X-ray collimators

How do the optics on Ginga differ from the more-modern X-ray missions? If you look at Ginga next to ASCA or Suzaku, it is clear that Ginga has a much different shape. Rather than being cylindrical, it is a large box. The difference in is the optics.

Ginga satellite in the clean roomASCA Satellite in the clean room
The Ginga (left) and ASCA (right) satellites.

When most people think of a telescope, they think of a fairly traditional picture where light enters one end of a tube, bounces off a mirror or two, then focuses on an eyepiece or instrument. The problem with this picture in X-ray astronomy is that X-rays don’t necessarily notice that a mirror is in their way! If an X-ray encounters a mirror nearly straight-on, it will most likely pass right through the mirror. Modern X-ray telescopes take advantage of grazing incidence optics to get X-rays to bounce off of mirrors and focus onto a detector. ASCA had a focal length of 3.5 meters and Suzaku has a focal length of about 4.5 meters. The satellite must then have space between the mirror and the instruments – 3.5 meters in the case of ASCA, 4.5 for Suzaku – which makes the spacecraft longer.

Ginga’s instruments, however, did not use focusing optics; instead, they used collimators. A collimator is essentially a way to block out light from unwanted sources without making the light “bounce” from mirror to mirror. Imagine using a paper towel tube to look at a distant object. It doesn’t magnify the object, but it blocks out light from other sources to help you see the object better. A collimator is constructed from lots of little tubes that each act like that paper towel tube.

* compare the optics on Ginga to those on ASCA – collimator versus focusing mirrors *
X-ray focusing mirrors were used as early as 1978 (on the Einstein satellite); however, the use of mirrors versus collimators is always a balance between the mass of the mirrors, the size of the spacecraft, and the science goals of the mission.