Next: Estimation of the Most Up: Determination of the OFAD Previous: Details of the Photographic
Next: Estimation of the Most
Up: Determination of the OFAD
Previous: Details of the Photographic
The values for the paraxial focal lengths predicted by Zemax at all
wavelengths were taken to be the ``f'' terms in the predicted OFAD.
Values for 
and 
were then obtained by fitting the OFAD
model to the positions given by Zemax for the centroids of a number of
images distributed uniformly throughout the field. This procedure
yields models which predict Zemax's theoretical image locations with an
rms error of less than 4 
in U and 2 in all other colors.
As previously mentioned, the parameters given in Table 2 are not those of the corrector as it was designed, but come from measurements made after construction. To estimate the effect of any error in these measurements, Monte Carlo (MC) calculations were done during which the mechanical and optical parameters of the system were varied at random within the tolerances which were maintained during manufacture and final assembly. Every MC iteration creates a new, slightly different optical system which represents the way the corrector might have been put together. Each MC design was modeled in the same way as the nominal configuration, optimizing the image quality by refocussing after each iteration.
Image quality after a MC perturbation always remained within or very
near to the design specifications, i.e. with the monochromatic
not exceeding .25'' in the center and .50'' at the edge. The
image center moved by as much as several hundred microns as a result
of tilts introduced by the Monte Carlo process, but after recentering,
a new OFAD model could always be found which was able to predict the
new theoretical positions to an rms precision of 6 or less. The
system focal length varied from the nominal value by an average of
mm from after each Monte Carlo calculation. The MC
perturbations changed by an average of 30,000 units.
Increases of were seen more often than decreases. Changes in
were usually accompanied by changes of in the opposite
sense.