Coded Masks

The Coded Mask Thing

X-rays are composed by high energy photons. It's difficult to get this photons to interact with matter in a way that they can be focused as we do with optical bands photons. Therefore, we need to use some trick to get our high energy images.

For photon energies up to about 10 Kevs, focusing can be done using different techniques like grazing-incidence reflection. There's an alternative way which can be used for imaging high energy photons like X-ray and gamma-ray ones. This techniques give a wider range of energies and also offer wider field of views and are known as multiplexing techniques. The direction and intensity of the incoming radiation is encoded, therefore it needs to be decoded later.  We can classify multiplexing techniques this way:

  • Temporal multiplexing: does not need position sensitive detectors. Examples of devices making use of this technique:
    1. scanning collimators
    2. rotation collimators
  • Spatial multiplexing: we do need a position sensitive detector. We can find two kinds of devices making use of this technique:
    1. fourier transform imagers: 2 or more collimator grids are placed in front of a detector plane
    2. coded-mask systems: one or more arrays ofopaque and transparent elements are placed in front of the detector plane

After decoding they permit the reconstruction of the original source image. Then these techniques allow imaging of high energy sources.

A coded-mask is a plate with areas that are transparent or opaque to photons in a given energy range following a defined pattern. The transparent and opaque areas are called mask elements. All of them have equal size and are distributed in a pre-determined pattern placed in a regular grid. The detector plane should have spatial resolution matched to the mask pattern grid size and must be sensitive to the range of energies of interest. The idea behind the coded-mask is the same as in the case of the pinhole camera. Letting light go through a little hole in an opaque surface. In fact the pinhole camera can be considered as the simplest case of coded-mask.


In the pinhole camera we had only one hole and, thus, only one image.

 

 

In a coded mask we will find a mixture of one image per trasparent element in the mask, as each of the transparent elements will work as an individual pinhole camera. We need to revert the process by means of computational techniques, i. e., with the help of our computers we have to find each one of the individual images wich compose the mixed one.

 

 
The goal to achieve is to get photons coming from a certain direction in the sky project the mask in the detector plane in a unique way. This projection has the same coding as the mask-pattern, but it will be shifted relative to the central position, this shift determine uniquely the direction of the incoming photons. The strength of the projection will determine the intensity of the source. The detector will accumulate, thus, the sum of several displaced mask patterns (originated from different sources in the field of view).. Therefore the accumulated detector image must be decoded matching every possible shifted mask pattern,  in order to recover the information on how many sources were in the field of view and its intensities.

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