i have a mystery item for you yes what
do you think this is
i would imagine some people would think
it's some kind of game or maze or like a
game of go or something it does look
like it doesn't it
would it help if i told you that the
little hexagons some of them are made
out of plastic and some of them are made
out of lead
people who know who you are are going to
assume this has something to do with
astronomy
it does and in fact the lead is a bit of
a giveaway because one of the things
that people everyone knows about lead is
that x-rays don't get through lead and
in fact this is part of an x-ray
telescope it's sort of the equivalent of
the lens
in an x-ray telescope so it's the bit
that goes at the top so it's the bit
that the x-rays first pass through
before they get to the detector yeah an
x-ray telescope tends to be it doesn't
have to be a closed tube but basically
you've got this thing kind of up there
somewhere and then you've got just got a
detector down here that just detects the
x-rays
that goes over the hole at the front yes
yeah it is i mean it's kind of the
equivalent of a lens but this comes back
to how you actually make pictures with
x-rays this is a thing called a coded
mask
and it's used on hard x-ray and
gamma-ray telescopes it's actually used
in medical imaging in other areas as
well but in the astronomical community
we put them on
telescopes they tend to be the kind of
telescopes that work at wavelengths that
don't get through the atmosphere so they
tend to be on satellites so for example
the integral satellite which is a
gamma-ray observatory an e-submission up
there at the moment has coded mask on
the front of it and so either you put
them in space or the alternative these
gamma rays get at least a little bit way
down into the atmosphere so if you have
a balloon you can you can actually have
a balloon born experiment with one of
these telescopes on as well so i
borrowed this from a friend of mine
a guy called tony bird works at the
university of southampton so this is
actually university of southampton coded
mask uh and he's an old mate so he lent
it to me for a week or so it does have a
symmetry it actually has this sort of
strange rotational symmetry that
actually if you if you rotate the thing
around you get back to the same pattern
you started with and this isn't it
really isn't the random pattern um the
the pattern of effectively holes the
bits that the x-rays can get through and
the the blocked out bits is is uh
carefully chosen this is a thing called
a hexagonal uniformly redundant array or
a
it's technical to name for it but it's
actually designed with a with a specific
objective in mind and i put it down
because it weighs a ton
we need to back up a little bit and talk
about um
how how you make pictures basically how
telescopes work how cameras work normal
optical wavelengths and actually across
a lot of the electromagnetic spectrum
the way a camera works is the way your
video camera works that you've got a
lens at the front which focuses the
light down onto a detector at the back
typically a ccd these days which records
the light that's come from us so
essentially you're taking all the light
from came from a particular direction
and you're focusing it down to a
particular point on the detector and
that works fine at optical wavelengths
because it relies on the fact that
you've got things like lenses or mirrors
that allow you to bend the path of light
so you can focus light down in this way
x-rays
at least at the soft x-ray end of things
you can still actually focus light you
have to start getting a little cleverer
because if you just put a mirror in the
way of an x-ray just go straight through
if you put a mirror at a very oblique
angle the light will still
get reflected off it so you can still
focus light even at x-rays you can do it
with the same material if you have the
same material viewed face on x-rays just
go straight through if you think about
the atoms within it as the little balls
then if you think about it when it's
face on there are gaps between the balls
but if you take the same thing and view
it very close to edge on now there are
no longer any gaps and so actually the
x-rays which penetrated perfectly easily
when it was face on when it said the
bleak incidence will
deflect off the surface and so you can
still focus now it's quite hard to do
because if you think about it if you've
got a given area of your mirror if you
then put it in an angle like that you
suddenly only have a very tiny area so
to build an x-ray telescope this way
they actually build these nests of
mirrors inside each other
each of which sort of deflects a little
bit of light so you can end up forming
an image that way but even that breaks
down by the time you you get to the hard
x-ray end and the gamma-ray end of
things nothing you can do will focus the
light effectively so you need some new
technique for imaging and this is what
this is this is a thing called i said
it's a thing called a coded mask and
essentially all it does is it throws a
shadow if this was a telescope supposing
there was only one source out there one
bright quasar massive black hole
emitting loads of x-rays and you wanted
to know where it was in the sky all
