| The atmospheric dispersion corrector |
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When objects outside the Earth's atmosphere are observed, a phenomenon called atmospheric dispersion is distorti the image. This effect, especially visible when the object of choice is situated near the horizon, manifests itself as a red rim' under and a blue rim above an object. It is particularly interfering with high resolution observations of the moon and planets, because it decreases the resolution of such objects and distorts fine detail. An atmospheric dispersion corrector, as described below, corrects for this interference Theory When light of any object (a star or planet) passes through the atmosphere of the Earth, it will be refracted because the refractive index of the air is not equal to the vacuum of space. The difference in refractivity is only small, but nevertheless it causes the light to pass through the atmosphere at a small angle rather than a straight line. The object of interest therefore appears at a slightly different position than it would appear without an interfering atmosphere. Refraction is strongest near the horizon, because the light has to pass a longer path through the atmosphere. There is no or almost no refraction the Zenit. The refractive index of air is different for different wavelengths of light. Shorter wavelengths are more strongly refracted than longer wavelengths. Since red light has a longer wavelength than blue light, it follows that red light is not refracted in exactly the same way as blue light, causing the two to diverge. Therefore, the red image of an object is not observed at the exact same position as the blue image. This effect makes the colours of the object to be dispersed somewhat, with red and blue displaced with respect to each other. This is called atmospheric dispersion. It is more strongly observed near the horizon. Figure 2 illustrates this with an image of the star Sirius near the horizon. This image shows the light of star to be dispersed like a miniature spectrum. Red light is less refracted than blue light, causing the blue image of the star to move away from the horizon when compared to the red image. |
Correction for the dispersion Most medium to large sized amateur telescopes can reach a resolution of up to 0.2 to 0.4 and the effect is a real problem when the dispersion of the different colours is larger than the resolution of the telescope. In practice, objects that are located below 60º altitude suffer from noticeable dispersion. Since planetary or lunar objects rarely reach this altitude (at least in countries situated at higher latitudes), compensation is needed to avoid loss of resolution. When observing objects near the horizon, dispersion can increase to values of 1 or 2, making high resolution imaging almost impossible. The problem can be controlled by limiting the wavelength range with the use of filters. In general, a RGB filter set can be use to restrict the wavelength range and therefore to limit the dispersion in each of the three colour channels. It is also possible to use filters that transmit in the near IR, where dispersion effects are far less pronounced. Restricting the wavelength area using filters therefore offers a good way of dealing with dispersion. As long as objects aren't observed at extremely low altitudes, the effects of dispersion can be suppressed. Since RGB filters are already used to make colour images, they simply offer this extra advantage. Yet there are disadvantages as well. Filters limit the amount of light that reaches the eye or CCD detector. In the case of RGB filters, the signal strength of an object is cut by a factor of three. Imaging the fainter planets like Saturn and Jupiter is therefore more difficult in comparison with bright objects like the moon or Venus. A lower surface brightness means longer exposures for a CCD camera, and less compensation for seeing effects. Using filters restricts the amount of light even further, requiring even longer exposures. A second disadvantage, although less pressing, is that filters only restrict the dispersion, but do not compensate for it. Especially the blue to violet part of the spectrum is more sensitive to dispersion effects, and at low altitude, the dispersion between blue and violet wavelengths may be a factor even when a blue filter is used. UV imaging of Venus may be even more sensitive, although to what extent this is problematic (UV imaging usually yields less resolution due to seeing effects) remains to be seen. The dispersion corrector Professional astronomers compensate for seeing effects with so-called dispersion correctors. Now, for the amateur, a simple corrector exist as well The dispersion corrector is a relatively simple instrument that is designed to correct for the effects of the atmospheric dispersion. The optics cause a controllable refraction that is opposite in direction to the atmospheric dispersion. It is therefore capable of compensating for the effect rather than restricting it, which makes it possible to view or record an object without being hindered by dispersion effects. The corrector consists of 2 round prisms that are placed with the plane sides facing each other. In resting position, both prisms are placed in such a way that each prism forms the half of a window with parallel facing surfaces |
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The image of Sirius illustrates the problems that occur when high resolution imaging is attempted without compensating for the dispersion. The dispersion difference of red, green and blue light causes multiple images of an object that are displaced with respect to each other. This causes a serious decrease in resolution and loss of fine detail. |
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