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 IceInSpace Does anyone know the definition of "wavefront"
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#1
17-04-2011, 07:18 AM
 gregbradley Registered User Join Date: Feb 2006 Location: Sydney Posts: 7,795
Does anyone know the definition of "wavefront"

I tried looking up the exact definition for wavefront on google and got a bunch of definitions that use the word but not define it.

Is it the shape of a mirror or optical surface or is it the waves of errors like ripples on a pond on a surface? Wavefront errors being the deviation away from a perfectly smooth curve to the tops of these ripples?

Greg.
#2
17-04-2011, 07:49 AM
 dannat (Daniel) daniel Join Date: Sep 2007 Location: TYLDEN, Australia Posts: 2,526
Greg I understood it o be the lines which make up what we represent to look like a single wave, the wavefront is the collection of the waves. A mirror or optical surface Changes the shape of the wavefront (& errors cab be intoduced here
#3
17-04-2011, 07:52 AM
 CraigS Unpredictable Join Date: Jul 2010 Location: Australia Posts: 3,022
Hi Greg;

In this case, a wavefront can be thought of as a line which connects points possessing equal phases, (across multiple parallel propagating waves).

Wiki definition here.

The ideal mirror would produce equidistant phase alignments (ie parallel wavefronts), from an initially coherent light source.

Errors in the surface, would produce misalignment of these 'wavefronts' such that they would no longer be parallel (when viewed over time). If a perfectly parallel set of lines could be drawn representing an ideal set of 'wavefronts', you would find that the phases of the light intersecting these lines, (from an errored mirror surface) would not be the same. If different phases come together at any point, constructive or destructive interference results, varying the intensity and amplitude, which gives rise to fringe patterns.

Wavefront errors could be thought of as deviations in phase angle, of the light about a common (mean) value.

Hope this helps.

Cheers & Rgds

Last edited by CraigS; 17-04-2011 at 08:19 AM.
#4
17-04-2011, 08:39 AM
 PeterM towards 2000 posts-slowly Join Date: Jun 2006 Location: Loganholme, Qld Posts: 1,073
Galaxy Optics site below may be of help also.
http://www.galaxyoptics.com/primarymirrors.html

PeterM.
#5
21-04-2011, 12:21 AM
 Danack (Dan Ackroyd) Registered User Join Date: Mar 2011 Posts: 125
Quote:
 Originally Posted by gregbradley I tried looking up the exact definition for wavefront on google and got a bunch of definitions that use the word but not define it.
When talking about light how you want to think about it really depends on what you're trying to do.

You're talking about 'wavefront errors' - which is a slightly unusual phrase - in what context are you talking about?

Quote:
 is it the waves of errors like ripples on a pond on a surface? Wavefront errors being the deviation away from a perfectly smooth curve to the tops of these ripples?
Light can be thought of as a wave - but no 'wavefront errors' wouldn't be any distortion in these waves - the only thing that I would imagine from 'wavefront errors' is the wavefronts getting out of phase e.g. Normally people only care about 'wavefront errors' if you've got a coherent light source and are trying to do something like a http://en.wikipedia.org/wiki/Double-slit_experiment

Hmm.

I just googled for wavefront errors and the first result is: http://www.telescope-optics.net/aberrations.htm

It's late and it's been a few years since I did physics in school but this terminology seems odd. I'm pretty sure that type of 'wavefront error' is more commonly known as http://en.wikipedia.org/wiki/Circle_of_confusion
but that's just a guess as to the context of what you're trying to understand.
#6
21-04-2011, 07:06 AM
 PeterM towards 2000 posts-slowly Join Date: Jun 2006 Location: Loganholme, Qld Posts: 1,073
I think that Galaxy Optics (a highly respected US mirror maker) definition of RMS wavefront error gives more than enough explanation that should satisfy most amateur astronomers and what they are hoping to achieve.

Quote from the Galaxy Optics website-
"RMS Wavefront Error:

RMS wavefront error is a statistical measure of how much a mirror's wavefront deviates from the ideal theoretical wavefront. The RMS wavefront error is calculated from all of the measured interferometric data points and is the best indication of a mirror's overall performance. To obtain the RMS value the fringe analysis software measures all of the data points to determine the error between the point positions on a theoretically perfect wavefront and their actual positions on the wavefront under test. The deviation between points is squared then averaged and the square root is extracted. The literature states and physical tests prove that an optic with a RMS wavefront value of 0.076 or less is diffraction limited."

PeterM.
#7
21-04-2011, 08:41 AM
 Satchmo (Mark) Registered User Join Date: Dec 2005 Location: Sydney Posts: 1,505
I'm always amazed that the Marechal's limit of 0.07 RMS ( equivalent to Lord Rayleigh's 1/4 wave P-V for primary spherical aberration) has been claimed by the commercial industry to mean `diffraction limited' which implies a standard at which not much improvement can be gained and suitable limit for commercial mirrors. These limits produce a Strehl ratio of around 0,8. As Lord Rayleigh found , at 1/4 wave or (0.07 RMS) primary spherical the effect on planetary detail and contrast was already `decidedly prejudicial'

I believe that Diffraction Limited means a wavefront that is only perturbed by the laws of diffraction that cannot be controlled ie the destructive effects of the circular aperture and the central obstruction. A wavefront that genuinely puts 96% or more of its available light into the Airy Disc will have a contrast transfer function that is nearly perfect and for me satisfies this definition.

Unfortunately while Strehl ratio's in the mid '90's are common place on documentation these days very few optics truly meet this criteria due to creative computing on the part of the manufacturers.
#8
22-04-2011, 01:10 PM
 TrevorW Zugzwang Join Date: Aug 2006 Location: Western Australia Posts: 6,866
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