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| content long | 19-May-2013 23:14:23 |
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Instruments |
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Coded MasksThe Imager, Spectrometer, and X-ray monitor share a common principle of operation: they are all coded-mask telescopes. Gamma-rays cannot be focalized with lenses or mirrors. To make an image of the gamma-ray sky, one is forced to use the coded mask technique. It consists to detect the shadow of a mask placed on top of the telescope and made of opaque and transparent elements. The gamma-rays project the shadow of the mask onto the detector plane. Since there are many holes on the mask the gamma-rays coming from one source produce many overlapping images of the object. A coded mask, however, is designed so that each object in the field of view casts a unique shadow. A computer can decode the composite image and translate it into a scene of the gamma-ray source in the sky.
If there is only one source, the shift of the projected shadow with respect to the telescope axis allows determination of the direction of the source and thus its position on the sky. If there are more than one source, different shadows of the mask overlap. The detected pattern of light and shadow has then to be analysed in order to assess the observed image of the sky (finding the position and intensity of the different sources). This procedure is called the image deconvolution. The coded mask technique has the advantage to allow an almost perfect removal of the background sky. The sides of the telescope are shielded to prevent the detection of photons which did not pass through the mask.
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Last Update: 06 September 2011 For further information please contact: SciTech.editorial@esa.int |
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