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.
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.
