors as well as spectrographs to observe the solar spectrum at high altitude.

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Figure 1.3: Thermospheric temperature variations (Banks and Kockarts 1973), re-produced from Dickinson (1986).

Figure 1.4 shows the first solar spectra obtained in this way. Results from theseunstabilized rocket systems were limited and special techniques were required todirect the solar VUV radiation into the spectrographs (de Jager 2001). Tousey et al(1947), who had photographed the spectrum shown in Figure 1.4, reported that “aLiF sphere of 2 mm diameter had been used in place of the [spectrograph entrance]slit.” 11 The development in the 1950s and 1960s of sub-orbital and orbital launchvehicles (Russo 2001) brought much progress.12 Alongside the improvement of thetransportation systems three-axis stabilised pointing systems were perfected in an-other most important effort. This not only permitted recording the VUV spectrumof the Sun with high spectral and spatial resolution (de Jager 2001), but also nowoffered the opportunity to observe more remote astronomical objects in the VUV(Savage 2001).

Although this will sound curious to today’s space researchers, there was no weight limitation: theA4 rocket, which was used by the military as the V2 missile, had been designed to carry a tonof explosives. However, the lack of a pointing system would have allowed exposures to be takenonly during the descent on a parachute. Diffusing optics, as described by de Jager (2001), wereforeseen to feed light into the spectrometers. To achieve a proper deployment of the parachute, alaunch up to an altitude of only ca. 50 km was planned. The payload had been delivered to thelaunch site, Peenemunde, in early January 1945 and its adjustments and functional tests had beencompleted on 18 January. At that time, however, the increasing severity of the war prohibited anA4-flight for scientific purposes (cf., DeVorkin 1993; de Jager 2001; Keppler 2003).

11Regener and Regener (1934) had earlier recorded the solar spectrum down to 294 nm on aballoon flight that had reached an altitude of 29.3 km.

12For a description of the early history of rocket developments see, for example, Bonnet (1992)and Stuhlinger (2001).

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Figure 1.4: The first photograph of the ultraviolet spectrum of the Sun, at analtitude of 55 km. Several exposures were made at different altitudes with a USNaval Research Laboratory spectrograph that was carried in a V2 rocket on 10October 1946 (from Tousey 1967).

Morton (1966) reported the first VUV spectra of bright hot stars, which wererecorded with a dispersion that was high enough for measuring the spectral linespresent. Absorption lines in these spectra revealed the existence of stellar windswith velocities ranging from 1400 km s−1 to 3000 km s−1. At that time the use ofobjective-grating spectrographs avoided the need for very high pointing accuracy.Pointing stability on the other hand was required to avoid the blurring of thespectrum. This could be achieved with the help of a fine-stabilisation system thatoperated in conjunction with the attitude control system, and kept the spectrographplatform steady to within ca. ± 16′′ with a few excursions to ± 50′′ (Morton andSpitzer 1966).13

How difficult it was to work without a three-axis attitude control system andwith only marginal spatial resolutions is also apparent from the paper describingthe first evidence of X-rays from sources outside the solar system (Giacconi etal 1962). This specific observation actually started X-ray astronomy on objectsbeyond the Sun and was eventually recognised by the first Nobel prize for spaceastronomy. On the rocket flight in question, the spectral information available fromthe two functioning Geiger counters with mica windows of two different thicknesseswas rather austere as well, so that a thorough discussion rationalising the exclusionof other celestial, near-Earth and solar-system sources was required to corroboratethe conclusion.

13It is remarkable that seven years later, the same group could report a stability of the Coper-nicus (OAO-3) satellite of better than ± 50 mas over a full orbit of ca. 90 min (Rogerson et al1973).

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The study of photons at γ- and X-ray energies

With nuclear physics being at the centre of contemporary physics, and moreoverbeing based mostly on electronic detection and analysis techniques, it was quitenatural that cosmic-ray research— and this meant also γ-ray astronomy— shouldbecome part of the early epoch of space research (cf., Chapter 3, Kanbach et al2010). The 31 first γ-ray photons detected from space (rather than from the terres-trial atmosphere, where they are generated through interaction with cosmic rays)were identified in 1961 by the Explorer-11 satellite. This was followed in 1968 byOSO-3 14 observations of 621 cosmic-ray photons (Kraushaar et al 1972). Figure 1.5shows the distribution of these photons with galactic latitude. As pointed out bythe authors “the peak at the [galactic] equator and the apparently flat plateau overthe range |bII | > 40◦ suggest the existence of two components in the cosmic γ-rayflux15 — one, which is clearly of galactic origin and another which is isotropic andpossibly of extragalactic origin.” OSO-7 then saw the first γ-ray lines generated bysolar flares in 1972 and — following the unfortunately abbreviated16 flight of SAS-2 — the ESA mission COS-B produced from 1975 until 1982 the first sky map,which showed diffuse radiation from the galactic disk caused by the interaction ofcosmic rays with the interstellar medium in the Milky Way and point sources suchas pulsars, as well as one individual extra-galactic source, namely 3C 273.

