----- Original Message -----
Sent: Sunday, March 28, 2004 6:42 AM
Subject: Re: [M]: Focus math


> That gives the range assuming that it's the EP that's being moved. In
this
> case the mirror is being moved, which moves the focal plane.
> -John
The same calculation works, but you have to use the focal length of the
primary. Obviously also, there is a question of what error level is
acceptable, which may differ (Suiter gives a formula that is a little less
'stringent' than the one quoted). The formula quoted is for a 0.5 wavlength
aberration. On the f/2 primary, the position has to be accurate to 0.0088mm
for the same error.
Typically screw threads on the focusser are something like 20TPI to 32TPI
(finer threads on the smaller scopes). Hence assuming 25TPI (1mm/turn),
gives a 3.2 degree 'zone'. This is very hard to achieve by hand with the
normal focus knob.
This is why micrometer adjusters, 'tin lid' attachments, or electric
focussers offering precise positioning (like the RoboFocus), are really
needed to get the best from an SCT when focussing with the primary..

Best Wishes

> >
> >At 2004-03-27 11:05 -0700, Keith wrote:
> >
> >>How much can the mirror be moved using a 6mm
> >>EP and still stay in focus?
> >
> >According to the bible (Born & Wolf, "Principles of optics"), 7 ed., p.
> >491, the focal tolerance is
> >
> > ±2(f/)²L
> >
> >with L the wavelength of light. This is assuming perfect optics, i.e.,
the
> >only degrading of the image is due to defocus. For your f/10 LX200 and
> >visual maximum L = 550 nanometers, the focal range is 400*550E-6 = 0.22
mm.
> >

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