![]() Most, but not all, other telescope types will have a central obstruction, sometimes supported by a ‘spider’ which gives rise to additional diffraction effects so, again, a refractor can give the highest quality images. If the aperture is unobstructed, as with a refractor, 84% of the light falls within the central disk and thus 16% lies in the rings − with the majority within the first ring whose diameter is about twice that of the central disk. The result is that a point source of light such as a star gives rise to a disc of light (whose size is determined by the aperture of the telescope) surrounded by concentric rings forming the Airy Pattern. In the case of a refractor the only diffraction effect is that caused by the fact that the telescope will have an aperture of a given size. In this case, light is only moved from its rightful place by angular distances measured in arc seconds or arc minutes and is thus particularly important in the case of observing planetary disks where the angular scale of the observed object is similar in scale and the features on the surface may have low contrast as well. The micro contrast, on the other hand, is determined by the effects of light that is diffracted by parts of the optical tube assembly. The micro contrast of a refractor’s imageĪs described above, the overall contrast of a refractor will be reduced by scattered light either from parts of the optical tube assembly or within the objective lens. My Cff Telescopes 127mm, f/7, refractor employs both as do other high quality telescope tube assemblies. Matt-black flock coatings can also be used to reduce any scattering. This is also why refractors can provide such high contrast images as a series of knife edge baffles reducing in size can be located within the optical tube to trap any scattered light. The use of my Takahashi FS102, f8, fluorite refractor enabled me to produce a very nice image of ‘Earthshine’ as the light from the illuminated part of the lunar disk did not ‘wash’ over the, far fainter, dark side of the Moon.Ī second reason for reducing the overall contrast is when light scatters off the interior of the optical tube assembly. If, as in the case of Fluorite doublets, one element is made of fluorite crystal – which essentially scatters no light – this is reduced to ~1% and a well baffled (see below) fluorite refractor can provide the highest overall contrast of any telescope. However, after dark, there may well be light pollution spreading light into the image and, even with no light pollution, there will still be some ‘air glow’ reducing the contrast a little.Ī low contrast (left) and high contrast (right) image of the Moon.Ī major reason why refractors give images with higher overall contrast than reflectors is that objective lenses may only scatter ~2% of the light passing through them. In this case, the reason was due to the fact that it was taken in twilight and so sky light was falling uniformly across the image. The lunar image at the left of the figure is obviously of low contrast – the blacks are grey, not black. So what is meant by the overall contrast of an image? In an ideal world, the light that is recorded by a CCD camera of a particular feature in an image would only have come from the light emitted or reflected by that feature alone. We will see that on both counts refractors are theoretically the best telescope type. I am not aware of any other author taking this approach but I honestly believe that this is by far the best way of considering this very important aspect of telescope design. ![]() The approach that I believe gives the best understanding of the subject splits the discussion into two parts: firstly that of the overall contrast of the image and secondly what I term the ‘micro-contrast’ of an image. The following two sections will, I hope, allow you to understand the various elements that come into play to determine what is termed the contrast of an astronomical image. ![]() Image contrast is perhaps one of the key properties of a telescope and a subject that is not too well understood, with erroneous statements often appearing in the astronomical press. ![]() In this essay I will try to explain why this is so. ![]() In the previous essay, I mentioned that refractors have the highest contrast of any telescope type – which makes them well suited to observing the Moon and planets. This is one of over 100 illustrated articles in the Author’s Astronomy Digest including two others about refractors. ![]()
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