High energy photons cannot be focused as their low energy counterparts due to their large penetrating power. Different approaches are thus needed: Grazing-incidence mirrors, Modulators, Compton telescopes.
The Snell law allows to use large incidence angles, greater than the critical angle at which radiation is totally reflected) to focus X-ray photons:
sin θ1/sin θ2 =n2/n1 →θcrit=arcsin(n2/n1)
Coded masks telescopes are based on the idea of imaging through a 'pinhole' but with larger collecting area to increase the efficiency.
Imaging with coded masks is a process that requires deconvolution through the mask pattern. The deconvolved image may not be unique and an optimal mask design will minimize the degeneracy.
The ability to reconstruct the image of the sky depends on the ratio between the size of the mask and the size of the detector.
Often a limited detector size or resolution can be associated with dithering techniques.
Left: Examples of coded masks and response patterns to a point source. Right: extreme example of an unusual mask pattern and of the image that can be reconstructed from it.
The first X-ray satellites had no focusing capabilities and relied on temporal modulation to infer the source of the incoming radiation.
Modulators could be created through the rotation of the satellite itself around one of its axis (e.g. Uhuru) or through more complex mechanical designs, such as those shown on the right and based on overlapping grids.
Compton scattering can be used to design telescopes capable of simultaneous imaging and spectroscopy, especially in the g-ray regime where it is one of the dominant scattering processes.
Compton scattering, followed by photoelectric absorption allows to infer the incidence angle of the incoming photons, once the impact point on each detector and the deposited energy is known.
The sky image reconstruction requires data deconvolution with the scattering pattern.
Energy-loss processes in the passive material surrounding the detectors can alter the spectral signature of the incoming radiation and significantly contribute to the detector background.
The image shows the spectral signature produced by the combined interaction processes occurring in the telescope structure and due to both photons and particles (cosmic rays).
Shielding significantly reduces the instrument background, but at the expense of reduced effective area, field of view, increased dead time and payload weight.
Integral telescope structure and its energy sensitivity range, compared to other high energy missions.
13. X-ray binaries