At what photon energy does the index of refraction start to increase again ?
Higher energy X-ray and Gamma - ray telescopes refrain from focusing completely and use coded aperture masks : the patterns of the shadow the mask creates can be reconstructed to form an image.X - ray and Gamma - ray telescopes are usually on Earth - orbiting satellites or high - flying balloons since the Earth 's atmosphere is opaque to this part of the electromagnetic spectrum .