07-12-2015, 09:17 PM
As a history lesson you can look up elsewhere for more detail, telephoto started off as a lens configuration which gave a physical size less than that of the focal length. This gives a size benefit. In modern common usage it generally refers to any lens of longer focal lengths, even if the size criteria might not be met.
Telescopes come in several classes.
1, Refractors - These use refracting optical elements just like most camera lenses. Cheaper telescopes may be as simple as 2 elements (not including eyepiece) to provide a degree of colour correction. And just like lenses, the more elements you use, the better the correction can be, if the design and construction are up to it. Astronomical ones can go as fast as about f/4 ball park, although most are slower.
2, Reflectors - these use mirrors which have a benefit of not introducing any colour problems. In typical designs there will be some obstruction in the light path from needing a secondary mirror, which can reduce contrast a bit. For bigger aperture sizes, this is the cheapest way to "go large". Again, astronomical ones can go as fast as about f/4 ball park, although most are slower.
3, catadioptric - these use a combination of mirrors and refracting elements. The more affordable astronomical scopes in this class tend to be Maksutov–Cassegrains for smaller apertures, and Schmidt–Cassegrain for bigger ones, but there are a lot of other types too. These two have a common characteristic of having relatively long focal lengths in a short body, but slower focal ratios. For an example of each, I have a Celestron NexStar 4SE (4" aperture) which is 1325mm f/13, and I have a Meade SCT 200mm aperture, or 2000mm f/10. But again because of the central obstruction, the contrast isn't as good as a refractor.
Putting aside the construction differences, if you can work out the focal length and focal ratio, it will do pretty much the same as a telephoto lens of the same specification. There are some "mirror lenses" in the photography world too, of which only Sony made one with AF. These also show another feature of reflectors and catadioptric systems, which is the "donut bokeh" caused from the centre obstruction.
Telescopes come in several classes.
1, Refractors - These use refracting optical elements just like most camera lenses. Cheaper telescopes may be as simple as 2 elements (not including eyepiece) to provide a degree of colour correction. And just like lenses, the more elements you use, the better the correction can be, if the design and construction are up to it. Astronomical ones can go as fast as about f/4 ball park, although most are slower.
2, Reflectors - these use mirrors which have a benefit of not introducing any colour problems. In typical designs there will be some obstruction in the light path from needing a secondary mirror, which can reduce contrast a bit. For bigger aperture sizes, this is the cheapest way to "go large". Again, astronomical ones can go as fast as about f/4 ball park, although most are slower.
3, catadioptric - these use a combination of mirrors and refracting elements. The more affordable astronomical scopes in this class tend to be Maksutov–Cassegrains for smaller apertures, and Schmidt–Cassegrain for bigger ones, but there are a lot of other types too. These two have a common characteristic of having relatively long focal lengths in a short body, but slower focal ratios. For an example of each, I have a Celestron NexStar 4SE (4" aperture) which is 1325mm f/13, and I have a Meade SCT 200mm aperture, or 2000mm f/10. But again because of the central obstruction, the contrast isn't as good as a refractor.
Putting aside the construction differences, if you can work out the focal length and focal ratio, it will do pretty much the same as a telephoto lens of the same specification. There are some "mirror lenses" in the photography world too, of which only Sony made one with AF. These also show another feature of reflectors and catadioptric systems, which is the "donut bokeh" caused from the centre obstruction.
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