# Why does e.g. Hubble's secondary mirror not block part of the picture?

The SOHO observatory has an opaque object in the light path that blocks the direct light from the sun making it possible to observe the corona. Hubble and many other telescopes have a big primary mirror in the bottom of a tube which bounces light on to a smaller secondary mirror at the opening of the tube. Why doesn't that other mirror cause a circular area of "no data" in the middle of the resulting images?

A particular point in the image is formed by the convergence of all the rays of light that leave from that point in the object, and bounce off the mirror. As long as there are some rays that can do that, then the only effect of the secondary is to block some of the rays, dimming those parts of the image slightly.

The function of a telescope or your eye is to translates angles into distances on a detector. You can see in the figure below (from Hyperphysics' excellent telescope page: http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/teles.html) that rays of light coming from a single object (such an unresolved point of light from a star in space) enter the telescope from a single direction (angle on the sky), in parallel. They pass through the telescope and an image is formed on the back of your eye (Or in Hubble's case, on an electronic detector, which you can think of being were the little red arrow is). The image is a representation of those angles as positions separated by some distance on a 2-D surface. On the detector all of the light from that single angle angle is light at a single point.

If your pupils constrict, you don't see less of the world, the angles entering your eye stay the same but the number of rays from each angle decreases and less light from every object (light from every direction) gets dimmer.

The opening at the front of Hubble is like your eye's pupil, if it were smaller, less light would come in, but just as many directions could be seen. The secondary mirror being in the way just means Hubble different shaped pupil from yours, a few rays from the object don't make it to the detector, but the image doesn't change, just the brightness.

If the secondary mirror were not there (as in a harder to manufacturer refracting telescope) Hubble would collect a little bit more light from each direction and could observe things a little quicker, but the view of the sky on the detector would be the same.

(SOHO blocks the sun by effectively placing a block in an image formed inside the telescope, for example where the arrow showing the image is in the simple telescope above, then SOHO's "eye" only sees the rays from around the edges of the sun, by blocking them at an "image plane", this is a "coronagraph", https://en.wikipedia.org/wiki/Coronagraph).

• This is a fairly good answer, but the diagram is not the best for explaining what is going on, as it represents a refractor, not a reflector. I could pop up an alternate answer, but maybe you could just replace the diagram with one that shows a secondary obstruction? Also, as the observatories being referred to are not observed with the eye, it may as well show a detector instead of an eyeball... – Jeremy Feb 27 '14 at 22:31
• Why then, if I hold up a mirror at arms length in front of my face, does that "secondary" mirror hide a specific part of my field of view, when on a telescope it doesn't? Because light rays from nearby sources are not parallel, as they are from a distant star? But then what about an apparently large object like the Moon? – LocalFluff May 23 '15 at 4:59