Quote:
Originally Posted by alistairsam
regarding what you mentioned above, i have read that scopes with higher F rations (slower) are suited for planetary imaging, and scopes with lower F ratios (faster) for DSO's. is this due to field of view? have searched and read a bit on faster and slower optics but did'nt really understand how they relate to DSO's and planets.
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Indeed, it is a result of the FOV.
As you would have read, the focal ratio is the focal length divided by the aperture.
Longer focal length instruments have smaller FOV's. So a scope with a long focal
length and large (slow) f/ratio works fine for photographing bright objects such as
the Moon and Jupiter as exposure times can be kept short. For extended objects,
the better choice is a scope with a smaller f/ratio (faster) as the imaging time
for these types of objects becomes smaller.
Fred Watson gives an interesting historical treatment of this topic in his book, Stargazer.
George Willis Ritchey was head of instrument construction at the Yerkes
Observatory in Chicago in 1901. Interested in astrophography, particularly that
of nebulae,
Quote:
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Originally Posted by Fred Watson, Stargazer, page 252
"Richey was aware of a curious and recently discovered (1882)
attribute of photographic image detection - namely that the speed with which
an extended object like a nebula or a comet can be recorded depends not on
the aperture of the telescope but only on is focal ratio, or f/number."
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Fred goes on to write -
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"It seems almost impossible to believe that the sensitivity could be independent of
aperture, but it is true - the lower the focal ratio (and therefore the stubbier
the telescope), the greater the speed with which the image of the nebula will
be detected. That is why low focal-ratio lenses or mirrors are often called 'fast'.
There are still very good reasons for making them as large as possible, however,
for their diameter determines both the amount of detail that is recorded and their
sensitivity to point like sources such as stars."
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