Hallo Beisammen,
Dale Eason schreibt, dass FFT Auswertung dem Fringetracing ueberlegen ist, weil man einfach mehr Datenpunkte hat.
"...FFT analysis can be more accurate at the edge than Fringe
Tracing. Part of that is because we use more fringes.
You may also have seen that just changing one point on a fringe trace close to
the edge can make a large difference to the edge analysis. The FFT method does
not have that problem but one must have more fringes to use it.
Unless one can not get many fringes for what ever reason I recommend to always
prefer FFT analysis to Fringe Tracing."
Und bzgl. Festlegung des Spiegelrandes:
"...The best way for OpenFringe's FFT analysis to outline the mirror is with a
perfect circle using either the "Edge and Edge" or "Center and Edge control".
Then move,expand,or contract the whole circle as necessary."
Weitere Details sind in der "Interferometry" Yahoo Gruppe unter Suchbegriff "edge fft".
Update: Ein Kommentar von Mike Peck zu dieser Frage.
"...If you're specifically comparing FFT based analysis to fringe tracing there's no systematic difference. In both algorithmic approaches you're assigning wavefront values and a pair of coordinates to each pixel in the sample. The difference is in fringe tracing you'll typically have a few to several hundred pixels in your sample that are irregularly located over the interferogram, while an FFT analysis yields wavefront values at every pixel. Everything else equal that means random errors will be larger in a fringe tracing analysis.
As to what primary aberrations depend on what, SA is sensitive to the measured interferogram diameter, coma is sensitive to position, and astigmatism to pixel aspect ratio. Higher order versions of those aberrations may be somewhat affected as well, but the sensitivity should be much smaller..."
Clear Skies,
Gert
