Re: [ATM] How fast is too fast?

["Bob May" <bobmay@nethere.com>]

I do know that F2 gets to be way too short!
The problem with the shorter focal ratios is that you are unable to get the
image out to the side with a decent sized flat.  With a F2, your flat is
about 1/2 the size of the primary just to get the image to the edge of the
input beam as 1/2 of the distance from the mirror to the focal plane is
needed just to get the beam to the side.  With a F3, 1/3 of the diameter of
the priimary is needed to get to the side and that is still a rather large
size for a secondary.  10" for a 30" mirror!
Something about a F3.5 is pretty close to what I'd more consider to be a
minimum focal ratio for a Newtonian telescope otherwise the secodary gets
too big and the obstruction to the incoming light too great.  How much
shorter you can get by doing relay optics is another question altogether
tho.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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