Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: GlentoranMark on 31/10/2009 15:02:42
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Hi all, I'm a seasoned astronomer and still don't really know what the above term means. Can anyone enlighten me? Has it anything to do with photography?
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It almost certainly refers to "lens speed," which is a really confusing term. Very generally it's a measurement of how much light the lens can put at the focus. In photography, the more light you get the faster your image forms, and the faster your shutter speed can be, hence the term "fast." More technically it's related to the ratio of the diameter of the lens to the focal distance.
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Many years ago, when I did a lot of photography, a 'fast' lens was exactly what JP described: a lens that could pass a lot of light. The amount of light passed by a lens depends upon the area of its aperture, so if a lens has a large aperture, equating to a large light collecting area, it can let in more light compared to a lens of the same focal length but with a smaller aperture.
The size of the aperture is denoted by the term f-stop and as JP says, it is proportional to the focal length of the lens. A 50mm focal length lens with an aperture diameter of 25mm would be described as an f2 lens. If you then 'stop' the lens down to f4 you've reduced the aperture diameter to 12.5mm.
However, cost and manufacturing difficulties play a practical role in the definition of a fast lens. It is relatively easy to make a 50mm f2 lens, needing just a 25mm aperture, but an f2 400mm lens would require an aperture diameter of 200mm, so not only do the primary lens elements need to be much larger, incurring greater cost and making them unwieldy, they have to ground to the same precision. In practice then, an f4 400mm lens would be considered very fast indeed, but a 50mm lens really needs to be f1.1-f1.2 to really be considered fast.
With auto-focusing optics though, fast might also refer to the speed of focus.
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There are several alternative possibilities GlentoranMark could you give us an example of the context in which you herd the term. It could be referring to speeds of adaptive optics
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This months Sky at Night magazine.
Quote: "Already up and running, the Californian Institute of Technology's PTF uses the 48 inch Samuel Oschin Telescope at Palomar Observatory, California - the fast Schmidt instrument that made a photographic survey of the sky in the 1950's."
I've always felt it was something to do with focal ratio, am I barking up the wrong tree?
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That quote referred to focal ratio. This instrument produced a "whole sky" photographic star atlas down to about the 21st magnitude. PTF refers I expect to the Palomar Transient Factory. A set of telescopes that are programmed to survey the sky automatically and regularly to look for small changes and raise alarms so that they can be observed. This is one of the ways in which electronic detection plus signal and image processing have completely revolutionised the science of Astronomy in the last few years.
Looking at the full paper http://xxx.lanl.gov/PS_cache/arxiv/pdf/0906/0906.5350v1.pdf shows that the 48inch aperture (72 inch primary mirror) operates at about F2.5 (faster than most small camera lenses these days) with a coverage of 8 square degrees (36 times the area of the full moon)with a greater than 100 mega pixel array that can resolve to better than one second of arc down to better than 20th magnitude using a 60 second exposure. This is an almost unbelievable data rate at this sensitivity.
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Thanks although you've answered the question with fast in your quote which doesn't really tell me what I want.
Does fast mean more light gathering abilities, wider field of view or a combination of both?
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Fast usually refers to light gathering properties but the very large field of view of this telescope is also important in this application most telescopes only manage considerably less than one degree
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Aha! If you're talking about 'fast' in the context of Schmidt cameras/telescopes have a look at:
http://en.wikipedia.org/wiki/Schmidt_Camera (http://en.wikipedia.org/wiki/Schmidt_Camera)
Schmidt cameras and telescopes provide a wide field of view, and a 'fast' Schmidt camera or telescope would be one with a large aperture. So 'fast' in the context of the original question does indeed refer to the light collecting ability.