—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
I PSI—Paul
Scherrer Institut, Villigen, Switzerland
Inc., Tiverton RI, USA
1 The name ‘photon’, derived from the Greek τ ò ϕω̃ς for ‘the light’, was coined by Lewis (1926),
more than 20 years after Einstein’s postulate.
II Nightsen,
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 similar 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 capable of dealing with, instrumentation. This kind of scientist is, however, needed to
innovate in space astronomy by advanced, more sophisticated or otherwise distinctive 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 spacequalified 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
2 LISA, 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 continuously 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 Electromagnetic 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, dispersionless, 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 material 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, transmission 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 temperature, 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,
whose wavelengths reach from λ ≈ 310 nm to λ ≈ 1 µm.3 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. At the
other end of the spectrum, galactic photons of extremely high energy (> 1 TeV)
can be indirectly observed from the ground by studying the secondary events they
cause in the atmosphere. In order to directly observe the impinging photons in all
spectral domains, however, one has to go to space. The same is true if one wants
to overcome the distorting influence of the turbulent atmosphere, the seeing, which
blurs our view in the optical window. This deleterious effect, however, can be significantly reduced by the indirect method of interferometry, and in direct imaging by
so-called adaptive optics, albeit up to now at infrared wavelengths only.
Another advantage of space astronomy that has already been mentioned, but
is pointed out less frequently, is the undisturbed and simultaneous access to opposite 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 schematically in Figure 1.1. Note that the interstellar medium, in addition, blocks a significant part of the vacuum-ultraviolet (VUV) spectral region, even if the observing
instrument is above the atmosphere. This absorption is dominated by the Lyman
continuum (λ < 91.2 nm) of hydrogen, the neutral species of the most abundant
element in the Universe. Owing to the proximity of the Sun this absorption is not
noticeable in front of our daylight star. In fact, the Sun provides the main example of a stellar spectrum in this domain (cf., Figure 2.1 in Wilhelm and Fröhlich
2010).4 The onset of atmospheric absorption at the long-wavelength end of the radio window is subject to diurnal variations: it depends on the electron density in the
ionosphere, plotted for day- and night-time in Figure 1.2. The figure also shows the
various subdivisions of the Earth’s atmosphere — troposphere, stratosphere, mesosphere, thermosphere and exosphere — and the general course of temperature and
electron density. The atmosphere expands with increasing solar activity through
the activity-related increase of the solar irradiance at VUV wavelengths. Thus, the
level of the solar VUV irradiance controls the exospheric temperature and hence
the extent and density of the upper atmosphere. The change in the thermo- and
exospheric temperatures with solar activity is shown schematically in Figure 1.3.
Increasing activity causes both an increased irradiance and subsequent absorption
of VUV radiation and an increase in the density at a given altitude in the terrestrial
atmosphere. Earth-orbiting satellites thus experience an increased drag and a more
3 The atmosphere also has some windows and stretches of only moderate absorption in the
wavelength range between ca. 1 µm and 30 µm (cf., Figures 1.1 and 1.7).
4 There is, in addition, a sample of nearby stars, where the column density of the interstellar gas
is exceptionally low along the lines of sight, and where, consequently, studies in this wavelength
region are possible as well (cf., EUVE, Bowyer and Malina 1991).
7
Figure 1.1: Lower panel: Absorption in the terrestrial atmosphere plotted vs. a
logarithmic wavelength scale in Ångström. Upper panel: The Lyman series and the
ionisation 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 the
Earth’s atmosphere. This latter absorption applies to the radiation of almost all
stars, but not to the Sun (from Aller 1963).
Table 1.1: Discriminating spectral domains according to the sub-disciplines of observational space astronomy.
Sub-discipline of observational astronomy
High-energy astrophysics
X-ray astronomy
Extra-terrestrial/vacuum-ultraviolet range
Visible and near-infrared spectral domains
Mid- and far-infrared range
Cosmic Microwave Background
Energies, wavelengths and frequencies
> 100 keV
from 100 eV to 100 keV
from 300 nm to 10 nm
from 30 µm to 0.3 µm
> 30 µm
from 30 GHz to 300 GHz
rapid decay of their orbits. This eventually leads to the destruction of a satellite
as it enters the denser lower layers and burns up.5
The sub-division of the spectrum into energy, wavelength, or frequency domains
varies among research communities.6 In this book we simply define them through
the sub-disciplines of observational space astronomy (cf., Table 1.1).
