A-Large, very clear lenses are harder to cast than more tolerant mirror blanks.
B-Large lenses deform under their own weight, but mirrors can be supported.
C-Large mirrors need only one optical surface, achromats four surfaces to grind.
D-Reflectors do not suffer from chromatic aberration like refractors do.
E-All of the above are correct
B-Large lenses deform under their own weight, but mirrors can be supported.
C-Large mirrors need only one optical surface, achromats four surfaces to grind.
D-Reflectors do not suffer from chromatic aberration like refractors do.
E-All of the above are correct
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All of the above! Grinding a lens and getting it to hold its shape is a much fussier process than making a mirror, and they suffer chromatic aberration to boot.
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C-Large mirrors need only one optical surface, achromats four surfaces to grind.
A, B. and C are all true. Perhaps you are not old enough to remember, but when it was discovered that the Hubble Space Telescope MIRROR had significant spherical aberration and could not focus, not only did the people at the University of Arizona who had spent THREE YEARS re-casting and grinding the mirror several times were disgusted, a LOT of scientists were VERY upset as well. Large MIRRORS are not easy to configure.
"...Flawed mirror Within weeks of the launch of the telescope, the returned images showed there was a serious problem with the optical system. Although the first images appeared to be sharper than those of ground-based telescopes, Hubble failed to achieve a final sharp focus and the best image quality obtained was drastically lower than expected. Images of point sources spread out over a radius of more than one arcsecond, instead of having a point spread function (PSF) concentrated within a circle 0.1 arcsec in diameter as had been specified in the design criteria.[48] The detailed performance is shown in graphs from STScI illustrating the mis-figured PSFs compared to post-correction and ground-based PSFs.[49]
Analysis of the flawed images showed that the cause of the problem was that the primary mirror had been ground to the wrong shape. Although it was probably the most precisely figured mirror ever made, with variations from the prescribed curve of only 10 nanometers,[21] it was too flat at the edges by about 2200 nanometers (2.2 micrometres).[50] This difference was catastrophic, introducing severe spherical aberration, a flaw in which light reflecting off the edge of a mirror focuses on a different point from the light reflecting off its center.[51]
A, B. and C are all true. Perhaps you are not old enough to remember, but when it was discovered that the Hubble Space Telescope MIRROR had significant spherical aberration and could not focus, not only did the people at the University of Arizona who had spent THREE YEARS re-casting and grinding the mirror several times were disgusted, a LOT of scientists were VERY upset as well. Large MIRRORS are not easy to configure.
"...Flawed mirror Within weeks of the launch of the telescope, the returned images showed there was a serious problem with the optical system. Although the first images appeared to be sharper than those of ground-based telescopes, Hubble failed to achieve a final sharp focus and the best image quality obtained was drastically lower than expected. Images of point sources spread out over a radius of more than one arcsecond, instead of having a point spread function (PSF) concentrated within a circle 0.1 arcsec in diameter as had been specified in the design criteria.[48] The detailed performance is shown in graphs from STScI illustrating the mis-figured PSFs compared to post-correction and ground-based PSFs.[49]
Analysis of the flawed images showed that the cause of the problem was that the primary mirror had been ground to the wrong shape. Although it was probably the most precisely figured mirror ever made, with variations from the prescribed curve of only 10 nanometers,[21] it was too flat at the edges by about 2200 nanometers (2.2 micrometres).[50] This difference was catastrophic, introducing severe spherical aberration, a flaw in which light reflecting off the edge of a mirror focuses on a different point from the light reflecting off its center.[51]
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