Figure 1.5: Galactic latitude distribution of cosmic γ-ray photons with energiesabove 50 MeV as observed by OSO-3 (Kraushaar et al 1972). Photons emergingfrom the galactic longitude range, −30◦ < ℓII < 30◦, had been excluded, as it wassurmised that sources in the direction of the galactic centre might be of a specialnature.

14The OSO series of NASA missions was primarily aimed at solar observations. Yet, the OSOsatellites not only provided a platform for instruments pointed at the Sun but also accommodateda set of instruments, which scanned the sky in the course of six months (see, e.g., Kraushaar 1972).

15In modern nomenclature: ‘radiance’16Because of a tape-recorder failure

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In 1967, the US Vela satellites observed the phenomenon of γ-ray bursts; this,however, was kept classified until 1973, because the Vela satellites had been builtin order to enforce the nuclear test ban treaty, and were meant to detect terrestrialnuclear explosions. The γ-ray bursts remained a puzzle until 1997, when the after-glow of a few such bursts could be followed in X-rays and by optical telescopes.From the redshifts of the γ-ray burst remnants it could be concluded that suchevents were a very remote phenomenon, which set free extremely high energies:assuming isotropic emission, the energy in a γ-ray burst can be of an order equiv-alent to a solar mass. It is not surprising then that the most remote object knowntoday was identified through a γ-ray burst—an object with redshift z = 8.2. Thisimplies an explosion 13× 109 light years away, which had occurred at a time whenthe Universe was about 1/20 of its present age. Indeed, this γ-ray burst was “aglimpse of the end of the dark ages” (Tanvir et al 2009).

A most significant inference of γ-ray astronomy is the absence of a backgroundof γ-rays from nucleon-antinucleon annihilation. This finding empirically excludesa matter-antimatter symmetric Universe (Cohen et al 1998; Pinkau 2009).

The impact of X-ray observations on the entire field of astronomy is summarisedin the Nobel lecture— The Dawn of X-ray Astronomy—by Giacconi (2003). Abreakthrough occurred, when — after several earlier attempts with threshold-limitedsensitivities to detect X-rays from sources other than the Sun — a rocket payload“intended to observe the X-ray fluorescence produced on the lunar surface by X-rays from the Sun and to explore the night-sky for other possible sources” had beenlaunched on 18 June 1962. With this rocket experiment, which was able to detect anX-ray photon irradiance of (0.1 to 1) cm−2 s−1, and thus was about 50 to 100 timesmore sensitive than any experiment flown before, Giacconi et al (1962) discoveredthe first X-ray source from outside the solar system. The detectors scanned thesky over a band covering the Moon’s disk in two energy bands. Figure 1.6 showsthe results. The source that gave the large signal was later on named Sco X-1.Since then the sophistication of X-ray observations has increased from scanners

Figure 1.6: The first observation of Sco X-1 and of the X-ray background in therocket flight of 12 June 1962 (from Giacconi et al 1962). The ordinate shows thenumber of counts accumulated in 350 s in angular intervals of 6◦.

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with one or a few Geiger counter detectors with mechanical collimators, i.e., bafflesrestricting the field of view, to high-resolution telescopes with imaging detectors,and with energy discrimination either by diffraction gratings that disperse the radi-ation according to wavelength, or by energy-discriminating detectors (cf., Chapter4, Culhane 2010). From the beginning in 1962 to the present, the sensitivity ofX-ray observing techniques has increased by more than nine orders of magnitude,a range comparable to that from the naked eye to the current 10 m ground-basedtelescopes (Giacconi 2003). High-energy astronomy— i.e., X-ray astronomy andγ-ray astronomy together— has led to the confirmation of fundamental, but orig-inally only surmised, phenomena, such as black holes, or the intergalactic plasmain galaxy clusters.