As we shall see at the beginning of the next section, even balloons provide
certain advantages to astronomical observations. More recently still, observing facilities are also placed in orbits around the Sun rather than the Earth.
5 In 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 use
of the Shuttle propulsion engines, can be lifted into a higher orbit. This possibility also allowed
astronauts, who visited Hubble five times, to repair and upgrade HST ; in the first instance by
auxiliary optics, which compensated the accidental spherical aberration of the prime mirror. The
ISS is regularly boosted, too.
6 The International Standard Organisation (ISO) has issued a highly-detailed list of definitions
of wavelength ranges. This list, however, is accessible only to paying customers.
8
1. Observing photons in space
Figure 1.2: The general structure of the terrestrial atmosphere and the diurnal
variation 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 below 300 nm, the atmospheric absorption limit,7 but measurements at shorter wavelengths require the use of sounding rockets or satellites operating at altitudes above
160 km. However, even at these altitudes residual absorption and emission by the
geocorona and by terrestrial airglow emission, respectively, can affect the measurements. The latter effects can be eliminated, if the observing instrument is placed
at L1 or L2, i.e., at Lagrange points of the Sun-Earth system, and thus in a heliocentric orbit. L1 and L2 lie on the Sun-Earth line 1.5 Gm away from the Earth,8 in
front of and behind the Earth, respectively. In general the spacecraft are, however,
not placed in the Lagrange points themselves, but guided along halo orbits around
L1 or L2, where their station keeping is energetically less demanding than if their
position were maintained in the Lagrange points themselves.9
The first sounding-rocket measurements were made in the mid 1940s with German V2 rockets that had been captured at the end of the Second World War.10
7 High-altitude balloons do not only extend the accessible wavelength range towards shorter
wavelengths, but also permit to overcome the seeing caused by turbulence in the lower, denser
atmosphere. 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 an
altitude of 24 km, i.e., above the tropopause (cf., Figure 1.3). With proper optical and thermal
design, the theoretical diameter of the diffraction disk, given by the diameter of the primary mirror
of the telescope, 1.12 (λ/D) ≈ 1.85×10−6 rad = 0.35′′ was actually achieved (Schwarzschild 1959).
8 1.5 Gm correspond to 1 % of the Earth-Sun distance.
9 For L1 this also avoids disturbances between the radio downlink of the spacecraft and radio
emission from the Sun.
10 Even 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 Wernher
von Braun. A barrel-shaped container (the ‘Regener Tonne’) was equipped with barometers and
temperature sensors as well as spectrographs to observe the solar spectrum at high altitude.
9
Figure 1.3: Thermospheric temperature variations (Banks and Kockarts 1973), reproduced from Dickinson (1986).
Figure 1.4 shows the first solar spectra obtained in this way. Results from these
unstabilized rocket systems were limited and special techniques were required to
direct 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 “a
LiF 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 launch
vehicles (Russo 2001) brought much progress.12 Alongside the improvement of the
transportation systems three-axis stabilised pointing systems were perfected in another most important effort. This not only permitted recording the VUV spectrum
of the Sun with high spectral and spatial resolution (de Jager 2001), but also now
offered 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: the
A4 rocket, which was used by the military as the V2 missile, had been designed to carry a ton
of explosives. However, the lack of a pointing system would have allowed exposures to be taken
only during the descent on a parachute. Diffusing optics, as described by de Jager (2001), were
foreseen to feed light into the spectrometers. To achieve a proper deployment of the parachute, a
launch up to an altitude of only ca. 50 km was planned. The payload had been delivered to the
launch site, Peenemünde, in early January 1945 and its adjustments and functional tests had been
completed on 18 January. At that time, however, the increasing severity of the war prohibited an
A4-flight for scientific purposes (cf., DeVorkin 1993; de Jager 2001; Keppler 2003).
11 Regener and Regener (1934) had earlier recorded the solar spectrum down to 294 nm on a
balloon flight that had reached an altitude of 29.3 km.
12 For a description of the early history of rocket developments see, for example, Bonnet (1992)
and Stuhlinger (2001).
10
1. Observing photons in space
Figure 1.4: The first photograph of the ultraviolet spectrum of the Sun, at an
altitude of 55 km. Several exposures were made at different altitudes with a US
Naval Research Laboratory spectrograph that was carried in a V2 rocket on 10
October 1946 (from Tousey 1967).
