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#An early telescope objective, such as those built by Hevelius and C. Huyghens , that utilized single small (2"-3") positive lenses with enormous focal lengths and tubes (up to 100 feet or more in length) that masked the uncorrected chromatic aberration within the large Diffraction pattern at focus. #An objective lens which is well-corrected for Spherical Aberration and off-axis Aberration s such as Coma and Astigmatism over the desired Field Of View at only one wavelength. Monochromatically corrected objectives are used for specific narrow-band applications such as solar observing at the Hydrogen Alpha spectral line of 0.6562725 micrometres. Other applications include monochromatic Laser applications such as Collimator s, Beam Expander s, and highly corrected Pupil Imaging for wavefront error sensors for Adaptive Optics . A common beginner mistake in designing monochromatic objectives to be used with near-zero field of view, such as collimators, is to design only with zero field of view. The Lens Design Program optimizer will only correct what it knows about any given lens. Such a lens will likely have sharp on-axis performance, but may have enormous coma, rendering it virtually un-manufacturable and unusable in practice. A zero-field lens should always be designed with a nonzero field of view so that its manufacturing Tolerance s and quality control are practical, and its alignment tolerance in the final system is reasonable. |
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