Re: parabolic / sperical mirror : How to tell

Bob May wrote:

Depends upon the focal length of the mirror.  You said that the image of the
sun was about a dime in size so it musn't be that much.
A parabolic mirror iwll make a more accurately focuse imag eof the sun.  The
problem with using the sun is the reflected image will be so bright that you
can't look at it.  Better to take it out at night and set it up on a stick
so you can put an eyepiece (a 10x eyeloupe will do fine for this) and look
at a distant light or a star.  If the image iw well focused (it will
probably have a blur in one direction as you aren't on the axis of the
paraboloid) then you probably have a paraboloid.  A spherical mirror at a
short focal length will have a very soft focus with a distant object.
Another test is to take a LED and put it behind a straight edge (hopefully
of a thin metal) and put it at the radius of curvature of the mirror and see
what the shadows look like when you start blocking the light (your basic
Foucault test).  You will be wanting to hold the mirror in a stand so it
doesn't move and then have the LED/KE on another stand so things don't move
aobut a lot.  Look up the Foucault test in google to see what is happening.
The test isn't hard to do even with even primitive wood equipment for the
parts.

--
Why do penguins walk so  far to get to their nesting grounds?




What I should have said is that the light from the sun focused into a point that had the diameter of a dime. Not the image of the sun. I was convinced that a parabolic mirror would not provide such a small sharp shape for the focal point.

Now I'm reading just the opposite. It's the parabolic that provides the better focusing. And I must admit, I'm still having a problem wrapping my mind around this concept.

A perfect spherical mirror should provide a single point of focus in 3d space. What's the shape of the focus [volume] from a parabolic? It can't be a single point, so it doesn't seem to be better. But if you tell me that the shape of the focus volume is more of a flattened ellipse and that this provides for better focusing over a larger area, then maybe I'm a believer.


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Relevant Pages

  • Re: parabolic / sperical mirror : How to tell
    ... A parabolic mirror iwll make a more accurately focuse imag eof the sun. ... paraboloid) then you probably have a paraboloid. ... Look up the Foucault test in google to see what is happening. ...
    (sci.optics)
  • Re: Where is the evidence pointing to Subh-i Azals succession?
    ... a Manifestation of God. ... The Sun you see in the Mirror is because of the Sun ... << commonly understood by the Bayanis. ...
    (talk.religion.bahai)
  • Re: Boat telescope 2.0
    ... magnifications will be a low of 15 X an a high of about 30X. ... You put grit on them and rub them together. ... The action of grinding naturally produces a spherical mirror. ... The proper shape for a telescope is a paraboloid and for a very long ...
    (rec.boats)
  • Re: Boat telescope 2.0
    ... magnifications will be a low of 15 X an a high of about 30X. ... You put grit on them and rub them together. ... The action of grinding naturally produces a spherical mirror. ... The proper shape for a telescope is a paraboloid and for a very long ...
    (rec.boats)
  • Re: How Archimedes Solar Death Ray Could Have Worked
    ... I think the large parabola would focus the light from the Sun to a ... Sun and wouldn't always be at the focus of the main mirror. ... Aiming can then be done by moving the secondary mirror, ...
    (rec.arts.sf.science)