Morton (1966) reported the first VUV spectra of bright hot stars, which were
recorded with a dispersion that was high enough for measuring the spectral lines
present. Absorption lines in these spectra revealed the existence of stellar winds
with velocities ranging from 1400 km s−1 to 3000 km s−1 . At that time the use of
objective-grating spectrographs avoided the need for very high pointing accuracy.
Pointing stability on the other hand was required to avoid the blurring of the
spectrum. This could be achieved with the help of a fine-stabilisation system that
operated in conjunction with the attitude control system, and kept the spectrograph
platform steady to within ca. ± 16′′ with a few excursions to ± 50′′ (Morton and
Spitzer 1966).13
How difficult it was to work without a three-axis attitude control system and
with only marginal spatial resolutions is also apparent from the paper describing
the first evidence of X-rays from sources outside the solar system (Giacconi et
al 1962). This specific observation actually started X-ray astronomy on objects
beyond the Sun and was eventually recognised by the first Nobel prize for space
astronomy. On the rocket flight in question, the spectral information available from
the two functioning Geiger counters with mica windows of two different thicknesses
was rather austere as well, so that a thorough discussion rationalising the exclusion
of other celestial, near-Earth and solar-system sources was required to corroborate
the conclusion.
13 It is remarkable that seven years later, the same group could report a stability of the Copernicus (OAO-3) satellite of better than ± 50 mas over a full orbit of ca. 90 min (Rogerson et al
1973).
11
The study of photons at γ- and X-ray energies
With nuclear physics being at the centre of contemporary physics, and moreover
being based mostly on electronic detection and analysis techniques, it was quite
natural that cosmic-ray research — and this meant also γ-ray astronomy — should
become part of the early epoch of space research (cf., Chapter 3, Kanbach et al
2010). The 31 first γ-ray photons detected from space (rather than from the terrestrial atmosphere, where they are generated through interaction with cosmic rays)
were identified in 1961 by the Explorer-11 satellite. This was followed in 1968 by
OSO-3 14 observations of 621 cosmic-ray photons (Kraushaar et al 1972). Figure 1.5
shows the distribution of these photons with galactic latitude. As pointed out by
the authors “the peak at the [galactic] equator and the apparently flat plateau over
the range |bII | > 40◦ suggest the existence of two components in the cosmic γ-ray
flux15 — one, which is clearly of galactic origin and another which is isotropic and
possibly of extragalactic origin.” OSO-7 then saw the first γ-ray lines generated by
solar flares in 1972 and — following the unfortunately abbreviated16 flight of SAS2 — 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 of
cosmic rays with the interstellar medium in the Milky Way and point sources such
as pulsars, as well as one individual extra-galactic source, namely 3C 273.
Figure 1.5: Galactic latitude distribution of cosmic γ-ray photons with energies
above 50 MeV as observed by OSO-3 (Kraushaar et al 1972). Photons emerging
from the galactic longitude range, −30◦ < ℓII < 30◦ , had been excluded, as it was
surmised that sources in the direction of the galactic centre might be of a special
nature.
14 The OSO series of NASA missions was primarily aimed at solar observations. Yet, the OSO
satellites not only provided a platform for instruments pointed at the Sun but also accommodated
a set of instruments, which scanned the sky in the course of six months (see, e.g., Kraushaar 1972).
15 In modern nomenclature: ‘radiance’
16 Because of a tape-recorder failure
12
1. Observing photons in space
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 built
in order to enforce the nuclear test ban treaty, and were meant to detect terrestrial
nuclear explosions. The γ-ray bursts remained a puzzle until 1997, when the afterglow 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 such
events 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 equivalent to a solar mass. It is not surprising then that the most remote object known
today was identified through a γ-ray burst — an object with redshift z = 8.2. This
implies an explosion 13 × 109 light years away, which had occurred at a time when
the Universe was about 1/20 of its present age. Indeed, this γ-ray burst was “a
glimpse of the end of the dark ages” (Tanvir et al 2009).
A most significant inference of γ-ray astronomy is the absence of a background
of γ-rays from nucleon-antinucleon annihilation. This finding empirically excludes
a matter-antimatter symmetric Universe (Cohen et al 1998; Pinkau 2009).
