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In the last lectures we were talking about the general background about the galaxy.

And in the last lecture Ingo was talking about detection methods

through the electromagnetic spectrum.

And he stopped talking about this when gamma rays came up.

If you think of the other detection methods

that you looked at, they always worked

with some kind of an optics.

In radio astronomy, for example, you

have these large dishes that focus electromagnetic radiation

onto a receiver.

In the optical, you have mirrors.

In the X-rays, Ingo was talking about Walter mirrors,

another way to focus radiation.

We always need to focus radiation in these cases

because astrophysical sources are weak sources.

So we need to collect as much radiation as possible.

Unfortunately, it is not possible to build

optics and energies, photon energies,

above 50 kiloelectron volts.

The reason for this is that grazing incidence optics,

the principle behind Walter mirrors,

do not work above that energy because material does not

reflect X-rays anymore or higher energetic radiation.

So from about 50kV upwards, we cannot use optics.

And if you want to do imaging, then

you need to use other methods.

And this mission here is one mission

that uses such an optics.

This is the Integral satellite by the European Space Agency.

But other missions that operate in this wave band

also use this technology.

And this is a mission that uses so-called coded masks.

Here's an explosion diagram of what Integral looks like.

Integral has two instruments, major instruments on board.

This box here, which is called EBIS,

and this box here, which is called SPI.

This is the imager on board the Integral satellite.

And this is spectrometer for Integral,

I think is the acronym here.

So how do these instruments work?

You see here on top, there is a so-called coded mask.

The coded mask is a fairly simple thing.

It's essentially a big box that contains holes.

And so X-rays that arrive on that mask that

hit one of the holes, they will just pass through and hit

the focal plane where we've placed gamma ray detectors.

X-rays that hit one of the closed places,

they get stuck in the mask.

And so in other words, a source that is at infinity