When I'm looking at the inside of a mirror telescope:

enter image description here

I'm wondering why the secondary mirror does not block half of the incoming light? Is it "transparent" in this direction?

  • $\begingroup$ Why in the world would you think it doesn't block incoming light? $\endgroup$ Commented Dec 10, 2016 at 17:18
  • $\begingroup$ @CarlWitthoft My guess is that netik thinks there should be a "hole" in your image where the secondary mirror is. This is nothing more than a basic misunderstanding with how the optics of a telescope works. $\endgroup$
    – zephyr
    Commented Dec 10, 2016 at 18:49
  • $\begingroup$ @zephyr you're probably right there. Shall we regal him w/ stories of Fourier Transforms? :-) $\endgroup$ Commented Dec 11, 2016 at 13:04
  • $\begingroup$ yeah @Zephyr that's basically what I thought haha. $\endgroup$
    – netik
    Commented Dec 11, 2016 at 17:33
  • $\begingroup$ Perfectly reasonable thought unless you really learn how optics works. Unfortunately, its fairly complicated stuff and there are entire textbooks just on optics. It looks like the answer you accepted didn't fully explain why there isn't a dark spot in your image. If you're still interested, I can try to provide a brief answer to explain that. $\endgroup$
    – zephyr
    Commented Dec 11, 2016 at 18:00

1 Answer 1


In a Newtonian reflector, as pictured, the secondary mirror does block some of the light, but maybe less than you think. Even if the secondary were half the diameter of the primary, it would only block 1/4 of the light ($\ (1/2)^2$). In a more typical case the secondary would be somewhat smaller - perhaps a quarter of the size of the primary (or less). Hence a $1/16th$ or less is blocked, which is not too bad.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .