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Gamma-ray Telescopes

Gamma-ray Telescopes

Artist's impression of the SAS-2 satellite

Artist's impression of the SAS-2 satellite. SAS-2 was the first dedicated orbiting gamma-ray telescope. (Credit: NASA)

Visible light can be focused by using mirrors or lenses to bend the paths of the photons and concentrate them in one place, which creates a sharper, brighter image. Focusing means to bend the photon's path without changing its energy too much. However, focusing gamma-rays is not easy. When a gamma-ray hits matter, it interacts in such a way as to destroy the gamma-ray or change its energy by a large amount. So traditional mirrors and lenses don't work for focusing gamma-rays.

As discussed in the introduction to X-ray Telescopes, X-rays are focused using "grazing incidence" mirrors, so that the incoming photons just skip across the surface of the mirror, like a stone skipping on water.

Gamma-rays, however, are even more energetic than X-rays, so they would have to come in at an even shallower angle, which makes grazing incidence mirrors impractical for gamma-ray astronomy. In fact, gamma-ray telescopes tend not to use focusing optics. Instead, scientists have developed other techniques to ensure that the light that enters the detector is from the correct area of the sky.

Gamma-rays can come toward a telescope from all directions, but when making astronomical observations, you want to be sure that you are only studying the gamma-rays from a particular source or region of the sky. Three common methods are collimators, anti-coincidence shields, and coded aperture masks. Telescopes generally combine two of these methods to ensure the photons used in scientific analysis are the ones form the correct direction in the sky.

Collimators

Photo of the collimator used on the RXTE satellite

Photo of the collimator used on the proportional counter array on the Rossi X-ray Timing Explorer satellite. (Credit: Craig Markwardt/NASA Goddard Space Flight Center)

Collimators are a workhorse of high energy astronomy, being used on X-ray and gamma-ray telescopes from the beginning. You've probably used a collimator yourself without realizing it. Imagine going outside on a sunny day and looking toward a flower. As you look, you are also seeing all kinds of other things in the periphery of your vision – maybe the sky, a yard, a house, a tree, or other flowers. Now, if you look at that same flower through a paper towel tube, suddenly you'll seem much less of your surroundings. That paper towel tube is acting as a collimator.

The way a collimator works is to only allow light coming from certain angles to make it to the detector (in the above example, the detector is your eye). The smaller the diameter of the collimator, the less light that is allowed in. For example, imagine that you looked at that flower through a straw instead of a paper towel tube. You would see even less of your surroundings, maybe only seeing a part of the flower. Making a collimator has the same effect. Replace your paper towel tube with a wrapping paper tube, and you'll see about the same thing as when looking with a straw.

On a telescope, collimators are not just single tubes, but a honeycomb of tubes. The tubes have to be made of material that will stop an incoming gamma-ray (or X-ray in an X-ray telescope), otherwise it would be as if the collimator wasn't there.

Anti-coincidence Shields

What happens if a gamma-ray was able to come directly in the side of a telescope into the detector? The detector wouldn't necessarily know the difference – it would record the presence of a gamma-ray. Many gamma-ray telescopes have an anti-coincidence shield to protect against those gamma-rays being counted as one from the object of interest. These also protect against cosmic rays that can enter the detector – cosmic rays can leave a signal in the detector that mimics a gamma-ray.

An anti-coincidence shield is essentially another detector. Gamma-rays and cosmic rays that pass through it leave a signal, and then are absorbed by the detector, leaving a second signal in the detector. Computer software that analyzes the data will see that there were two signals within a very short amount of time of each other, one in the anti-coincidence shield and one in the detector. That signal in the detector is then rejected, so that scientists only use the "good" photons that come from the right direction.

Coded Aperture Masks

A coded aperture mask doesn't exactly keep out unwanted photons, but provides a way to determine the direction photons entering the detector came from. It works by casting a gamma-ray a shadow on the detector.

Imagine that you are in a building with a skylight. For part of the day, the sun shines through the skylight, leaving a patch of light on the floor. You can tell which direction the sun is in relation to the skylight by where that patch of light lies on the floor.

Diagram showing how a coded aperture mask works Photo of an engineer working on the coded aperture mask
	used for the Burst Alert Telescope on Swift

Diagram of how a coded aperture mask works and a photo of an engineer working on the coded aperture mask used for the Burst Alert Telescope on Swift. (Credit: NASA's Imagine the Universe and NASA/Swift)

A coded aperture is a mask positioned in front of the gamma-ray detectors. It is made of a material that will stop gamma-rays, but only about half of the mask is covered in that material. The rest of the mask is open, allowing gamma-rays to reach the detector. When a gamma-ray source shines gamma-rays on the detector, about half of them are stopped by the coded aperture mask, and about half get to the detector. By looking at the pattern of gamma-rays on the detector, computer analysis will show where those gamma-rays passed through the aperture. Just like the patch of sunlight in a skylight, those computers can then tell which direction the signal came from.


Updated: October 2013




 

A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC

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