6.5. MTF 2

FIGURE 46 (continued):

(e) Turned down edge effect on the PSF and MTF. The P-V errors for 95% zone are 2.5 and 5 waves as needed for the initial 0.80 Strehl (the RMS is similarly out of proportion). Lost energy is more evenly spread out, and the central disc becomes enlarged. Odd but expected TE property - due to the relatively small area of the wavefront affected - is that further increase beyond 0.80 Strehl error level does almost no additional damage.

(f) The effect of ~1/14 and ~1/7 wave RMS  wavefront error of roughness, resulting in the peak intensity and contrast drop similar to those with other aberrations. Due to the random nature of the aberration, its nominal P-V wavefront error can vary significantly for a given RMS error and image quality level. Shown is the medium-scale roughness ("primary ripple" or "dog biscuit", in amateur mirror makers' jargon) effect.

 (g) 0.37 and 0.74 wave P-V of wavefront error caused by pinching having the typical
3-sided symmetry (trefoil). The aberration is radially asymmetric, with the degree of pattern deformation varying between the maxima (red MTF line, for the pupil angle θ=0, 2π/3, 4π/3), and minima (green line, for θ=π/3, π, 5π/3); (the blue line is for a perfect aperture). Other forms do occur, with or without some form of symmetry.


(h) 0.7 and 1.4 wave P-V wavefront error caused by tube currents starting at the upper 30% of the tube radius. The energy spreads mainly in the orientation of wavefront deformation (red PSF line, to the left). Similarly to the TE,
further increase in the nominal error beyond a certain level has relatively small effect  Contrast and resolution for the orthogonal to it pattern orientation are as good as perfect (green MTF line).
 

(i) Near-average PSF/MTF effect of ~1/14 and ~1/7 wave RMS wavefront error of atmospheric turbulence. The atmosphere caused error fluctuates constantly, and so do image contrast and resolution level. Larger seeing errors (1/7 wave RMS is rather common with medium-to-large apertures) result in a drop of contrast in the mid- and high-frequency range to near-zero level.