Re: ATM A scope like Alice

["Michael J. Coslo" <mjc5@psu.edu>]

>I've been planning my second telescope for awhile now, and I think
>I've decided to try making one like Alice, with whatever changes I
>can think of....my main question is this: is there any reason I
>couldn't make an 8" f/6 ? I want to make the mirror myself (first
>one) and though the f/6 would be easier...that aside, will this work
>well for planetary viewing and a little deep sky? I'm still new at
>this and unsure of all the effects the focal ratio has on
>performance. Thanks for your help, keep up the interesting posts!

You certainly can, Jason. F6 is a good compromise, size wise.

        In general, (very general) the effects that focal ratio's have on
scope performance is that:

higher focal ratio's have less coma,

higher focal ratio's have less difference between a sphere and the correct
parabola. If the difference is so small that a spherical mirror is within
1/4 wavelength, you don't need to parabolize unless you want to.

higher focal ratio's are longer in body, they can become a problem to use.

lower focal ratio's suffer from coma, correctable with guess what, a coma
corrector. Coma is a natural "defect" that is just inherent in the
steepness of the curve.

lower focal ratio's are nice and short, great for altitude challenged people.

For large mirrors, a lower focal ratio approaches being a must have. There
are exceptions to this, but those scopes are so darn long, you risk a
nosebleed climbing up the ladder 8^)

        - Mike -