From: hermit 
CC: ATM Archives 
Reply-To: hermit 


Chuck Taylor wrote:
>
> > > The trick here is that for a given star only a tiny spot of
the diagonal
> is
> > > used.   Surface errors beyond the boundaries of that spot are
> meaningless.
> >
> > When looking at stars, I'm not much concerned with detail. ;-)  What
> > about extended objects?
>
> But extended objects are effectively a collection of points, if those points
> are not smeared (smeared=highly technical term), the detail in the extended
> object is not smeared. If those points are smeared, then the detail in the
> extended object is smeared.
>
> Although I suppose someone could argue that a point is not smeared, but has
> been moved by a small area deformation in the diagonal. But even then, it
> would have smeared any point that was only half imaged through that small
> deformation area.
>

But if there is an error in the system, wouldn't those "points"
end up in the wrong place and there by reduce resolution of the object? 
Plus, I just don't buy "But extended objects are effectively a
collection of points".  Even a point source hits the primary objective
across the entire surface.  It is not a point when it hits the diagonal.
You are not diverging or converging or warping a point.  So the error is a
function of the distance from the focal plane and how much error you have
in the focused image area and the distance off axis of the image? Since the
image is smaller at this point, does it take less error to make it more
noticeable?

A long time ago I seem to remember someone saying that errors in the
figures were additive as squares or rms values or something like that?

Is this the "Anti Thread" to the "My mirror has a pv of
1/12345 wave? If you can't figure a primary, that's ok.  Just do the
diagonal? ;-)

Ken

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