Re: Smaller hole in telescope's cover


"Chris L Peterson" <clp@xxxxxxxxxxxxxxxxxx> wrote in message news:g78573l8s4e1qej0u5ikqp8atp0rv29gc2@xxxxxxxxxx
On Fri, 15 Jun 2007 16:56:40 +1000, "Peter Webb"
<webbfamily@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Why do they make one hole with an area of (say) 1 square inch, instead of
three smaller holes spaced as far apart as possible? That way you would get
the full resolution of the telescope and reduced light, instead of reduced
light and reduced resolution.

A circular aperture produces the smallest diffraction pattern. The more
complex diffraction produced by occluded or multiple apertures can cause
reduced contrast at a fine scale (the MTF is degraded). So while there
are special cases where unusual apertures are useful, as a general rule
circular is best.


I know you are correct, because that is how telescopes are made (and telescope makers are not idiots), and you obviously know far more about this than I do.

But I still don't understand why. As I don't have a telescope, I am reduced to theory (which is my interest anyway).

Here are some statements, and I would like a true or false statement on each. Lets assume we have an 8" reflector, no spider, perfect world, etc:

1. An 8" telescope stopped down to 1" aperture has the same light gathering power and resolution as a 1" telescope of the same focal length, so you are chucking away 63/64th of the light and 7/8 of the angular resolution.

2. An 8" telescope with a cover with 1,000 evenly spread tiny holes adding to the same area as hole of 1" diameter chucks away 63/64th of the light, but none of the resolution, and indeed performs the same as an 8" telescope with a filter at the front-end which blocks 63/64ths of the light.

3. The images taken of an object taken with a single hole will be diffraction limited to the size of the hole, but if multiple holes are used will be diffraction limited to the spacing of the holes (not their diameter).

4. If the total amount of light is not an issue (as for example when viewing the moon or terrestrial objects), then the only other thing a larger telescope is giving you is angular resolution - which is thrown away when you "stop" down the aperture with a single hole.

5. Radio telescopes use the same technique - multiple widely separated "holes" (antennae) instead of one big hole (or lots of little holes clustered together). So, for that matter, does Keck, but regrettably only on one axis.

What am I missing?

TIA


Peter Webb

.

Relevant Pages

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