Although opening the X-ray window to the Universe beyond the Sun was, ofcourse, a momentous discovery, we recall that X-ray observations of the Sun havegiven us an early insight into the large radiance variations associated with flaresand the structure and dynamics of the solar corona, which is consisting of highlyinhomogeneous plasma that is contained by magnetic field structures of widely dif-ferent stability (cf., Blake et al 1963; Friedman 1963; Vaiana and Rosner 1978). Andsolar γ-rays continue to play an important role in γ-ray astronomy (cf., Chapter 3,Kanbach et al 2010).

Space astronomy in the VUV — an observationallydelicate region

The early VUV and soft X-ray spectra and images of astrophysical objects wererecorded by photography. Conventional photographic emulsions, however, were notsuitable for recording the interesting VUV wavelength range below 200 nm, becausethe protective gelatine layer absorbed the radiation in question before it reachedthe silver-halide grains. Schumann (1901) developed emulsions on glass plates withjust enough gelatine to hold the silver halide grains. Such emulsions, however, wereextremely sensitive to physical contact and, particularly if on a glass substrate,were not suitable for use in space, given the launch environment of space vehicles(cf., Chapter 40, Kent 2010). For the manned Skylab flights in 1973 and 1974,which offered the opportunity to have the exposed films returned to Earth bythe astronauts, VanHoosier et al (1977) developed suitable containers with specialfilm-based Schumann emulsions.

Photographic emulsions have several drawbacks beyond the paramount obstacleof recovering them from orbit after they have been exposed, namely their limiteddynamic range, and the need to develop them before the data can be viewed.17

This meant, for example, that film-based science operations during the first of the

17NASA’s early lunar exploration programme was based in part on the use of the unmanned‘Lunar Orbiter ’ spacecraft. These were designed to obtain detailed photographs of potential Apollolanding sites in the years 1966 to 1967. As no suitable electronic cameras were available at thattime, the Moon’s surface was photographed from lunar orbit. The exposed film was developed andphotoelectrically scanned on board. The images could then be sent back to Earth by telemetry(Nelson and Aback 1971).

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three manned observing periods on Skylab had to be carried out blind, becauseresults became available only after recovery.

On the other hand, photographic material is inherently two-dimensional, andaccordingly permits the simultaneous recording of an image, for example, a field onthe sky, or a stigmatic spectrum, i.e., a spectrum which reflects the radiance vari-ations along the entrance slit of the spectrograph. This was, in the first instance,a definitive advantage for the photographic techniques. Nevertheless, for rapidlyvarying objects, one would need short exposure times and ample photographicmaterial. With the advent of imaging detectors the advantage of photography dis-appeared; now images could also be downlinked to Earth in near-real time. Inaddition, photoelectric measurements eventually became the adopted solution foraccurately calibrated observations.

Initially, photoelectric detectors were not even one-dimensional, but recordeda single pixel only. For space astronomy they had to be rugged and this requiredspecial developments. When used to register the weak solar VUV radiation, theyalso had to be ‘solar blind’, i.e., insensitive to the strong visible light stemmingfrom the solar photosphere, which invariably would appear as stray light. In fact,this is also critical for observations of other, relatively cool stars.18

Further difficulties arise if one progresses to even shorter wavelengths in theVUV, namely to below ≈ 116 nm, where no rugged window materials are available.Hence the need for purely reflective optical systems and detector systems that canoperate without windows.19

For the detection of visible and infrared radiation, as well as for the detectionof X-rays, the most common detector today is a solid-state device — the charge-coupled device (CCD, see Chapters 23 through 25, Waltham 2010; Holland 2010;Schuhle 2010), which however has a relatively slow read-out. As CCDs are not sen-sitive to ultraviolet radiation, appropriate image intensifiers or fluorescent materialhave to be added to achieve the desired results for observations of ultraviolet andVUV radiation; and for X-rays thin-film metal filters have to be added to rejectradiation of longer wavelength.

Another important imaging detector system, which, on the other hand, is oper-ating in vacuum, is the microchannel-plate (MCP) detector. If operated as a photoncounter this detector can also time-tag single photons (cf., Chapter 22, Timothy2010). The origin of this detector can be traced to the magnetic electron multi-plier (MEM), which was based on a continuous-dynode structure. The concept ofa continuous-dynode electron multiplier had first been proposed by Farnsworth(1930), but practical devices only became available in the 1960s. The first practi-cal continuous-dynode multiplier was the crossed-field Bendix MEM (Heroux andHinteregger 1960; Timothy et al 1967, cf. also, Chapter 22, Timothy 2010, where

18As there were no ‘solar blind’ emulsions, alkali films for wavelengths in the VUV or thin metalfilters for X-rays had to be used to reject visible light in photographic apparatus, unless the objectwas a young star, with peak radiation in the ultraviolet. For photographic X-ray observations thinmetal films were used not only to reject the visible radiation, but also to selectively transmit agiven wavelength region (cf., Vaiana and Rosner 1978).

19Finally, the reflectance of normal-incidence optical systems falls dramatically below 50 nm,and use of grazing-incidence telescopes (Brueggemann 1968) and spectrometers (Samson 1967),or synthetic multilayer interference coatings (Spiller 1974) is mandatory (cf., Chapters 9 and 10,Lemaire et al 2010; Lemaire 2010).

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the intermediate stage between MEM and micro-channel plates, namely the chan-neltron, is described as well).

The path towards space telescopes observing in the

visible and infrared

Today’s most important space telescope, the HST has its roots in the firstquarter of the 20th century. In his book ‘Die Rakete zu den Planetenraumen’20

Hermann Oberth (1923) had written that “Telescopes of any size could be usedin space, since the images of stars do not twinkle...” (cf., Stuhlinger 2001). Inreviewing the path of HST, then called ‘Space Telescope’, Lyman Spitzer (1979)explained: “... the engineering possibilities of launching a complicated device intospace and keeping it in operation seemed rather remote at the time of Oberth’ssuggestion, and few astronomers took these possibilities seriously.” However, afterWorld War II, Spitzer (1946) discussed a number of scientific programmes thatcould be carried out with large reflecting telescopes in space, namely “a) pushingback the frontiers of the Universe and determining galactic distances by measuringvery faint stars, b) analysing the structure of galaxies, c) exploring systematicallythe structure of globular clusters, and d) studying the nature of planets, especiallytheir surfaces and atmospheres” (quoted after Spitzer 1979). We recall here thatanother Princeton astronomer, namely Spitzer’s colleague Martin Schwarzschild(1959) made the first high-resolution observations in the visible by photographingthe granulation of the solar atmosphere from a stratospheric balloon in the late1950s (cf., footnote 7).

Much emphasis today is on the cool parts of the Universe and on the red-shiftedCosmos. As a consequence many, if not most, current and currently-planned majorspace observatories are aimed at the infrared and sub-millimetre domains: Spitzerand Herschel are in orbit, JWST is scheduled for launch in 2014 and SPICA is un-der advanced study. Moreover, a facility installed on an aeroplane, SOFIA, is nowentering service. The road to this state was arduous, as described by Harwit (2001).Certain segments within the infrared domain are accessible, depending on wave-length, from high mountains, aeroplanes or stratospheric balloons (cf., Figure 1.7).This has often led to extensive debates as to whether a given observational problemreally required a telescope in space, and has caused serious rifts in the astronomicalcommunity. Moreover, infrared detectors and the cryogenics needed to suppress thethermal background of the equipment itself at longer wavelengths, are technicallydemanding and required long developments. In brief, the long fruition time of in-frared space observations can be explained by both observational and technologicalreasons. This has now been overcome, although operating infrared observing fa-cilities is still a demanding task, especially because of the limited lifetime of thecooling agent, or the need to fly a cooling apparatus (cf., Chapter 37, Rando 2010).

The development of microwave observations in space, in particular the develop-ment of COBE (cf., Chapter 8, Lamarre 2010) seems to have steadily progressed

20Literally: “The Rocket to the Space of the Planets”.

16 1. Observing photons in space

Figure 1.7: Atmospheric absorption in the infrared at a mountain-top site likeMauna Kea and at altitudes of aircraft and balloons (from Harwit 2001).

quite separately from the development of infrared space observatories, and has ledto the 2006 Nobel prize being awarded to John C. Mather and George F. Smoot forthis work in space astronomy. In the meantime COBE was followed by WMAP, an-other NASA mission, and now the ESA mission Planck has reached its orbit aroundL2, after a joint launch in May 2009 together with the infrared/sub-millimetre ob-servatory Herschel.

Innovative space technology meets conservative

attitude

It might be constructive in the long run to look back to the time when spaceastronomy was in its infancy, and was considered by many as a somewhat extra-vagant exercise— the early epoch of space science. Seen from today’s astronomyscene, where ground-based and space astronomy are viewed as being complemen-tary, it may be interesting (and, if not disconcerting, now and again amusing)to recall the early attitudes of ‘traditional’ astronomers towards the opportunitiesoffered by space technology (similarly, by the way, to those of ‘traditional’ geophysi-cists and oceanographers). Three quotes from history studies of space astronomyshall illustrate this mercifully forgotten issue.

