> What takes the place of a light source and slit in this situation?

The Sun.

(and the 150.000.000 km between it and us)

From the NASA page you link:

> The light is passed through a slit which is placed such that the reflected light from the mirror forms parallel rays that pass through the test section.

A light source placed at 150.000.000 km distance will obtain the same effect without any slit or mirror: the rays coming from it will appear to be substantially parallel to one another.

The fact that the Sun does not appear point-like in the sky has a small effect on this, as each point of the surface of the Sun will generate a family of parallel rays, and each family will not be parallel to the others, possibly blurring the image.

A careful construction of the receiving optical system (on the right of the slide you took from the NASA page) can obviate to this. Details on how to achieve such a result are more in the realm of photography, optics and physics in general rather than aviation.