Re: ATM Focus questions for my 4.25" f/10 Newtonian

[epabcc@epa.ericsson.se (Bratislav Curcic)]

Kevin wrote :

> 4.25" blank into a f/15 or so sphere, and make a Herschilian
> reflector.  

Unfortunately, not quite. F/15 is too short for a spherical
Herchellian. It would need to be an off axis parabola ... not
a beginners project.

> The shallow mirror would be quickly generated, and be easy to test.

... and be bastard to figure. Long f/ratios can be quite tricky.

>  But I will argue with anyone who says a sphere comes automatically...
> not for me it doesn't!

It _does_,  but at fast f/ratios. At f/2 it is almost impossible to get 
anything else but sphere !

> Inexpensive Ramsden & Kellner eyepieces should work well saving him $$'s.

No. The effective f/ratio is less than half of what your mirror is.
So with f/15 primary, eyepieces are working at ~f/7 (not quite, but close
enough). It can be made a bit easier for eyepieces by tilting them 
towards the center of primary.

> What is that formula for figuring what f# is needed to meet Raylegh
> criterion with spherical mirror?

Now we're coming to the crunch ... with 4.25 primary you will need to
look at ~10 inch "effective" primary diameter. A 10 incher must be
something like ~f/10 to meet "1/4 wave" (I'm reluctant to call this
"Rayleigh" criterion. Note that at this f/ratio mirror doesn't fulfill
Danjon & Couder criteria). 
This makes our scope 100/4.25 or f/23 ... yeah, at this f/ratio Ramsden
& Kellner will work OK; but at more than 8 feet from the ground when 
pointed at zenith, not quite what one may call "practical little scope".

Bratislav