Dr. L. Fletcher
Room 618
Kelvin Building
A1Y OBSERVATIONAL METHODS
Handout – High Energy Optics
Ultraviolet and Extreme Ultraviolet range: 400nm – 20nm
At wavelengths as low as 20 nm or so, it is possible to use standard reflecting optical configurations
of the type we have discussed previously (generally known as normal incidence optics) to focus
photons – provided that the reflecting surfaces are coated to enhance the reflectivity in the desired
wavelength range (necessary at wavelengths shortward of about 300nm). Using coatings will
inevitably reduce the reflectivity outwith the desired wavelength range, so that the observations may
be limited to a narrow band. For example, the Transition Region and Coronal Explorer satellite
(TRACE) observes in bands about 0.2nm wide, centred at 17.1nm and 19.5 nm, using Cassegraine
optics. It should be realised that coatings cannot be produced with high reflectivity for use at every UV
or EUV wavelengths, so some parts of the spectrum cannot be observed. Of course, these optical
systems must be kept in vacuum to minimise photon absorption.
Soft X-rays: 10nm – 0.1nm
At higher photon energies, from about 10nm down, it is not possible to make reflecting surfaces which
are smooth enough (compared to the photon wavelength). Photons are scattered by irregularities in the
surface, and will not be properly focussed. A different focussing technique, called grazing incidence
optics, is used.
The refractive index, nair, of air is 1.0. The refractive index nmetal of metal is less than 1 (this occurs
since it is a conducting medium and free electrons in the medium interact with the photons in such a
way as to bend the direction of travel away from the normal). To see what happens in this case,
consider optical photons moving from a medium of high refractive index to one of low refractive
index – e.g. from water into air. Snell’s law tells us that these photons will experience total internal
reflection at the water-air interface, if they arrive at an angle to the surface normal which is greater
than the critical angle given by
Sin crit 
nair
nwater
Similarly, high energy photons can undergo total internal reflection when they arrive at the (low n)
metal surface from air/vacuum, at an angle to the surface normal which is greater than some critical
angle, given this time by
Sin crit 
nmetal
nair
The grazing angle, gr = 90- crit, so that Cosgr = Sincrit = nmetal/nair. Since the refractive index of
metal is rather close to 1 (about 1.01), the grazing angle is typically small – on the order of 2-4
degrees. The process is illustrated below. This is often referred to as total external reflection of the
photons from the metal surface. It is not the same process as ‘ordinary’ reflection.
Normal to surface
gr
nair
nmetal
This process can be used to direct photons, and therefore to focus them, which is done using metal
surfaces having the form of a parabola or hyperbola of revolution (i.e. the surface described when a
parabola or hyperbola is rotated about an axis). This is how X-ray telescopes are made. Cross sections
through two configurations, the Wolter I and Wolter II configurations, are shown below.
Since light has to be prevented from passing
‘straight through’ the telescope, a baffle is
put in the middle (used as a reflecting
surface in the Wolter II configuration, lower
sketch). This means that the effective area of
the telescope is much smaller than its actual
area. To increase the effective area, nested
paraboloids
are
sometimes
used
(Kirkpatrick-Baez design, not shown, but
used for example in the Chandra X-ray
telescope.)
Collimating Optics for Hard X-rays (0.1nm- 0.001nm, or ~10 to 1000 keV)
At very high energies, the use of grazing incidence optics is also not possible – the grazing angle
becomes very small and the focal length impractically long. Collimating optics are then used.
Collimating optics does not involved focussing photons, rather photons are blocked or transmitted to a
detector, depending on the angle at which they arrive at the telescope. Angular information can then be
deduced.
Photon not transmitted
Photon transmitted
d
h

tr
detector
Collimating optics are made by constructing arrays of metal slats which are opaque to photons, and
separated by small distances. Typical slat thicknesses and separations are 20 – 40 microns
(comparable to a human hair): measurements and manufacture have to be very precise! In the above
figure, a photon arriving at a small angle  would be transmitted by the slats, and reach the detector at
the base, whereas one arriving at an angle larger than some critical transmission angle, tr, would not.
The value of tr is given by geometry:
Tantr 
d
,
h
where d is the slat spacing and h is the height of the slats. By using a number of grids with different
ratios d/h, one can observe photons from a range of different angles – i.e. different positions on the
sky. By mounting the grids in different orientations, or so that they spin (for example as is done on the
Ramaty High Energy Solar Spectroscopic Imager Satellite), a two-dimensional picture of the source
on the sky can be built up. It should be noted here that the process of working out what the source
looks like armed only with this rather convoluted and ambiguous angular information is not
straightforward, involving advanced mathematics. Nevertheless, it can be done, and is used for
imaging high-energy astrophysical sources, including the Sun.