Soft X-ray Telescope (SXT)
X-ray Mirror Background
In order to capture and focus X-rays, "grazing incidence" optics must be used. X-rays are so energetic (and have such a small wavelength) that they tend to pass through most matter. Mirrors are no different. Most of the X-rays in a beam shot directly at a mirror would simply pass right through – they don't even see that the mirror is there. This means that traditional lenses and mirrors just don't work for X-ray astronomy. However, if an X-ray encounters a mirror at a "grazing incidence", i.e. if it just glances the surface of the mirror, then it will bounce, behaving just as an optical photon would.
Illustration of grazing incidence. X-ray telescopes require that the X-ray photons encounter the mirror at a "grazing incidence", where they just glance the surface. The scale in this image is exaggerated, as the angle of incidence (the angle between the mirror surface and the X-ray) would be shallower.
This means that the mirrors in an X-ray telescope have to be oriented such that their surfaces are nearly parallel to the incominglight. However, if we simply turn a regular optical telescope mirror on its side, we would miss a lot of the incoming X-rays – the "cross-section" of the mirror to the X-rays would be very small.
To solve this problem, X-ray telescopes use cylindrical mirrors and nest them, one inside the other. This is the general design used by both the Soft X-ray Telescope and Hard X-ray Telescope on Constellation-X.
Photo of the XMM-Newton mirrors.
Constellation-X SXT Mirrors
Each of the four Soft X-ray Telescope (SXT) mirror assemblies will be consist of 163 nested shells, with the largest shell having a diameter of 1.3 meters. Each mirror shell will be made in several segments, so that each of the inner 66 shells will have 5 segments and the outer 97 shells will have 10 segments. Each mirror assembly, though, will have two mirrors – a hyperbolic and parabolic pair, like some optical telescopes. This means that each mirror assembly will need 2600 total shell segments.
Making that many shells quickly and accurately is quite an engineering challenge. Our engineers have developed a "glass slumping" technique that brings this engineering feat within our grasp. The basic idea is that they start with a mandrel that is shaped just right for the mirror segment. On this mandrel, they put a flat piece of thin glass. Then they heat it up, and as they do, the glass will "slump" onto the mandrel, taking the shape of the mandrel. Once the glass cools, the engineers take it off the mandrel and apply a reflective coating. Once perfected, our engineers can make the mirror shells by repeating this process 2,600 times with the proper mandrel sizes to make the different mirror shells.
Cartoon of the glass slumping method for producing mirror shells. Place a flat piece of thin glass on a mandrel, then heat up the system. As the glass heats up, it will begin to slump onto the mandrel, leaving it with just the right shape.
