> "Dr. Michael Blaber" a écrit :
>
> Hello
>
> here is a dumb question that I would like an answer to...
>
> If I focus on a star, and then point the scope at a planet, or the
> moon, will the focus be appropriate for these objects, or will an
> adjustment be required?
>
> thanks
>
> Mike

The formula which gives the position of the focus (call it p) versus
the position of the object (call it p') for a lens of focal distance f
is :
1/p + 1/p' = 1/F
When looking at an object which is at infinite (say a star, or even
better a distant quasar ), 1/p' = 0, and p = F, i.e. you should be
focused exactly at the focal point of the objective lens or mirror.
Now, for a practical calculation, if you are using a 20cm F/10 telescope
(i.e. 8"), and looking at the moon (for simplification, on a day where
it is located 400000km from us, i.e. 400,000,000,000 mm from us), you
get :
1/p = 1/2000 - 1/400,000,000,000 = 0.0004999999975, i.e. p =
2000.00001mm, or 10 nanometers away from exact focus.
To me it looks like exactly in focus. Focus should not be changed. Even
for satellites which are 300km away, focus is the same (same
calculation, 13.3 microns away from exact focus, still in focus :-)).
Now, we can do the reverse calculation. If I remember well, the diameter
of the Airy disk for a given telescope is given by 1.22 x lambda (i.e.
0.5 microns) x F/D (damned, or is it the radius ?, maybe I am making a
mistake there).
An F/10 system gives 6 microns images (or 12 ? ). At F/10, this should
be obtainable on a 2x10x6 = 120 microns range (60 microns each side of
focus). Suppose that you are at the exact focus, you should be able to
see an object with the required sharpness 60 microns away in the focal
plane, or 2000.06mm. This gives 66km away. Everything closer should be
in need for refocussing.
Anyway this gives the correct idea that everything above the atmosphere
is at the same focus (not talking about focus position variation because
of flexure, temperature, etc... ).
Alain

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