Spherical Aberration

If a CAT's corrector plate is not doing its job, not completely eliminating the primary mirror's spherical aberration, the system is said to be undercorrected. The effect of this aberration is to make the diffraction pattern look brighter on one side of focus than the other. More light is being thrown into the rings on one side of focus. The centers of the bull's-eyes on either side of focus are different, with the intrafo-cal image showing a little disk or fuzz ball in its middle. The extrafocal image shows a darker spot dead center. What is the effect of spherical aberration on in-focus images? Looking at the image of the star in Figure 4b, it does not seem too bad. The diffraction ring around the star is a bit brighter, but that appears to be all. The Jupiter image in Figure 4c tells the tale. At one-third wave of spherical aberration, much detail has been lost.

Telescope optics can also be overcorrected, and that is just as bad as undercorrec-tion. Figure 4a shows that going from under- to overcorrection swaps the way the intra- and extrafocal images look. Otherwise, the bull's-eyes are identical in appearance to the undercorrected patterns. The result of overcorrection in focus is still as much a disaster as undercorrection.

Perfect a: Inside/outside Focus

b

Undercorrection a: Inside/outside focus

b

a: Inside/outside Focus

Worn

b

a: Inside/outside focus

io

b

c

Figure 4. (Star Test 1) Star test diffraction patterns page one. Credit: Author.

Figure 4. (Star Test 1) Star test diffraction patterns page one. Credit: Author.

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