The impact of X-ray observations on the entire field of astronomy is summarised
in the Nobel lecture — The Dawn of X-ray Astronomy — by Giacconi (2003). A
breakthrough occurred, when — after several earlier attempts with threshold-limited
sensitivities 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 Xrays from the Sun and to explore the night-sky for other possible sources” had been
launched on 18 June 1962. With this rocket experiment, which was able to detect an
X-ray photon irradiance of (0.1 to 1) cm−2 s−1 , and thus was about 50 to 100 times
more sensitive than any experiment flown before, Giacconi et al (1962) discovered
the first X-ray source from outside the solar system. The detectors scanned the
sky over a band covering the Moon’s disk in two energy bands. Figure 1.6 shows
the 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 the
rocket flight of 12 June 1962 (from Giacconi et al 1962). The ordinate shows the
number of counts accumulated in 350 s in angular intervals of 6◦ .
13
with one or a few Geiger counter detectors with mechanical collimators, i.e., baffles
restricting the field of view, to high-resolution telescopes with imaging detectors,
and with energy discrimination either by diffraction gratings that disperse the radiation according to wavelength, or by energy-discriminating detectors (cf., Chapter
4, Culhane 2010). From the beginning in 1962 to the present, the sensitivity of
X-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-based
telescopes (Giacconi 2003). High-energy astronomy — i.e., X-ray astronomy and
γ-ray astronomy together — has led to the confirmation of fundamental, but originally only surmised, phenomena, such as black holes, or the intergalactic plasma
in galaxy clusters.
Although opening the X-ray window to the Universe beyond the Sun was, of
course, a momentous discovery, we recall that X-ray observations of the Sun have
given us an early insight into the large radiance variations associated with flares
and the structure and dynamics of the solar corona, which is consisting of highly
inhomogeneous plasma that is contained by magnetic field structures of widely different stability (cf., Blake et al 1963; Friedman 1963; Vaiana and Rosner 1978). And
solar 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 observationally
delicate region
The early VUV and soft X-ray spectra and images of astrophysical objects were
recorded by photography. Conventional photographic emulsions, however, were not
suitable for recording the interesting VUV wavelength range below 200 nm, because
the protective gelatine layer absorbed the radiation in question before it reached
the silver-halide grains. Schumann (1901) developed emulsions on glass plates with
just enough gelatine to hold the silver halide grains. Such emulsions, however, were
extremely 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 by
the astronauts, VanHoosier et al (1977) developed suitable containers with special
film-based Schumann emulsions.
Photographic emulsions have several drawbacks beyond the paramount obstacle
of recovering them from orbit after they have been exposed, namely their limited
dynamic 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
17 NASA’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 Apollo
landing sites in the years 1966 to 1967. As no suitable electronic cameras were available at that
time, the Moon’s surface was photographed from lunar orbit. The exposed film was developed and
photoelectrically scanned on board. The images could then be sent back to Earth by telemetry
(Nelson and Aback 1971).
14
1. Observing photons in space
three manned observing periods on Skylab had to be carried out blind, because
results became available only after recovery.
On the other hand, photographic material is inherently two-dimensional, and
accordingly permits the simultaneous recording of an image, for example, a field on
the sky, or a stigmatic spectrum, i.e., a spectrum which reflects the radiance variations along the entrance slit of the spectrograph. This was, in the first instance,
a definitive advantage for the photographic techniques. Nevertheless, for rapidly
varying objects, one would need short exposure times and ample photographic
material. With the advent of imaging detectors the advantage of photography disappeared; now images could also be downlinked to Earth in near-real time. In
addition, photoelectric measurements eventually became the adopted solution for
accurately calibrated observations.
Initially, photoelectric detectors were not even one-dimensional, but recorded
a single pixel only. For space astronomy they had to be rugged and this required
special developments. When used to register the weak solar VUV radiation, they
also had to be ‘solar blind’, i.e., insensitive to the strong visible light stemming
from 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 the
VUV, namely to below ≈ 116 nm, where no rugged window materials are available.
Hence the need for purely reflective optical systems and detector systems that can
operate without windows.19
For the detection of visible and infrared radiation, as well as for the detection
of X-rays, the most common detector today is a solid-state device — the chargecoupled device (CCD, see Chapters 23 through 25, Waltham 2010; Holland 2010;
Schühle 2010), which however has a relatively slow read-out. As CCDs are not sensitive to ultraviolet radiation, appropriate image intensifiers or fluorescent material
have to be added to achieve the desired results for observations of ultraviolet and
VUV radiation; and for X-rays thin-film metal filters have to be added to reject
radiation of longer wavelength.
