11/3/2023 0 Comments Parabolic mirror priceThe Strehl value can vary from 0 – 1, where one is a perfect mirror without aberration. Strehl value of the coefficient varies from zero to unity as a unit is the amount of perfect mirror without aberration. This is the ratio between the light intensity in the central part of the disc of Airy, of a mirror with a particular aberration to the light intensity in a perfect mirror without aberration. Perhaps the best way to measure the error of each optical surface is through the Strehl ratio. Despite the obvious advantage of this indicator, however, there is no accepted standard for the value of the “mean-square” error. It is assumed that a mirror with a smooth surface, which was a difference in its wave front correction of 1/4 lambda, correspond to 1/14 RMS. This can only be done with an interferometer, which can examine wave fronts over the entire surface of the mirror. It is important to note that in order to be applied correctly, this method should take measurements from a large number of uniform mirror segments. The same method is used in statistical analysis of random variables, known as the “root mean square deviation”. This is done by making measurements over the entire surface of the mirror. The purpose of this indicator is to better describe the entire surface of the mirror taking into account the relative size of the defects. From this, it is clear that the PV error measurement is not enough to give us the full picture about the quality of a mirror, since it does not take into consideration the number of errors over the entire surface. Both meet the quality assurance requirement since they have the same PV indicator. However, one of the mirror only has one such error on its surface while the other has several. Now imagine that you have two mirrors, both with an error 1/4 lambda in wave front (a 1/8 lambda difference in the surface). Thus, the mirror meets the requirements for being of good quality. If a mirror is one with an error PV 1/4 lambda in wave front, it means that the difference between the highest and lowest point on the surface of the mirror is 1/8 lambda. So knowing this how much would be a permissible error and how can we tell if a mirror is of high quality? The best we can expect is that 84% of the light will fall in the central part of the disc and the remaining 16% will dissipate in the diffraction circles. ![]() This image is known as a “Airy disk” after astronomer George Airy. As a result, a star will appear as a bright central disk, surrounded by faint diffraction rings. Due to the wave nature of light, each image formed by an optical system suffers from diffraction. If it is possible to produce a mirror with zero spherical aberration, it is contrary to expectations and light rays would not be precisely focused at one point. Not all telescopes use parabolic mirrors, but whatever surface the mirror of the telescope is, it must be designed so as to eliminate spherical aberration or in the best case, to reduce it to acceptable limits. That is why parabolic mirrors are widely used in certain types of telescopes, for example “Newton” telescopes. ![]() If the spherical surface however, is parabolic, then all light rays which reach the mirror will focus in one common focus and produce a quality image. This phenomenon is known as “spherical aberration”. This unusual behavior is due to the fact that the light rays which reflect from the end zones of the spherical mirror are focused closer to the center, when compared to those which reflect off the center zones. Unlike radial rays, these parallel rays of light can not be focused by the spherical surface in one common focus. As cosmic objects are at very large distances, the light rays coming from them reaches Earth parallel to each other, rather than radial, as it happens with earthly objects.
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