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.