17

In her description of starting the space-astronomy programme of NASA, Ro-man (2001) writes: “Astronomers, practitioners of a very old science, deal withlong-lived objects and thus tend to be conservative. Hence, it is not surprising thatthere were social as well as technical problems to be met in the development of thenew NASA astronomy program.” Also Stuhlinger (2001) mentions that the valueand cost of “ten Palomars vs. one Space Telescope” were often used as argumentsagainst building a space telescope. A special impediment has been the infraredcommunity. Harwit (2001) reports: “Since rocket payloads were relatively expen-sive, the U.S. ground-based observers feared that the National Aeronautics andSpace Administration (NASA) or National Science Foundation (NSF) support forrocket work would drain away funding from their own efforts. Uncertainties aboutthe precise limitations of ground-based techniques often led to acrimonious disputesbetween ground-based observers and those who wanted to launch sensitive infraredastronomical telescopes above the atmosphere on rockets. Time and again, the moretraditional astronomers bluntly recommended that rocket-borne efforts be slashedor pared back to an absolute minimum. Pioneering infrared rocket research was nota happy venture!” Harwit, however, then goes on with a very useful description ofhow the difficulties were overcome by uniting the infrared space community behindone single project.

There was no great difference between the sceptical attitudes on the two sidesof the Atlantic, although the politically motivated support of all space activitiesin the US led to a great disparity in funding for space science. Space astronomyin Europe started— with the exception of the already mentioned γ-ray missionCOS-B — relatively late, i.e., long after atmospheric and magnetospheric research.This may be ascribed to similar attitudes. The lack of adequate funding in Europe,however, led advanced practitioners to enter into fruitful collaborations with NASA;this joining of forces did, although it led to brain drain initially, ultimately helpboth sides. Indeed, observing photons in space— like other space ventures— wasan early manifestation of international collaboration beyond continental bound-aries. Given the cold war, however, bilateral collaborations across the iron curtainremained restricted to only a few western countries, such as the USA, Austria,France and Sweden, although the Committee on Space Research (COSPAR) heldbiennial conferences where scientists from East and West could easily meet.

Overall structure of the book and conventions used

The chapter following this introduction presents the fate of photons from sourceto detector, i.e., the physics of the interaction of photons with matter with an em-phasis on the principles of physics being used in detecting and analysing photons.The next six chapters deal with both the physical processes and the characteristicsof the radiation that can be observed in the energy-, wavelength- or frequency-domains from γ-rays to radio waves. Examples of the relevant astrophysical ob-jects and the early missions devoted to their study are presented together withresulting key findings. The subsequent eleven chapters are devoted to descriptionsof the techniques and systems used for imaging and spectral analysis. An in-depthdiscussion of detectors used in space astronomy, subdivided into 13 chapters, then

18 1. Observing photons in space

precedes three chapters on polarimetry and an additional four chapters on tech-niques of general importance to space observations, such as calibration, cryogenicsystems, laser-aligned structures, and the peculiarities of Earth observations, in-cluding radar measurements. Two final chapters, namely a pragmatically-orientedessay on the implications of the space environment that must be considered whenbuilding space instrumentation, and a summary and outlook, conclude the book.

In an Appendix we present a brief overview of the most pertinent rules of theSysteme International. The SI has been used as systematically as possible in thetext. Figures which show historical plots, however, were not redrawn, and thereforemay show obsolete nomenclature and units. Conversions to SI units are also given inthe Appendix. Abbreviations and acronyms of spacecraft are not introduced; theyare collected in a list of acronyms at the end of the book. References to internetresources and URLs have been checked to be valid in August 2009. However, theinternet remains a volatile resource and its contents and locations are subject tochange.

Acknowledgements

On behalf of all editors, we want to gratefully acknowledge the contributions ofAndre Balogh who provided the liaison to ISSI. Thjis de Graauw participated withuseful advice at the early stages of the book. Anthony J. Banday, Walter Gear,Hugh Hudson, Anthony Marston and Claudia Wigger acted as outside referees.Many of the book’s authors also gave valuable advice as referees for chapters of theircolleagues. Karen Fletcher of ESA’s Communication and Knowledge Departmentis thanked for sound professional editorial support. We also wish to thank BrigitteSchutte-Fasler, Andrea Fischer, Saliba F. Saliba, Katja Schupbach, Silvia Wengerand the other members of the staff at ISSI for actively and cheerfully supportingus whenever we needed to use the infrastructure of ISSI.

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