Another important imaging detector system, which, on the other hand, is operating in vacuum, is the microchannel-plate (MCP) detector. If operated as a photon
counter this detector can also time-tag single photons (cf., Chapter 22, Timothy
2010). The origin of this detector can be traced to the magnetic electron multiplier (MEM), which was based on a continuous-dynode structure. The concept of
a continuous-dynode electron multiplier had first been proposed by Farnsworth
(1930), but practical devices only became available in the 1960s. The first practical continuous-dynode multiplier was the crossed-field Bendix MEM (Heroux and
Hinteregger 1960; Timothy et al 1967, cf. also, Chapter 22, Timothy 2010, where
18 As there were no ‘solar blind’ emulsions, alkali films for wavelengths in the VUV or thin metal
filters for X-rays had to be used to reject visible light in photographic apparatus, unless the object
was a young star, with peak radiation in the ultraviolet. For photographic X-ray observations thin
metal films were used not only to reject the visible radiation, but also to selectively transmit a
given wavelength region (cf., Vaiana and Rosner 1978).
19 Finally, 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).
15
the intermediate stage between MEM and micro-channel plates, namely the channeltron, 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 first
quarter of the 20th century. In his book ‘Die Rakete zu den Planetenräumen’20
Hermann Oberth (1923) had written that “Telescopes of any size could be used
in space, since the images of stars do not twinkle...” (cf., Stuhlinger 2001). In
reviewing the path of HST, then called ‘Space Telescope’, Lyman Spitzer (1979)
explained: “... the engineering possibilities of launching a complicated device into
space and keeping it in operation seemed rather remote at the time of Oberth’s
suggestion, and few astronomers took these possibilities seriously.” However, after
World War II, Spitzer (1946) discussed a number of scientific programmes that
could be carried out with large reflecting telescopes in space, namely “a) pushing
back the frontiers of the Universe and determining galactic distances by measuring
very faint stars, b) analysing the structure of galaxies, c) exploring systematically
the structure of globular clusters, and d) studying the nature of planets, especially
their surfaces and atmospheres” (quoted after Spitzer 1979). We recall here that
another Princeton astronomer, namely Spitzer’s colleague Martin Schwarzschild
(1959) made the first high-resolution observations in the visible by photographing
the granulation of the solar atmosphere from a stratospheric balloon in the late
1950s (cf., footnote 7).
Much emphasis today is on the cool parts of the Universe and on the red-shifted
Cosmos. As a consequence many, if not most, current and currently-planned major
space observatories are aimed at the infrared and sub-millimetre domains: Spitzer
and Herschel are in orbit, JWST is scheduled for launch in 2014 and SPICA is under advanced study. Moreover, a facility installed on an aeroplane, SOFIA, is now
entering service. The road to this state was arduous, as described by Harwit (2001).
Certain segments within the infrared domain are accessible, depending on wavelength, from high mountains, aeroplanes or stratospheric balloons (cf., Figure 1.7).
This has often led to extensive debates as to whether a given observational problem
really required a telescope in space, and has caused serious rifts in the astronomical
community. Moreover, infrared detectors and the cryogenics needed to suppress the
thermal background of the equipment itself at longer wavelengths, are technically
demanding and required long developments. In brief, the long fruition time of infrared space observations can be explained by both observational and technological
reasons. This has now been overcome, although operating infrared observing facilities is still a demanding task, especially because of the limited lifetime of the
cooling agent, or the need to fly a cooling apparatus (cf., Chapter 37, Rando 2010).
The development of microwave observations in space, in particular the development of COBE (cf., Chapter 8, Lamarre 2010) seems to have steadily progressed
20 Literally:
“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 like
Mauna Kea and at altitudes of aircraft and balloons (from Harwit 2001).
quite separately from the development of infrared space observatories, and has led
to the 2006 Nobel prize being awarded to John C. Mather and George F. Smoot for
this work in space astronomy. In the meantime COBE was followed by WMAP, another NASA mission, and now the ESA mission Planck has reached its orbit around
L2, after a joint launch in May 2009 together with the infrared/sub-millimetre observatory Herschel.