you'd have to do is point this telescope
somewhere in vaguely the right direction
and then the x-rays from that quasar
will come through uh the mask here and
you know where they hit the lead they'd
be stopped where they hit the plastic
they just go straight through um and so
you've now got your detector behind
which records the x-rays it'll just see
a picture essentially of this mask
but where the mask appears on the
detector will tell you what direction
the light was coming from just like if
you were looking you know if you're
looking at shadows normally you can tell
where the sun is in the sky just from
the pattern of shadows so this is doing
exactly the same thing from the pattern
of shadows that this mask casts on your
detector you can tell where that source
was so couldn't you
just make that a big x or a straight
line why is it why is it this funky
groovy pattern that it is so that's
because there is more than one source
out there so if there were only one
source you it would probably be very
dull but actually almost any pattern
would work because you could just see
the pack of shadows what's clever about
this is that actually even if you have
multiple sources it can still produce
tell you where they all are and how
bright they all are so i'm going to yes
give you a demonstration of how these
things work with a nice little optical
model so actually i've made
uh
one of these coded masks here this is
actually the same design as the one on
the integral satellite up there in space
at the moment but rather than making out
of blocks of tungsten or lead i've made
it out of a transparency sheet when a
photocopier here's my high-tech
gamma-ray x-ray telescope made out of an
old cardboard box and there's the coated
mask that goes on the top
and obviously then the detector will be
down here at the bottom
and so we can very easily see what
happens
and shine it through you can see
basically you get just the shadows of
the
of the mask and when the light's in
different places obviously the pattern
moves around so by recording where the
pattern is you essentially know where
the the light source was so the clever
part is that this is a trivial thing to
do because actually if there's only one
source out there it's pretty easy to
figure out what the pattern is
the neat part is if there's more than
one source as we find astronomically and
you want to actually look at more than
one thing at once we can still do it so
i need to borrow your phone for this all
right
so for now if there's more than one
source out there in space
so you can see what happens is you get
overlapping shadows
so as you can see in some places you
have white in some places you have black
and in other places you have shades of
grey
just depending on you know exactly how
things line up depending on where the
sources are you end up with these
different overlapping patterns
okay dancing patterns
and it's just and it's a pretty simple
simple job to figure out what's doing
what is it
the clever part about these whether it's
these uniformly redundant arrays whether
it's one like this which is hexagonal or
whether one it's one like this which is
designed around squares
the clever thing about them
is that that pattern of blacks whites
and grays
you can use to interpret whatever is
going on in the sky there's never any
ambiguity so in fact there's a if you
like there's a unique way you can go
from the things you detect that patterns
of shades of grey that you detect on
your detector plane you can go all the
way back to saying exactly what the sky
must have been
so there's no degeneracy there's no
confusion there's no possible multiple
combinations of things that could
produce the same patterns of shades of
grey there's a unique way to go from
what you actually detect here very
simply back to what an image of the sky
looks like does that mean designing
these things is a bit of an art form
in the mathematical sense of the word it
is indeed an art form so there's a
there's a
rather clever mathematical algorithm
that allows you to just define one of
these optimal arrays that really has no
ambiguity at all between what you detect
and what the sky was doing and the
second question is why aren't they all
the same surely once you've got a
pattern that does that job they must all
just have the exact same pattern on them
so in fact quite a lot of them do have
the same patterns on them but there's
for example different sizes of array you
might want in terms of what resolution
of images you're trying to make what
size of field of view you're trying to
make if i have a really let me show you
a picture a very pretty picture so
here's a whole series of these hexagonal
uniformly redundant arrays and you can
see a different order so basically
they're making smaller and smaller
little blocks which allows you to make
finer and finer detailed pictures
and actually the thing that strikes me
about them is they're really rather
beautiful
so special is the quality of that mirror
that mirror was engineered to be so
perfectly optically smooth that if you
took this 2.4 meter diameter mirror
scaled it up to the radius of the earth
the largest bumps on that surface would
only be six inches high