Re: Smaller hole in telescope's cover

On Sun, 17 Jun 2007 12:37:49 +1000, "Peter Webb"
<webbfamily@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

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.

Yes. But you should keep in mind that in many cases the actual
resolution you can achieve with a telescope is far less than the
theoretical resolution of the telescope as determined by aperture. Many
things conspire to reduce resolution, such as atmospheric motion, the
inherent low resolution of the human eye at low light levels, etc.


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.

A telescope like this does not have the same resolution as one with an
8" unobstructed aperture. In fact, it will have really poor resolution
for most objects. The interference patterns produced by 1000 individual
circular apertures will overlap and interfere with each other, producing
a seriously degraded MTF. Except for the special case of objects with
spatial frequencies the same as that produced by the hole pattern
(assuming the hole spacing is regular), you will not have a high
resolution result. This is easy enough to test by making a mask like you
describe.


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).

A set of widely spaced holes will provide for higher resolution along
the axis of those holes than you would get from a single hole, but along
other axes the resolution won't be as high. When you combine these
asymmetric resolutions with diffraction, the result is not something
most people would be happy with, certainly not for "aesthetic" viewing.


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.

That is true, but most people don't stop down their telescopes enough
for this to be significant. For instance, when viewing the Sun, it is
common for apertures larger than just a couple of inches to be seeing
limited anyway. I regularly stop my 12" scope down to 4" when viewing
the Sun, and have never detected any drop in 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.

They do. But they also mathematically reconstruct the image, just as is
done with optical interferometers. If you simply looked at the raw radio
data, or put your eye at the focal plane of the Keck interferometer, you
wouldn't see anything that really looked like a normal telescopic view.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
.

Relevant Pages

  • Re: The end of Seti
    ... Telescopes are theoretically limited in resolution by the diffraction limit ... divided by the diameter of the telescope d. ... eliminates the objective for ET to send out any radio signals seems rather ... We have scanned only a minute fraction of the radio spectrum for ETI beacon ...
    (sci.astro.seti)
  • Re: Questions about future of space telescopy
    ... What is the next resolution level that can bring ... is not sufficient to shield such a telescope, ... The main reason for moving such a telescope away from Earth is ... multielement interferometers like the VLT. ...
    (sci.astro.amateur)
  • Re: Questions about future of space telescopy
    ... What is the next resolution level that can bring ... is not sufficient to shield such a telescope, ... The main reason for moving such a telescope away from Earth is ... multielement interferometers like the VLT. ...
    (sci.astro)
  • Re: Update on laser distance meter project (master thesis) and question
    ... On my own for my Master Thesis and for my former ... My Thesis used a coarsed resolution Laser rangefinder: ... (Laser Power and optical telescpoe Limited) ... telescope: Refractor (Gold plated mirrored telescope would have been ...
    (sci.electronics.design)
  • Questions about future of space telescopy
    ... Hubble's Telescope has < 0.1 arcsec resolution in visible and ... ultraviolet ranges. ... What is the next resolution level that can bring ... some active cooling can be used? ...
    (sci.astro)