Why does the secondary mirror within the body of a reflecting telescope not produce a blind spot for the viewer. Since the light rays are being reflected from the primary mirror onto the secondary (which is before the primary), surely this smaller mirror would prevent some of the light rays from reaching the primary mirror which would result in a blind spot? enter image description here


This is a common misconception, and to find the answer one only has to think what adjusting the aperture size in a camera does.

I will illustrate my answer using a lens as it is easier for me to draw the diagram.

enter image description here

The incoming rays from the top of a distant object form an image of the top of the distant object in the focal plane of the lens.

If the bottom part of the lens $PB$ is covered up, the image of the top of the distant object is still formed in the focal plane of the lens with the rays which pass through the top part $AP$ of the lens.
There is no part of the image which is "missing" – rather the image formed is less bright as less light is responsible for its formation.

So the secondary mirror does obstruct some of the light reaching the primary mirror, but the effect is that the image is less bright than if the whole of the primary mirror was illuminated.
As the diameter of the secondary is much less than the diameter of the primary mirror and it is the area of the mirrors which is important, the reduction in intensity of the final image is not that great.

And there is no "blind spot" as the image of the whole of the distant object can still be seen.

  • $\begingroup$ Thank you, excellently explained $\endgroup$ – JDF Jun 13 '19 at 12:38
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    $\begingroup$ I don't quite follow -- how is this different from a coronagraph, which purposely blocks a portion of the visible field? $\endgroup$ – costrom Jun 13 '19 at 20:36
  • $\begingroup$ @costrom Imagine in the focal plane of a convex lens you put a black disc of the same diameter as the image of the Sun. The light which misses that disc is that which comes from the solar corona. Using more lenses the light which misses the disc can be focussed with a black centre (the image of the disc) onto a screen/detector. $\endgroup$ – Farcher Jun 13 '19 at 20:56
  • $\begingroup$ @Farcher thanks! It still took me a few readings of your comment, but I think the gist is that the outer portion of the reflector/lens/what have you still receives light from the entire object of interest, so the image can still be produced onto the screen/detector. $\endgroup$ – costrom Jun 13 '19 at 21:48
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    $\begingroup$ @costrom The whole of the lens collects light and forms an image in the focal plane. The light which forms the central part of the image is blocked off allowing only light which formed the outer part of the image to continue onward to be focussed on a detector/screen. $\endgroup$ – Farcher Jun 14 '19 at 5:10

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