NASA’s novel x-ray telescope
The Nuclear Spectroscopic Telescope Array (NuSTAR) was launched yesterday, with the aim of finding distant super-energetic black holes, towards understanding the structure of the universe.
Just about every part of the space craft shows US engineering at its best.
Even the launch vehicle is neat – not a conventional shot-off-the-ground rocket, but Orbital Science’s air-launched Pegasus rocket, which is dropped from its TriStar airliner at almost 12km altitude – much like this other launch – and leaves the satellite in 600km-high orbit.
NuSTAR is looking for high-energy (6 – 79 keV) x-rays, for which optics are:
a, tricky – see below
b, long – over 10m
b, is interesting because Pegasus’ payload bay is only 2m long and 1m in diameter, so the whole satellite had to fold up considerably.
NASA’s answer was to make the optics and detector as two separate pieces, held apart by a long collapsible hollow mast – see picture.
Mind-boggling to construct, the optics consist of 133 thin tubular mirrors, nested together like Russian dolls
– and there are actually two sets of 133 mirrors per telescope, and two identical telescopes next to each other to collect more light.
These change the direction of the x-rays by glancing angle reflection, with front-end paraboloid mirrors, immediately followed by hyperboloid mirrors – see diagram – Wolter type 1 optics, originally invented by Hans Wolter for microscopes.
Optical assemblies end up 450mm long, 191mm in radius, and have a 10m focal length.
Mirror substrates start out at thin flat sheets of glass that are heated until they slump over precision polished cylindrical quartz mandrels to achieve the right curvature – at NASA’s Goddard Space Flight Center in Maryland.
Even the reflective layers are exotic – contracted from 200 stacked depth-graded layers.
The multi-layers alternate between low-density and high-density – platinum (high) and silicon carbide (low). And some parts of the mirrors use an alternative tungsten – silicon combination.
Platinum, absorbing x-rays above 79keV, puts the upper limit on bandwidth – previous x-ray telescopes have only got to 15keV, said NASA.
Multi-layer coating was done by DTU-Space – at the Danish Technical University in Copenhagen.
The optics were then are assembled from the inside out, shell upon shell, spaced with graphite blocks and glued with epoxy – at Columbia University in New York.
Once the satellite is extended, an adjustment mechanism will be used for a one-time compensation for mast deflection.
Laser metrology will govern that adjustment, and to correct x-ray images for blurring due to movement in the mast.
Science will start in a month and last for two years.
As well as black holes, NuSTAR will observe exploded star remnants, compact dead stars, and clusters of galaxies.
Amongst other things, it will map young supernova remnants to understand how stars explode and how elements are created, as well as help to understand the relativistic jets of particles emerging from active galaxies around super-massive black holes.