Re: ATM 21" F/6: response about sagitta equations

[Dwight Elvey <elvey@hal.com>]

Dominic-Luc Webb molmed <Dominic.Luc-Webb@molmed.ki.se> wrote:
> 
> If the shorter equation is for a parabola, 
> why does Howard claim it will not work to use 
> that equation for all focal ratios whilst an 
> equation for a sphere will? 
> 

Hi Dominic
 I'm sure what he is talking about is that for rough
grinding, you are trying to make it spherical. Since
at fast f/ratios, the spherical deviates from the
parabolic by measurable amounts, I believe this is
why he makes these statements.
 For a finished mirror, it will always follow the
parabolic formula, assuming it is parabolic.
Dwight