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02-22-2009, 03:16 PM   #16
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QuoteOriginally posted by Ben_Edict Quote
The bigger scope has a slower (much slower) f-number. So, despite having obviously the larger diameter, it will show much less of a faint astronomical nebulae, which the smaller 1:4 instrument makes easily visible. [/INDENT]
The situation is different for point sources (= stars), where the pure diameter, independent of the f-number plays its strength. Here the 30cm scope will show much fainter stars (and accordingly much more stars), than the 20cm scope, because the focal length needs not to be taken into account with these point sources.

But the faint nebulae in the f4 instrument would be tiny, despite it being brighter.

I'm sure you delibrately left the eyepiece out of the equation as it is a moot point in imaging.

Once setup to give the same magnification at the eyepiece, the 30cm 'slow' scope will still resolve more detail and yield a brighter nebulae than the 20cm 'fast' scope.

02-23-2009, 02:32 AM   #17
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QuoteOriginally posted by boom Quote
But the faint nebulae in the f4 instrument would be tiny, despite it being brighter.

I'm sure you delibrately left the eyepiece out of the equation as it is a moot point in imaging.

Once setup to give the same magnification at the eyepiece, the 30cm 'slow' scope will still resolve more detail and yield a brighter nebulae than the 20cm 'fast' scope.
You are right - but you won't get an extended nebula in the field of view with the bigger, slower scope. This is the reason, why many astronomers use different scopes for different purposes or why some sellers advertise certain scopes as planetary scopes and others as deep sky scopes. I myself have five different telescopes with different primary focal lengthes and diameters, but am limited for deep sky observing by my bright, suburban nightsky.

The eyepiece is indeed an important part of the whole optical train of a telescope. It is one of the importantn differentation factors between a telescope and a photographic lens, as you wrote.

regards
Ben
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