Image from XMM-Newton Guest Observer Facility webpages.
There is one consideration which must be made before detecting photons. You want to make sure that you are getting photons from the right place in the sky! Since X-rays and gamma rays are emitted in many places in the sky (as will be covered in the Sources sections), you want to make sure that the photons you are collecting and counting are those from the source you want to study.
A first step is to shield the sides and back of the detector assembly. Some detectors are able to determine the angle from which the photon originated accurately enough that shielding is enough. However, many detectors need a bit more help.
For X-ray observations, there are two main ways to do this: grazing incidence mirrors and collimators. Mirrors are used for smaller detectors, like CCDs and microcalorimeters. Collimators are used for larger detectors, like proportional counters and microchannel plates (these detectors will be covered on the next page).
As mentioned in a previous section, X-rays can only be redirected by having them hit a mirror at grazing incidence (at a very shallow angle - almost parallel to the mirror). This means that X-ray mirrors cannot be constructed as mirrors for optical telescopes. In an optical telescope, you put the mirror up in front of the photons and they bounce off to focus at your detector. In an X-ray telescope, you need to orient the mirrors so that they are nearly parallel to the source. To cover as much area in this configuration as an optical mirror, the mirrors must be nested, one inside the other. As a result, mirrors for X-ray telescopes look kind of funny. Below is a picture of the mirrors used for XMM.
Image from XMM-Newton Guest Observer Facility webpages.
There are 58 wafer-thin mirrors in the above picture. They are shaped like cylinders and are nested one inside the other like Russian dolls.
One type of X-ray collimator uses tubes of material that absorb X-rays to ensure that the detected X-rays are those that have come from a very narrow part of the sky. The animation below illustrates how this type of collimator works.
Notice that only photons coming from nearly straight up will make it to the detector.
For gamma-ray observations, two different methods are commonly used: coded aperture masks and Compton telescopes. The Compton telescope will be covered in the Gamma-ray Detectors. Coded aperture masks are briefly described below.
One other consideration to make with gamma-ray observations is that cosmic rays mimic gamma-ray signals in the detector. To determine whether the signal was a gamma ray or cosmic ray, an anti-coincidence detector shield is used.
A coded aperture mask is a plate mounted in front of a detector array made of tiles of material opaque to gamma rays (typically lead). These tiles cover up to half of the area and are commonly arranged in a random pattern. Then, when gamma rays from a source hit the mask, a shadow is cast on the detector array. A computer can use this shadow pattern and the tile pattern to determine the direction of the gamma rays.
Cosmic rays can interact with gamma-ray detector in the same manner as gamma-rays, creating a false signal. To distinguish between the two signals, an anti-coincience detector shield is used. This shield is transparent to gamma-rays, but will detect a "hit" by a cosmic ray. Then, if the anti-coincidence shield detects a hit at the same time the gamma-ray detector records a "gamma-ray", that detection can be rejected.