Innovative space technology meets conservative
attitude
It might be constructive in the long run to look back to the time when space
astronomy was in its infancy, and was considered by many as a somewhat extravagant exercise — the early epoch of space science. Seen from today’s astronomy
scene, where ground-based and space astronomy are viewed as being complementary, it may be interesting (and, if not disconcerting, now and again amusing)
to recall the early attitudes of ‘traditional’ astronomers towards the opportunities
offered by space technology (similarly, by the way, to those of ‘traditional’ geophysicists and oceanographers). Three quotes from history studies of space astronomy
shall illustrate this mercifully forgotten issue.
17
In her description of starting the space-astronomy programme of NASA, Roman (2001) writes: “Astronomers, practitioners of a very old science, deal with
long-lived objects and thus tend to be conservative. Hence, it is not surprising that
there were social as well as technical problems to be met in the development of the
new NASA astronomy program.” Also Stuhlinger (2001) mentions that the value
and cost of “ten Palomars vs. one Space Telescope” were often used as arguments
against building a space telescope. A special impediment has been the infrared
community. Harwit (2001) reports: “Since rocket payloads were relatively expensive, the U.S. ground-based observers feared that the National Aeronautics and
Space Administration (NASA) or National Science Foundation (NSF) support for
rocket work would drain away funding from their own efforts. Uncertainties about
the precise limitations of ground-based techniques often led to acrimonious disputes
between ground-based observers and those who wanted to launch sensitive infrared
astronomical telescopes above the atmosphere on rockets. Time and again, the more
traditional astronomers bluntly recommended that rocket-borne efforts be slashed
or pared back to an absolute minimum. Pioneering infrared rocket research was not
a happy venture!” Harwit, however, then goes on with a very useful description of
how the difficulties were overcome by uniting the infrared space community behind
one single project.
There was no great difference between the sceptical attitudes on the two sides
of the Atlantic, although the politically motivated support of all space activities
in the US led to a great disparity in funding for space science. Space astronomy
in Europe started — with the exception of the already mentioned γ-ray mission
COS-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 help
both sides. Indeed, observing photons in space — like other space ventures — was
an early manifestation of international collaboration beyond continental boundaries. Given the cold war, however, bilateral collaborations across the iron curtain
remained restricted to only a few western countries, such as the USA, Austria,
France and Sweden, although the Committee on Space Research (COSPAR) held
biennial 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 source
to detector, i.e., the physics of the interaction of photons with matter with an emphasis on the principles of physics being used in detecting and analysing photons.
The next six chapters deal with both the physical processes and the characteristics
of the radiation that can be observed in the energy-, wavelength- or frequencydomains from γ-rays to radio waves. Examples of the relevant astrophysical objects and the early missions devoted to their study are presented together with
resulting key findings. The subsequent eleven chapters are devoted to descriptions
of the techniques and systems used for imaging and spectral analysis. An in-depth
discussion 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 techniques of general importance to space observations, such as calibration, cryogenic
systems, laser-aligned structures, and the peculiarities of Earth observations, including radar measurements. Two final chapters, namely a pragmatically-oriented
essay on the implications of the space environment that must be considered when
building space instrumentation, and a summary and outlook, conclude the book.
In an Appendix we present a brief overview of the most pertinent rules of the
Système International. The SI has been used as systematically as possible in the
text. Figures which show historical plots, however, were not redrawn, and therefore
may show obsolete nomenclature and units. Conversions to SI units are also given in
the Appendix. Abbreviations and acronyms of spacecraft are not introduced; they
are collected in a list of acronyms at the end of the book. References to internet
resources and URLs have been checked to be valid in August 2009. However, the
internet remains a volatile resource and its contents and locations are subject to
change.
Acknowledgements
On behalf of all editors, we want to gratefully acknowledge the contributions of
André Balogh who provided the liaison to ISSI. Thjis de Graauw participated with
useful 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 their
colleagues. Karen Fletcher of ESA’s Communication and Knowledge Department
is thanked for sound professional editorial support. We also wish to thank Brigitte
Schutte-Fasler, Andrea Fischer, Saliba F. Saliba, Katja Schüpbach, Silvia Wenger
and the other members of the staff at ISSI for actively and cheerfully supporting
us whenever we needed to use the infrastructure of ISSI.
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