Dave Rowe’s FringeXP

 

Hello Again,

 

Let me start off with a rather blunt statement.  “If Dave Rowe had not written his Fringe Reduction software, I would NOT have built an interferometer!”  Bottom line, with out reduction software, Interferometry fringe images are near worthless (especially if you trying to test Parabolic mirrors at RoC with out an optical null!)  Previously, the Fringe reduction software was “OH MY” expensive, and put interferometry pretty much out of the realm of ATM’s.

 

However, Dave did author his software, and He’s going to make it available to the public!  You can download his software from Peter Ceravolo’s website here:

 

http://www.ceravolo.com/fringe/index.htm

 

In the mean time, let me review the general process of how to go from Images of interference Fringes to final results.

 

  1. First you need some Fringes (the more the better!)
  2. Once you have the Fringes, you need to Know:
    1. The accurate diameter of the optic
    2. The Accurate Radius of Curvature of the optic
    3. The desired Conic Constant of the optic
  3. One by one we load the fringe images into Dave’s software
    1. First we define the edge of the mirror in the image
    2. Next we carefully trace each fringe in order
    3. After the fringes are traced, the software mathematically reduces the images in to a set of Zernike Polynomials

                                                              i.      Zernikes are odd little creatures, each value describes an aspect of the optic

                                                            ii.      For more information on Zernike Polynomials, see Dr. Wyant’s web page here:

                                                          iii.      http://www.optics.arizona.edu/jcwyant/Zernikes/ZernikePolynomials.htm

    1. Now that we’ve turned the Fringe image into Zernikes we have two options

                                                              i.      We can turn these Zernikes directly into a surface profile

1.      While this sounds like a good idea, its not really

2.      The problem is a single fringe image has the following possible errors

1.      Aperture definition

2.      tracing the fringe centers

3.      Local “seeing” effects from air turbulences

4.      The number and orientation of the fringes may be insufficient

                                                            ii.      Alternatively, we should take SEVERAL fringe images

1.      Find their individual Zernike values

2.      Average the Zernike values into a final set

3.      use the averaged values to create our surface map

4.      See the below MS Excel file showing the deviations between individual fringes

1.      http://lerch.no-ip.com/atm/Inter/Fringes_and_Foucault/252x2994_Don_A/Fringes/Individual_V_average_results.xls

2.      My current reason for such a wide distribution of results is my LACK of a proper test tunnel.

 

I guess that’s it for the moment, I imagine once Dave gets his Web-Site up and running, he will have a substantially more accurate and complete description of his methodology.

 

In any event, I don’t claim to be an interferometry guru (yet J). And I have a LOT to learn.   So you’ll have to bear with me for a bit…..

 

Take Care,

James Lerch