From: "John Sherman" 
To: 
Reply-To: "John Sherman" 





Hi List,

> >>"What are the optical advantages of short focal length.
>
>
> >I have never heard of one.
>
> How about wider field of view with practical eyepieces.


Is a wider field of view an optical advantage, or a personal one? The
original question was about optical advantages.  Yes, there are practical
advantages, like you say, and like getting the thing in the trunk of the
car....

You can get a wide field of view with an f/4. But you have coma out the
ying-yang  (and it is harder to figure well, and the collimation is very
critical, and the depth of focus is almost non-existant, etc).  So you can
make a f/6 and have a bit less true field, but you can end up with a wider
amount of field where the stars are actually sharp (according to an article
I once read, which I will look up later if request is made). So why bother
with a very wide field that requires a correcting lens to look at?


>I don't know whether you would consider this an advantage,
>but short FL scopes give brighter (but smaller) images.

Nope, not brighter. Only smaller. If you're doing photography, there is
more density per pixel, which fools you into thinking the image is
brighter.



John

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