Re: Smaller hole in telescope's cover




Peter Webb wrote:

Chris L Peterson wrote...

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:

I am going to give these a shot, but keep in mind that I am just an
Electronics Engineer with some optical experience (mostly UV lasers),
so please forgive me if I get it wrong and please pay attention if
someone corrects my errors.

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.

It seems to me that it *is* a one inch telescope. Enclose the shaft
of light with a tube and cut away the portion of the mirror that is
in darkness, and you have a one inch off-axis reflector.

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.

I don't think so. The filter doesn't have any edge diffraction effects.
The holes do. Do a web search on "Hartman Screen" / "Hartman Mask",
then do a search on "Photon Sieve" for a taste of the kind of effects
that this can cause.

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).

Wouldn't the two holes, if small, would be a variation on the classic
double-slit experiment, and thus would look less like an image and
more like an interference pattern as the holes got smaller?

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.

Ignoring central obstruction and spider, the single hole seems to me
to actually be a smaller telescope. Which means less light gathering
and lower maximum resolution.

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.

I believe that the radio telescopes and Keck use aperture synthesis
by combining fourier transforms. Simply adding the two waveforms on
a retina, film, or CCD array loses information compared to aperture
synthesis. Also, doesn't Keck use the rotation of the Earth to get
a second dimension? Or am I mis-remembering and/or confusing it
with another system?

BTW, take a look at C.O.A.S.T. (Cambridge Optical Aperture Synthesis
Telescope) at http://www.mrao.cam.ac.uk/telescopes/coast/

What am I missing?

Diffraction and scattering effects?


Guy Macon
<http://www.guymacon.com/>

.

Relevant Pages

  • Re: Smaller hole in telescopes cover
    ... power and resolution as a 1" telescope of the same focal length, ... chucking away 63/64th of the light and 7/8 of the angular resolution. ... theoretical resolution of the telescope as determined by aperture. ... to the same area as hole of 1" diameter chucks away 63/64th of the light, ...
    (sci.astro.amateur)
  • Re: Diffrection limit for APS-C DSLRs
    ... Diffraction spreads as a linear function of distance ... from aperture to recording media. ... Now project that 1-degree spread onto a surface ...
    (rec.photo.digital)
  • Re: Looking at 130mm reflector . . .
    ... > the telescope specs so I have been doing some reading. ... the aperture, the brighter everything appears, and the more "resolved". ... Dob, especially at first. ... roll the scope out of the garage on a handtruck. ...
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  • diffraction curiosity questions
    ... I am curious about diffraction's role in photographic imaging, ... I noticed in a few classic optics texts that a chapter on diffraction ... interest to photographic imaging is the circular aperture. ...
    (sci.optics)
  • Re: 20 Years and Still Going
    ... we'd have a very high res, low noise sensor. ... and that is diffraction limiting. ... The higher the pixel resolution, ... has to do with the physical size of the aperture. ...
    (rec.photo.digital)