Re: Shor Focal Length AND Aperture?

From: Jon Isaacs (jonisaacs_at_aol.com)
Date: 10/10/04 

Date: 10 Oct 2004 11:43:30 GMT

>Is there a scope that combines the wide-filed views of a short tube
>(400mm or so) and light-gathering ability (8" or so). Or is the best
>of both worlds too much to ask for?
>
>Thanks

The limitation here is on the human eye, it only opens so far. In young people
this seems to be a maximum of about 7mm and reduces gradually as you grow
older.

So what this means is that there is a practical limitation on the lowest
magnification for any "effective" aperture.

The "exit" pupil of the telescope/eyepiece is calculated by dividing the
aperture of the telescope by the magnification. A 200mm (8 inch) scope at 20X
will produce an exit pupil, the beam of light leaving the eyepiece, that is
200mm/20=10mm in diameter.

Since you eye is only open 7mm maximum, all the light from the telescope does
not enter your eye and a significant amount is unused.

For an 8 inch telescope, the minumum magnification that will allow you to use
all the light is 200mm/7mm=29X.

As far as the 8 inch F4 scope goes, I don't recommend it. At F4, you will
reach that 29X with a 28mm eyepiece which is OK but the problem with an F4
Newtonian (the only thing made at F4) is that the stars are only sharp in the
center of the Field of view. With some extra money and some high quality
eyepieces, this can be improved but those big wide fields of view on wants will
never be sharp across the field of view.

In an 8 inch scope, one is better of IMHO with F5 or F6, at F6, things are
pretty nice with normal eyepieces and additional correctors ($300) are not
needed.

Bottom line: Best of both worlds is too much to ask for.

Jon

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