Well now as I try to implement this I come upon the problems that probably caused me to resist it in the first place.
It gets very complicated. If anyone is interested here are the issues.
If OpenFringe only delt with zernike based wavefronts it would be easy. Just remove the Bath astig part from the calculated Z4.
Surfaces can be derived from two methods, FFT and Fringe Tracing.
It has to support FFT analysis that are not based on Zernike values but produces a set of Zernike values that the zernike based routines think came from fringe analysis. That way the user can display the surface data either as a FFT wavefront or Zernike Based wavefront.
Here is the problem. The FFT produced wavefront needs to have the Bath astig removed from it and that is not hard. However the zernike values it then passes on to the Zernike based routines will now not have that Bath astig in it. Those routines do not know that and will subtract the Bath astig once more. Thus adding a negative version of the Bath astig into the Zernike based surface.
For various reasons there a many places in the code that convert wavefronts into zernike values. The code becomes way too complex to try to untangle it all.
The other way out is to subtract it only from the FFT based wavefronts. That means the people who do fringe tracing will never get that option.
The Bath astig only becomes objectionable for small fast mirrors. Most ATM's are going bigger and not smaller. It is an obscure option for the most part and I'm not sure that it is worth the trouble. Those that want it try and convince me.
--- In email@example.com
, Michael Peck <mpeck1@...> wrote:
> At 10:36 PM 12/7/2010, atmpob wrote:
> >I will add the ability to remove the Bath astig. What I need to know
> >is how to use the value that Dave Rowe calculated in the Wiki for Z4.
> >In it he states to be careful and use the correct sign. Should it
> >be positive.
> >Second what is the scaling on his value. Should I use it as is or
> >multiply it by the sqrt of 6.
> Just in case Steve didn't answer privately, if you're looking at the
> section titled "Aberrations in the Bath Interferometer" it's the
> value given in equation 9 for the Zernike polynomial scaling you use
> (the coefficient is half the OPD for astigmatism). The sign should be positive.
> Assuming the X axis of the image is parallel to the axis defined by
> the separation of the beams it's Z4 that is affected.
> Mike P.
> Michael Peck